ia__intro

Introduction To

Instrumentation &Process Control

Edition 7

Instrumentation & Process Control

This manual is intended for instructional purposes only and shouldnot be relied upon for technical detail. Always consult manufac-turer or other authoritative source before making design changesor performing maintenance on equipment.

NTT shall not be responsible for the accuracy of, use of, or inability touse any information provided in this student manual. NTT makes nowarranties regarding the information contained in this guide anddisclaims all implied warranties including but not limited to those ofmerchantability and fitness for a particular purpose.

Training sessions are not to be tape recorded or video taped withoutprior approval.

Copyright Notice

© 1998, 1999, 2000, 2001, 2006, 2007 by National TechnologyTransfer, Inc.All rights reserved.Edition 7 © October 2007Printed in the United States of America

Published By

National Technology Transfer, Inc.P.O. Box 4558, Englewood, CO 80155-4558800-922-2820 • 303-649-9980 • www.nttinc.com

Acknowledgments

NTT wishes to express appreciation to the following individualswho have assisted in development of this book.

Author: Robert M. Dombek

Field instructors who reviewed the text and offered comments and suggestions: Jeff Grovom,Carl Michael,Vernon Miller, AlanPike, Neil Finch

Art Director: Cynthia Rishko

Illustrator: Derek Ricks

Layout Production: Lynn Kehr

Copyeditor: Eliza Gibbons

AUTHOR’S NOTE:

I would like to dedicate this book to my wife Michelle, daughtersRebekah and Chloe, son Benjamin and to my loving parents Janiceand the memory of George who,through the grace of God, broughtme into this world supporting and guiding me in all my endeavors.

—Robert M.Dombek

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Welcome

I hope that your time invested in this seminar proves education-ally rewarding and personally enjoyable for you.

Our experienced instructors and supportive training materials aregeared to provide you with the information you need and to helpincrease your skill level for when you return to your responsibili-ties on the job.

Over a quarter million people have attended NTT seminars andhave benefited from, as well as contributed to, the success andquality of these courses. Your job experiences and applications maybe as beneficial and relevant as any part of the seminar. Iencourage you to offer examples or ask questions.

On the following page is a list of other seminars and on-sitetraining programs we offer. See for yourself if there are any othertopics that could help further your career, then call us toll free toregister at 1-800-922-2820. Or visit our website: www.nttinc.com.

Thank you for attending this seminar. I invite you to let me knowyour thoughts and impressions of this program. You can write tome directly at [email protected].

Sincerely,

Robert M. DombekElectrical Trades Program Manager

Table ofContents

Pre-test For Instrumentation & Process Control

Post-test For Instrumentation & Process Control

Chapter 1 Basic Concepts

Instrumentation...................................................................1-1

Purposes for Modern Control Systems ...............................1-2

Terminology..........................................................................1-3

Characteristics of Instruments and Measurement ....1-6

Signal Range vs. Span .................................................1-8

Components of a Feedback Control Loop....................1-9

Signal Types ...............................................................1-13

Signal Errors ..............................................................1-14

Signal Prescription.....................................................1-14

Shielding and Grounding...........................................1-15

Examples of Analog Signals ......................................1-16

Signal Converssion Example.....................................1-17

Controller....................................................................1-18

Signal Converters.......................................................1-18

Final Control Elements .............................................1-19

Piping & Instrument Diagrams (P&IDs)..........................1-19

Discrete Instruments .................................................1-20

Shared Display, Shared Control ................................1-20

Computer Function ....................................................1-20

Programmable logic Control......................................1-21

Line Symbols ..............................................................1-21

Instrument Identification Tags .................................1-22

Instrument Bubbles ...................................................1-24

Piping and Device symbols used on Piping &Instrument Diagrams ................................................1-25

Loop Diagrams ...........................................................1-26

Control Example ................................................................1-34

Control Technologies..........................................................1-35

Local Manual—On/Off or Modulated........................1-35

Remote Electrical—On/Off ........................................1-36

Local Pneumatic Modulated......................................1-36

Remote Analog Electronic—Modulated ....................1-37

Remote Digital Electronic—Modulated ....................1-38

Basic Electrical and Math Concepts .................................1-40

Applications to Instruments......................................1-42

Electrical Principles and Symbols.............................1-43

Series Circuits ............................................................1-44

Parallel Circuits .........................................................1-45

Variable Resistors ......................................................1-46

Instrument Math .......................................................1-48

Chapter 2 Pressure Instrumentation & Measurements

Pressure Measurement Devices ..........................................2-3

U-tube Manometer.......................................................2-3

Bourdon Gauge.............................................................2-3

Bellows Gauge..............................................................2-5

Piezoelectric..................................................................2-6

Pressure Summary ......................................................2-7

Chapter 3 Temperature Instrumentation & Measurements

Measurement Devices and Techniques...............................3-2

Bimetallic Temperature Measurement.......................3-2

Filled Capillary & Bulb Temperature Measurement.3-3

Thermocouple Temperature Meassurement...............3-3

Resistance Temperature Detector TemperatureMeasurement................................................................3-9

Thermistor Temperature Measurement ...................3-11

Thermowell.................................................................3-12

Infrared Temperature Measurement ........................3-13

Chapter 4 Flow Instrumentation & Measurements

Flow Measurement Methods...............................................4-2

Factors that Influence Flow Measurement ........................4-3

Type of Fluid ................................................................4-3

Velocity Profile .............................................................4-3

Differential Pressure Flow Measurement ..................4-5

Piping considerations...................................................4-6

Line Size .......................................................................4-7

Inferential Flow Measurements..........................................4-8

Orifice Plates ................................................................4-8

Venturi Tube...............................................................4-14

Flow Nozzle ................................................................4-14

Elbow Taps .................................................................4-15

Pitot Tube ...................................................................4-15

Parshall Flume...........................................................4-16

Velocity Flow Measurements ............................................4-17

Magnetic Flowmeter (Mag Meter) ............................4-17

Vortex Shedding Meter ..............................................4-19

Turbine Meters...........................................................4-20

Target Flowmeter.......................................................4-21

Ultrasonic Flowmeter ................................................4-22

VariableArea Rotameter ............................................4-23

Mass Flow Measurements.................................................4-24

Coriolis Meters ...........................................................4-24

Volumetric Flow Measurements .......................................4-26

Nutating Disc Positive Displacement Flowmeter ....4-26

Chapter 5 Level Instrumentation & Measurements

Level Measuremment Types ..............................................5-1

Sight Glass ...................................................................5-2

Differential Pressure Level Measurement .................5-2

Bubbler .........................................................................5-5

Displacer Level Sensor ................................................5-6

Float Level Sensors......................................................5-7

Capacitance Level Sensors ..........................................5-9

Radiation-Based Level Sensors.................................5-11

Radar and Ultrasonic Level Sensors.........................5-12

Chapter 6 Density, Specific Gravity & Analysis

Density and Specific Gravity...............................................6-1

Measurement Types and Principles............................6-1

Monitoring & Analyzing Techniques...................................6-4

Conductivity .................................................................6-4

pH..................................................................................6-4

Chapter 7 Manipulating The Process

Final Control Element .........................................................7-1

Globe Valve...........................................................................7-2

Flow Characteristics ............................................................7-3

Actuators ..............................................................................7-4

Valve Failure Modes.............................................................7-4

Valve Positioner ...................................................................7-5

Pneumatic Device Operation—I/P ......................................7-6

Split Range Control Valves..................................................7-7

Butterfly Valves ...................................................................7-8

Ball Valves............................................................................7-8

Variable Speed Drive and Motor .........................................7-9

Chapter 8 Troubleshooting

Testing for Open Circuits ....................................................8-1

Testing for Short Circuits ....................................................8-4

Troubleshooting Suggestions ..............................................8-8

Installation Example Drawings ........................................8-11

Chapter 9 Controllers

Control Modes ......................................................................9-2

Proportional Gain.........................................................9-3

Integral Control............................................................9-6

Derivative Control........................................................9-7

Control Modes Example ......................................................9-8

Tuning Feedback Controllers ............................................9-111/4 Decay Ratio.............................................................9-11

Zeigler-Nichols ...........................................................9-11

Damped Oscillation....................................................9-12

Examples ............................................................................9-12

Ratio Control ..............................................................9-13

Cascade Control .........................................................9-14

Feedforward Control ..................................................9-15

Chapter 10 Control Systems

Control System Overview..................................................10-4

Central Processing Unit ....................................................10-5

Programming Interface Device .........................................10-7

Power Supply .....................................................................10-9

Input and Output (I/O) Sections .....................................10-11

Fixed I/O PLC...........................................................10-11

Modular I/O PLC......................................................10-12

Input Section ............................................................10-15

Output Section .........................................................10-20

Addressing........................................................................10-24

Module Location-Dependent Addressing .......................10-24

Processor Scan Routine ...................................................10-26

Summary of Scan Routine ..............................................10-29

Glossary

Appendix A Thermocouple Conversion Tables

Appendix B Resistance vs.Temperature

Appendix C Instrument Loop Diagram Symbols

Appendix D Instrumentation-Related Internet Addresses

Appendix E Conversions

Appendix F

Appendix G

Pre-test ForInstrumentation& Process Control

1. The difference between the measured variable’s value and the desiredset point is? __________________________________

2. T F RTD’s work on the principle of two dissimilar metals, whichwhen in contact produce a potential.

3. ________________ is defined as the amount of change in the input thatcorresponds to a 100% change in output.

4. T F Another term used to describe derivative control is resetcontrol.

5. A type of control strategy in which one feedback loop is placed insideanother feedback loop:a. Cascade.b. Ratio.c. Feedforward.d. None of the above.

6. The __________________ describes the necessary equipment to convertthe controller output signal into the signal needed to properly controlthe process.

7. The most common or frequently used temperature detector in usetoday is:a. RTD.b. Thermistor.c. Thermocouple.d. Integrated circuit transducer.

© 2007 National Technology Transfer, Inc. PRE-1

8. Which type of control is often associated with process operations inwhich it is necessary to mix two or more streams together continu-ously to maintain a steady composition in the resulting mixture:a. Feedforward.b. Cascade.c. Ratio.d. None of the above.

9. Name three types of flow elements that produce a differential pressure.1. _______________________2. _______________________3. _______________________

10. A thermowell protects the temperature element but degrades:a. Accuracy.b. Maximum temperature measured.c. Minimum temperature measured.d. Response time.

11. It is difficult to tune a pH control loop when fluid pH is:a. Highly acidic.b. Slightly acidic.c. Neutral.d. Slightly alkaline.e. Highly alkaline.

12. A magnetic flow meter requires that the fluid being measured have acertain amount of ____________________________________________

13. T F Proportional Band as related to Gain is expressed as PB% =100 / Gain.

14. Which of the following variables would most likely require derivativeas a mode in its controller:a. Flow.b. Level.c. Pressure.d. Temperature.

Introduction To Instrumentation & Process Control

PRE-2 © 2007 National Technology Transfer, Inc.

15. What is the most common analog signal used in industry:a. 0–10 AC volts.b. 4–20 DC volts.c. 10–50 DC milliamps.d. 4–20 DC milliamps.

16. What will be the voltage across a shorted load in a series current loop?

________________________________________________________________.

17. In a pneumatic control system, the common cause of misoperation is

________________________________________________________________.

18. To minimize noise problems in a computer based control system, use:a. A signal ground.b. Twisted pair.c. A shield or drain.d. All of the above.

19. A shielded signal cable must have its shield:a. Grounded at one end.b. Isolated from ground.c. Grounded at both ends.d. Reinforced.

20. For a pressure transmitter monitoring a gas and located at a higherelevation than its process connection, the instrument line must:a. Slope up from the process to the transmitter.b. Slope down from the process to the transmitter.c. Be as level as possible.d. Not be drained periodically.

21. Offset can be eliminated in a proportional only feedback controlsystem by:a. Changing the setpoint.b. Changing the percentage proportional band.c. Retuning.d. Changing the manual reset.

Pre-test For Instrumentation & Process Control

© 2007 National Technology Transfer, Inc. PRE-3

22. A 4-20 dcma transmitter’s highest output is 18 dcma. Which of thefollowing cannot be the cause?a. A load in the loop is shorted.b. The transmitter needs recalibration.c. The power supply voltage is too low.d. Too much total resistance is in the loop.

23. Which of the following devices is typically a part of a process control loop?a. Final control device.b. Primary element.c. Controller.d. All of the above.

24. Hysteresis is when the:a. Process variable does not equal the set point value.b. Indicated value increases proportionally to the measured variable.c. Indicated value will be different for a given signal when the signal

is increasing vs. when the signal is decreasing.d. Indicated value will not change for a part of the range that the

input changes.

25. T F A P/I transducer will convert current into pressure.

26. What is the most common and inexpensive pipe connection type fororifice plate applications? ______________________________________

27. The Proportional Band or Gain adjustment will determine the__________________ of the output change for a given error.

28. What is the lower range value for an indicated calibrated 70°F to 250°F?

_________________________________________________________________

29. In an automatic mode, if a controller’s output value increases whenthe process variable decreases?a. The proportional band is not adjusted correctly.b. The controller action is set to reverse.c. The controller has adaptive gain.d. None of the above.

30. T F The output of an ON/OFF controller will slowly change from alow to high value.


PRE-4 © 2007 National Technology Transfer, Inc.

Post-test ForInstrumentation &Process Control

1. To be considered a closed loop a circuit needs ________________.

2. T F Thermocouple extension wire is always copper wire that hasspecial tolerances in the manufacturing process.

3. ____________________ responds to the PV or error signal rate of change.

4. T F To increase the effect of Proportional Band the PB value needsto be increased.

5. A type of control strategy that can be regarded as an open loop is:a. Cascade.b. Ratio.c. Feedforward.d. None of the above.

6. The ___________________ describes the necessary equipment toconvert the controller output signal into the signal needed to properlycontrol the process.

7. The temperature detector with the highest accuracy is the:a. RTD.b. Thermistor.c. Thermocouple.d. Integrated circuit transducer.

8. Which type of control is considered a form of open loop control?a. Manual.b. Automatic.c. Cascade.d. None of the above.

© 2007 National Technology Transfer, Inc. PST-1

9. Name three flow meters that use velocity to determine flow?1. ______________________________2. ______________________________3. ______________________________

10. A thermowell’s internal diameter needs to be:a. The same as the device outside diameter being inserted.b. Twice the diameter of the device being inserted.c. Can be anything that is available.d. None of the above.

11. The span of a transmitter calibrated to 50 psig to 250 psig is ________ .

12. T F A flow nozzle has a lower accuracy capability but more stabilitythan an orifice plate.

13. In North America, for a thermocouple negative wire, the insulationcolor is __________.

14. When a 4-20 DC milliamp signal is applied across a 62.5Ω signalresistor the voltage signal will be:a. 1.0–5.0 DC volts.b. 0.4–2.0 DC volts.c. 0.2–1.0 DC volts.d. 0.25–1.25 DC volts.

15. A two-wire transmitter in a control loop will function like a:a. Power supply.b. Fixed resistor.c. Variable resistor.d. All of the above.

16. T F A three-wire RTD uses the third wire to compensate for thelead resistance.

17. For a primary flow element that develops a differential pressure theflow profile needs to be:a. Transition flow.b. Turbulent flow.c. Laminar flow.d. None of the above.


PST-2 © 2007 National Technology Transfer, Inc.

18. T F A meter that can accept multiple thermocouples and correctlyconvert the signal to indicate temperature has softwarecompensation.

19. For normal operation and measurement, the equalizer valve on athree-valve manifold, used with a differential pressure transmitter,should be left in the _____________ position.

20. The voltage signal from a pH sensor is:a. Linear.b. Logarithmic.c. Unusable.d. None of the above.

21. T F When a load becomes a short in a two-wire transmitter circuit themilliamps in the circuit will go extremely high and blow a fuse.

22. The integral component of a three-mode controller will stop changingthe controller output when the ______________ becomes zero.

23. For a steam valve adding steam to a heat exchanger the fail safe positionshould be:a. Open.b. Closed.c. Last position.d. None of the above.

24. A proportional only controller will always have _____________ afterload changes.

25. T F A four-wire transmitter has four wires so that the lead resis-tance can be measured and compensated.

26. A Globe valve will have the following flow characteristic.a. Quick opening.b. Linear.c. Equal percentage.d. Any of the above.

27. In Auto, if a P & I controller’s PV is oscillating the proportional bandwill need to be:a. Increased.b. Decreased.c. Unchanged.d. Any of the above.

Post-test For Instrumentation & Process Control

© 2007 National Technology Transfer, Inc. PST-3

28. T F A valve positioner installed on a control valve is a closed loopsystem.

29. An orifice plate should be installed so the information engraved on thehandle will be facing _______________.

30. Before changing an instrument which of the following should be checked:a. Electrical power supply.b. Air supply.c. Configuration.d. All of the above.


PST-4 © 2007 National Technology Transfer, Inc.

Basic ConceptsObjectives

Upon completion of this chapter, the student should be able to:• Understand what a closed loop is and the parts of it.• Understand common signal types.• Know instrumentation terminology.• Recognize and understand piping and instrument drawings and

process flow diagrams.• Understand basic electrical and math concepts as applied to

instrumentation.

Introduction

The content and organization of this text is designed to be used as a toolto introduce novice maintenance technicians to the world of instrumenta-tion and process control.

Complex and vague concepts and principles will be simplified into easy-to-understand and useful information. Common process and control systemterms will be defined and application examples will illustrate keyconcepts, preparing the reader to be better qualified and capable ofworking with process control systems and devices.

Instrumentation

The field of instrumentation and process control analyzes, monitors, andadjusts the various devices used to measure properties of forces that canaffect a process. When we describe a process we are describing a systemsuch as an oil refinery, chemical plant, beverage or pharmaceuticalfacility, etc., where ingredients are blended through various means andmeasured and adjusted to create an end product.

© 2007 National Technology Transfer, Inc. 1-1

1

Figure 1-1Process facility

Automatic control or process control is achieved by safely regulating aprocess using various feedback and computing devices to monitor andmeasure variables that can affect the end product.

When diving into the process control world, you will come across manydifferent devices and systems that are designed to measure and controlvarious variables affecting the process.

Purposes for Modern Control Systems

As process control systems are put in place to control and manage thevariables affecting an end product, the individual methods and technolo-gies that are integrated can have various benefits to the overall system.The underlying factors that determine which process control devices andequipment are used are cost, time and safety.

• Economics/Cost – the number of personnel required to accomplisha task has been reduced due to the advent of automated processes.

• Time – faster performance of routine tasks, replacement of peoplein hazardous tasks – fewer lawsuits, health concerns.

• Safety – quicker response to emergencies – issues with operatortraining

• Consistancy – in doing tasks.


1-2 © 2007 National Technology Transfer, Inc.

Along with these factors, other benefits from modern computer-basedcontrol systems (whether PLC, DCS or computer) allow measurementdata to be remotely monitored and adjusted by supervisors and operators.

Terminology

As a person studies any particular field or trade in order to becomecompetent, he will need to learn the language and terminology used.Common instrumentation terms will be presented and defined andemphasized in bold throughout this text. Where applicable, examples willbe given to help further explain the context of how the terms pertain toreal circumstances.

Let’s start with a rudimentary heating example of a campfire and developit into a modern HVAC control system using and defining terminologyalong the way.

Figure 1-2Campfire

For this first example of a process, the campfire, the process that wewould like to control is temperature. Main control for heat in this exampleis the regulation of air. In this process it is a manual process in which theoperator can add fuel to the fire or fan the fire to cause it to burn hotter.This is manual control.

Manual control or open loop control is when a human has to interact withthe process in order to regulate or adjust the process.

Automatic control or closed loop control uses feedback devices (in place ofhuman control) in order to close the loop. Don’t think of an automaticcontroller as super-human control. A human can control the process just

campfire b.eps

Chapter 1: Basic Concepts


about as well as an automatic control, however automatic control is moreconsistent, reacts quicker, and is more economical to implement.

If we modify the heating system further by placing it into a box (like awood burning stove) to heat the interior of a house, we still have manualcontrol. In this example we can open the air inlet to allow more air intothe system causing the fire to burn stronger (and therefore consume morefuel) and produce more heat.

Figure 1-3Wood burning stoveheat control

If the heat decreases and we want to increase the heat, we must increasethe output (let more air into the furnace). The direction in which weadjust the output (increase or decrease) is called controller action.Controller action determines the way the controller is going to respond toa process change. Direct and reverse. In direct mode, if the process vari-

able – PV (which is one particular measured variable) increases, thecontroller will increase the output (also if PV decreases, the controller willdecrease the output). In reverse mode, if PV increases the controller willdecrease the output (also if the PV decreases, the controller will increasethe output).

woo

d bu

rnin

g st

ove

heat

con

trol b

.eps



Figure 1-4Direct and reversecontroller action

In your process, your operator might run into a situation where it seemsthat as soon as he puts the system on automatic, the results go the wrongway. In this situation the operator would just have to go into the controller’sparameters and make sure the controller action is set to the proper mode –direct or reverse. If controller action is not set correctly, it will drive the PVto one limit (upper or lower temperature) until action is taken.

Let’s improve our heating application further by installing a furnace inour home. This furnace will have an automatic control system. Thefurnace uses a feedback control system, or a thermostat to measure thetemperature of the home’s interior environment.

Figure 1-5Furnace control

Notice that we can have disturbances in the processes. Disturbances inthis example can be outside air temperatures, doors opening/closing, andwindows opening/closing. If the house door were left open on a cold day

0%

100%

ControllerOutput

Time

Time0%

100%

0%

100%

ProcessVariable

0%

100%

ControllerOutput

Direct

ControllerAction

Reverse

ProcessVariable

direct n reverse controller b.eps



(without your knowledge) the first action you might take is to raise thesetpoint of the thermostat. If the temperature still doesn’t reach thesetpoint, you might say the furnace is broken; however, the underlyingproblem is that the system isn’t designed to deal with the disturbance,which in this example is the door being open. The disturbances need to bemanaged for process control-related issues.

Notice also that for this furnace control example it is an on-off auto-matic control.

Characteristics of Instruments and Measurement

This section defines and describes the terms related to describing thecharacteristics instruments demonstrate.

The accuracy of an instrument is a measure of how close the signalreading of the instrument is to the correct value.

Precision is a term that describes an instrument’s degree of freedom fromrandom errors. High precision does not imply anything about measurementaccuracy. The following figure illustrates differences in precision and accuracy.

Figure 1-6Precision, accuracy andrepeatability

Repeatability describes the closeness of output readings when the sameinput is applied repetitively over a short period of time, with the samemeasurement conditions, same instrument and observer, same locationand same conditions of use maintained throughout.

Tolerance is a term that is closely related to accuracy and defines themaximum error that is to be expected in some value. When used correctly,tolerance describes the maximum deviation of a manufactured componentfrom some specified value. For instance, resistors have tolerances of 5%.

Y

Low AccuracyLow Repeatability

X

Y

Low AccuracyHigh Repeatability

X

Y

High AccuracyHigh Repeatability

X

accu

racy

and

repe

atab

ility

b.e

ps



One resistor chosen at random from a batch having a nominal value of1000Ω and tolerance of 5% might have an actual value anywhere between950Ω and 1050Ω.

The sensitivity of a measurement is a measure of change in instrumentoutput that occurs when the quantity being measured changes by a givenamount. It is the ratio of “scale deflection” to “value of measurandproducing deflection.”

Hysteresis is a term that describes the non-coincidence between loadingand unloading curves. Hysteresis is commonly found in instruments thatcontain springs such as a passive pressure gauge. The following figure illus-trates the output characteristic of an instrument that exhibits hysteresis.

Figure 1-7Hysteresis characteristics

If a quantity measured is steadily increased from a negative value, thenthe output reading varies according to curve a in Figure 1-7. If the quan-tity measured is then steadily decreased, the output varies according tocurve b in Figure 1-7. The fact that the two curves do not follow the samepath illustrates the effects of hysteresis.

Dead band is defined as the range of different input values over which thereis no change in output value. Any instrument that exhibits hysteresis alsodisplays dead band as seen in Figure 1-7. Some instruments that do notexhibit significant hysteresis can still exhibit a dead band.

Input

10% Input Change

Hysteresis

Dead Band

Hysteresis plusDead Band

50% Input Change 100% Input ChangeO

utpu

t

(a) (a) (a)

(b) (b)(b)

(c) (c) (c)



Signal Range vs. Span

Range is the extent of a signal’s lower value to its maximum value. Forexample, if we were expressing a range of flow we would say it rangesfrom zero to 100 gallons per minute. A signal’s span is the differencebetween the minimum and maximum values. For example, if we were toexpress a span for temperature that ranges from 200 to 500 degrees, thespan would be 300 degrees.

As another example, in a 4 – 20 mA signal, remember from the definitionof range that it goes from the lower range value/minimum: 4mA to theupper/maximum range value 20mA. The span for this example would be16mA. Keep in mind that range and span can define the same quantity,but not always. The following is a summary of range and span:

Lower Range Value LRV (minimum) – can be zero, but doesn’thave to be

Upper Range Value URV (maximum)

vs.

Span is the difference between the minimum and maximumvalues.

The following two names are what manufacturers use for adjustments:

Zero and span

Zero and range

When you read literature for your devices, realize that the terms “span”and “range” are both used, but they may not always be used properly.

Looking ahead at Figure 1-15, notice that the dashed line (yellow) has alower range value other than Ø. In this example it is 4mA. The reason itdoesn’t start at zero is because by starting at 4mA when we takemeasurements, we know that if there is a value (of 4mA), then there ispower on the signal. If we had zero mA when we took a measurement wewould know that somewhere power was interrupted along the circuit. So,in summary, having a minimum range value that is other than zero givesus important information when troubleshooting. Having a minimumrange value other than zero is often called a live zero.

Another benefit of having an elevated lower range value is that when thecircuit’s power is a positive DC signal, we have no means of quantifying anegative number with a positive power supply. With an elevated lowerrange value we can quantify a negative error. For example, in the case inwhich we read 3.9mA with a 4-20mA signal, we know we have a negativeerror. The signal is below the specified minimum range value of 4mA. Bythis we achieve accuracy and troubleshooting benefits.



Components of a Feedback Control Loop

One thing to note is that for a particular control loop we can only controlone variable. This is why in a majority of applications we must havemultiple control loops. The following figure shows an example of thecomponents typically seen in a feedback control loop.

Figure 1-8Components of afeedback control loop

Let’s define some more terms related to the previous figure.

A process is a means or method of producing a product.

A process variable is one measured element of the process.

Secondary elements or transmitters are used to convert the sensor’s orginaloutput signal into an electrical or pneumatic signal that can send data to aremote user over a long distance without signal degradation. The trans-mitter’s output signal must be much larger than any interference so thatthe signal will not be overpowered and corrupted by it.

In this example here’s how we measure: the primary element or sensor firsttransmits a low-level signal (a signal that doesn’t travel long distances).



Figure 1-9Secondary elementsor transmitter

Since the signal doesn’t travel long distances we need a secondaryelement/transmitter that can convert the low-level signal to a high-levelsignal (a signal that can travel long distances without signal degrada-tion), which in many cases is a 4 to 20mADC current signal. Secondaryelements or transmitters are used to convert the sensor’s output signalinto an electrical or pneumatic signal that can send the data to a remoteuser over a long distance without signal degradation. The transmitter’soutput signal must also have much higher amplitude than any interfer-ence so the signal will not be overpowered or corrupted by it. As a sidenote, according to OSHA we don’t need to wear special safety equipmentwhen working with currents in this range.

A controller is a device that takes the information from a secondaryelement/transmitter, compares it to the setpoint and changes the output.

Figure 1-10Single loop controller



A recorder is a printing device used to record or log information about aprocess. There are two main types of recorders: continuous trace and digital.

Figure 1-11Bailey trace recorder

For continuous trace recorders there should be a separate non-clogginginking system for each pen with sealed and replaceable ink cartridges.

Figure 1-12Yokogawa digital recorder

Digital recorders should typically display the point number that is beingprinted, and the descriptive data (date, time, scale range, messages)should be printed as the recorded value is being printed.



An indicator is a device used to display the real-time measurement valueof a particular process variable.

Alarms are methods for indicating when a component has failed or ameasured variable is reaching (or has reached) a value that can behazardous or can cause damage.

A signal converter is a device used to convert a measured value from onemeasurement method to another. I/P is a current (electrical symbol forcurrent is I) to pressure (P) converter or pneumatic signal. Other abbrevi-ations of signal conversion methods include P/I and E/P.

A final control element is the device that is used to affect the process,which in this example is a control valve. It is important to note that themanipulated variable is not necessarily the same variable as the processvariable (what you’re measuring).

Figure 1-13Process

Some examples of processes are:1. Production of steam and electricity in a power plant.2. Production of a chemical in a chemical plant.3. Canning beer in a brewery.4. Pumping heavy rainfall runoff from New Orleans to Lake

Ponchartrain.

Figure 1-14Electrical andmechanical variables

In order to achieve and/or maintain a process a measurement is made andconverted into a proportional variable of another type. These days mostmeasurements are converted to an electrical variable. Examples of elec-trical variables are millivolts or volts, resistance or impedance, frequencyor phase shift. Examples of mechanical variables are movement, force,torque or displacement. The methods and steps involved in a feedback



control loop are still the same for measurements, however the technolo-gies available for how we make the measurements are different.

When measuring a condition of a process, a sensor’s output variable repre-sents the process variable, but is more conveniently processed. It isdesired that the sensor’s output variable be linear, however, many are onlyapproximately linear, or they may even be highly non-linear.

Linear means that the change in the process variable is reflected in thesensor output in a proportional (straight-line) manner. In the followingfigure notice that there are two linear signals and two non-linear signals.

Figure 1-15Linear and non-linear signals

The dashed line represents how the output signal would look for a differ-ential pressure transmitter when measuring flow.

The red line is a very non-linear signal and has poor characteristics fromzero to 100 percent. Some measurements like this one, for example measurefrom 40% to 100%, where the signal is somewhat linear and useful.

Notice all four signals increase with an increase in process variable. Thisis a positive coefficient of change. Be aware that there are signals with anegative coefficient of change, in which the output decreases the processvariable increases.

Signal Types

Analog signals have two limits and any change between those two limitsis meaningful. An analog signal is one that varies continually between



high and low limits. An example of an analog quantity is temperature.The temperature varies continually; it is never on or off.

Figure 1-16Analog anddigital signalcharacteristics

Discrete or digital signals have two values and any other value is of nomeaning; these are usually on/off or 1/0 or high/low. Analog signals havemuch more information associated with them.

Since analog signals have much more information packaged, the elec-tronic equipment to process and transmit analog signals costs much morethan that needed to process and transmit digital signals.

Signal Errors

Realize that there are acceptable error limits. For a 4-20mA signal this istypically about .05mA, this is important when calibrating your instru-ments. For example, if your upper range value is greater than 20mA youwould first have to adjust your instrument back to 20mA, then you wouldhave to measure the lower range value so that it is outputs at the lowerrange, for example, 3psi at 4mA. This process is repeated until the errorsare minimized. In common calibration methods a 5-point test is takenbecause when we take measurements, we are somewhere in the middle ofour range (say 9psi). A 5-point test verifies the linearity of the signal overthe whole range, not just the upper and lower range limits.

As a rule of thumb we want to choose our instruments so that the quan-tity that we are measuring (pressure, flow, temperature) typically residessomewhere in the 40%-70% range of the span.

Signal Prescription

Notice in the following figure that we have a 4-20mA loop. Remember thata controller has an input and an output. We have a 4-20mA signal on the

Max

Min

Voltage

Voltage

Time

Max

MinTime

Analog

Digital

analog vs digital b.eps



input (FT5 – a flow transmitter) and another separate 4-20mA signal onthe output that powers up FY5 and FIC5 and FI5. The nice thing with aDC signal is that it can be easily manipulated.

Figure 1-17Example loopconnections

In this example the DC power source is 24VDC. With DC the signal ispolarity-sensitive, so we must make sure the positive and negative termi-nals are connected properly to the device.

In this example notice that the devices in the control loop are connected inseries.

Shielding and Grounding

An important concern in a 4-20mA loop is the way we protect the elec-trical signal that travels along the loop. Due to the sensitivity of thesignal and to prevent harmful signals, other electrical devices such asvariable frequency drives, UPS, or switching DC power supplies that youmay have in most industrial facilities, it is highly recommended that youuse twisted shielded pair cable. This cable is specifically designed andmanufactured to have two signal wires twisted together and surroundedby a non-insulated shield. By having the two signal wires of the twistedpair cable twisted, it reduces the chance that a harmful signal can consis-tently act upon the area of a specific location of the wire.



Figure 1-18Wire types

On a diagram like the one shown in Figure 1-17, notice the signal wiresare shown as solid and the shields are shown as dotted. Also notice thatthe shield is connected to ground only at one end.

NOTE

The cardinal rule for connecting analog signals is that the shield isconnected to ground only at one end of the loop.

Another important item of concern is that for signal grounds for electronicinstrumentation devices such as PLCs, DCSs, it is always recommendedthat the grounding point not be connected to a power ground (like thegrounding point for motors, drives, etc. inside a cabinet) but to connect tothe source of the ground, which would be the grounding grid for a partic-ular machine or plant depending on how your facility is designed.

As you are running your twisted shielded pair cable from your instrument(which is typically out on the machine) back to your electrical panels, if atall possible do not run it in the same conduit or cable trays as your controlor power cables.

Examples of Analog Signals

Electronic analog signals have been used to convey information betweencontrol system components for quite some time. There are several varia-tions of the electrical properties of how an analog signal can betransmitted. The 4-20mADC signal is probably the most common analogsignal used in process industries. When we say mA, this tells us that thesignal is a current signal (milli-amps). As an example a 4-20mA signal canrepresent a process pressure ranging from 0 to 100 psig.

When process electronics began, the 10-50mADC signal was the typicalsignal range. An example of how this signal was used might be to repre-

Unshielded Twisted Pair (UTP)

Shielded Twisted Pair (STP)

Coaxial

Twinaxial

Shield

Shield



sent a process temperature with a range of 100 °F to 500 °F. The benefitsof using a 4-20mA range versus a 10-50mA range is that if you have aprocess in which you might have hundreds to thousands of control loops,the maximum value of 20mA of the 4-20mA range uses much less powerthan the maximum value of 50mA in the 10-50mA range.

When using current signals in the field, if the analog signal is beingconverted to a digital signal (such as converting a 4-20mA flow signal intoa PLC analog input module), then the module or instrument doing theconverting will convert the analog current signal into a voltage signalusing analog to digital (or A/D) signal processing. This 4-20mA signal isconverted to a 1-5VDC signal using a 250Ω resistor. The details of thiswill be covered in the Basic Electrical Concepts portion of this chapter.

A 0-10VDC signal is typically used to represent rotational speed, such asfor adjustable frequency drives that can rotate a motor from 0 to 3600 rpm.

In older control systems or control systems used in hazardous areas youstill might find pneumatic signals such as a 3 to 15 psig pressure signalthat might represent a process temperature signal ranging from 100 °F to500 °F. Similarly you might find a 6 to 30 psig pneumatic signal from anI/P transmitter to a control valve diaphragm to fully stroke the valve from0% to 100% open.

Signal Conversion Example

Let’s say we have a process temperature that has a range of 100 °F to 500°F and a transmitter that will send a 4-20mADC signal. The following isthe formula used to convert the actual process temperature into milliamps:

Formula 1-1 mA = ((PV - LRV) x 16)..+ 4Span

where, PV = Process Variable

LRV = Lower Range Variable

To determine what amount of milliamps the transmitter should be sending,we first need to know the actual value of the process variable, which in thisexample is temperature. Let’s say the current temperature equals 300 °F. Ifwe put the numbers into Formula 1-1 we get the following:

mA = (((300 °F - 100 °F)) x 16) + 4

400 °F

mA = (( (200 °F)) x 16) + 4

400 °F

mA = 8 + 4 = 12mA



So, for a process temperature that currently equals 300 °F, having a rangeof 100 °F to 500 °F, the process would be represented as 12mADC in a 4-20mADC signal.

Controller

A controller measures an input signal, compares it to a setpoint, computeshow it should react, then sends an output signal to the appropriate deviceto adjust the process. The setpoint is the desired value at which thecontroller tries to keep the process variable maintained.

Figure 1-19Controller

How much and the timing of how the output signal should change or reactis determined by the controller’s PID (Proportional, Integral andDerivative) settings. How much an output signal changes is largely depen-dent on the proportional setting or proportional band or gain. The timingof how the output signal changes is primarily dependent on the integral ordecent. These terms will be explained further in Chapter 9.

Signal Converters

Signal converters convert an input signal to a linearly proportionaloutput signal. For example, a 4-20 mADC signal into an I/P converterproduces a proportional 3-15 PSIG output signal that can be used tostroke a control valve.

Figure 1-20Signal converters



Final Control Elements

The final control elements manipulate the process in such a way as todrive the controlled variable back to its setpoint. The manipulated vari-able (or controlled variable) may not necessarily be what we’re measuring.It is a variable that once manipulated will affect the process.

Figure 1-21Final controlelements

Piping & Instrument Diagrams (P&IDs)

On any drawings, symbol consistency is probably the most importantissue to address. As a general rule of thumb, typically the signal linesymbols are shown with a lighter weight line than the piping or processline symbols.

Most of the instrument symbols that will be presented in this text weredeveloped by standards developed by ISA. The following figures describethe different types of general instrument or function symbols.



Figure 1-22General instrument or function symbols

Discrete Instruments

Field mounted means an instrument is located near the process it is asso-ciated with. Discrete means it is a stand-alone instrument. This istypically a symbol for a panel-mounted transmitter with a display.

Auxiliary location. This is an instrument located away from the processequipment it is associated with. For example, this might be a boiler appli-cation. It is a secondary indication for someone that is not typicallyinvolved in the control, such as an operator.

A square around the symbol means it is a shared display or shared control.

Shared Display, Shared Control

Shared Display, shared control symbols represent HMI graphics andcontrollers.

Computer Function

Computer functions, as the term implies, are functions, logic and tasksperformed by the computer.

Primary LocationNormally Accessible

to the OperatorField Mounted

Auxiliary LocationNormally Accessible

to the Operator

Located Behinda Panel or May

Not be Accessible

DiscreteInstruments

Shared DisplayShared Control

ComputerFunction

ProgrammableLogic Control

instrument function symbols b.eps



Programmable Logic Control

Programmable Logic Control (PLC) symbols indicate PLC functions anddevices. It is not uncommon to see Programmable Logic Control andShared Display, Shared Control symbols being used interchangeably, espe-cially since the differences between PLC and Distributed Control Systems

(DCS) are becoming less visible. These symbols are often interlocks thatare inputs to the PLC.

Line Symbols

Line symbols are used to designate signal types between control compo-nents and/or piping.

Figure 1-23Line symbols

Undefined Signal

Hydraulic Signal L L

Pneumatic Signal

Pneumatic Binary Signal

Electric SignalUS International

or

US InternationalorElectric Binary Signal

Capillary Tube

Electromagnetic or SonicSignal (Guided)

Electromagnetic or SonicSignal (Not Guided)

Internal or Software Link

Internal or Software Link line

sym

bols

b.e

ps



Instrument Identification Tags

According to ISA standards, an instrument identification tag consists ofthe following two parts: a functional identification and a loop number(such as TT-001). There is also an expanded tag number consisting of afunctional identification, a loop number and an optional prefix and/or anoptional suffix. An example of this is shown in the following figure.

Figure 1-24Identification tags

In this case the optional prefix 100 represents the P&ID page number ofthe complete drawing set.

The ISA (Instrument Standards of America) has developed standard iden-tification lettering for piping and instrument diagrams. The followingfigure is a table of identification lettering according to ISA standards.

The total number of letters in an identification tag should not exceed four.The following figure is what ISA lists as the identification letters andcombination of letters to represent the instrument devices.

Using the following table of instrument identification lettering, what dothe following devices represent?

100-TIC-001 __________________________________________

101-PT-004 __________________________________________

422-FQI-021 __________________________________________

325-LALL-003 __________________________________________

100-TT-001Loop Number Temperature Transmitter

Loop Number 001, on P&IDPage Number 100.

Functional IdentifierPID Page

instrument tag numbers b.eps



Figure 1-25General instrument or function symbols(Courtesy N.E. Battikha– The Condensed Handbook of Measurement and Control)

MEA

SURE

D O

R IN

DIC

ATIN

G V

ARI

ABL

EM

OD

IFIE

RRE

AD

OU

T O

R PA

SSIV

E FU

NCT

ION

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TPU

T FU

NCT

ION

MO

DIF

IER

AM

RAL

A)91 ,5 (

S IS

YLA

NA

B)1(

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C S

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)1( E

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CL

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C)1(

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IFFE

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AG

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(13,

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Instrument Bubbles

Instrument bubbles will be displayed in two different formats. On a P&ID,they will be shown with a partial tag number. The P&ID identifier will beomitted. The P&ID number is assumed to be the same as the page it is onunless otherwise noted.

Figure 1-26Instrument bubbles

Instrument bubbles on loop sheets detail a full tag number so that youwill easily be able to associate it with a P&ID, or a system.

Figure 1-27Instrument bubbles

Identify the devices and signal lines in the following control loop.

Instrument Bubble

Loop or InstrumentNumber

Functional Identifier

P&ID Page Number100TT001

Instrument Bubble

Loop or InstrumentNumber

Functional IdentifierTT001

Instrument Bubbles will be displayed in two different formats



Figure 1-28Elements of acontrol loop

Piping and Device Symbols used on Piping & Instrument Diagrams

Piping and device symbols used on P&IDs illustrate the physical connec-tions of the process. Keep in mind that the piping and device symbols onlyrepresent the connection points (similar to how symbols on a ladderdiagram represent the electrical connection points) and do not indicatethe physical location of the individual components with respect to eachother in the system.

Figure 1-29Example pipingand device symbols

TY001

TT001

TE001

TV001

TIC001

Controlling Element

Measuring Element

Primary Element

FinalElement

AS25 PSIG

I/P

elements of a control loop b.eps



Loop Diagrams

A loop diagram is a roadmap that traces process fluids through thesystem and designates variables that can disrupt the balance of thesystem. Loop diagrams show the details that are not shown in the P&ID.

Figure 1-30Details of loop diagrams



From the piping and instrument diagram, the loop diagrams use the samesymbols, but show the details of each individual loop. The following figuresshow a few loop diagrams.

Figure 1-31Example P & ID

TankT-100

P-104-1 P-101-1CGS

CGR

P-102-1

CGR

P-101-1

PV

006

LV

005

LY

005

LE

005.2

LT

005

LE

005.1

PY

006 TV

001

PT

006

P-103-1

PIC

006

LAHH

005

LAH

005

HS

003

YI

003

SI

003

HIS

003

HIC

003

FIC

002

LALL

005

LAL

005

LIC

005

HS

004

ZSL

004

ZSH

004

Pump P-001FE

002

SIC

003

HV

004

TE

001

FT

002

TY

001

ZSH

001

ZSL

001

HS

001

TIC

001

ZSH

001

ZSL

001

TT

001

AS25 PSIG

AS25 PSIG

HS

004

ZSL

004

ZSH

004

I/P

I/P





Figure 1-32 Individual control loops

The following figure is an example of a piping and instrument diagram.

TankT-100

FE

002

FT

002

FIC

002

HIC

003

flow control loop b.eps

TankT-100

TE

001

TV

001

TY

001

HS

001

TIC

001

ZSH

001

ZSL

001

P-101-1

P-101-1CGR

TT

001

ZSH

001

ZSL

001

tem

per

atu

re c

on

tro

l lo

op

b.e

ps

TankT-100

HV

004

HS

004

ZSL

004

ZSH

004

HY

004

ZSL

004

ZSH

004

P-102-1

Pump P-001

valve control b.eps

TankT-100

SIC

003

HIC

003

FIC

002

HIS

003

SI

003

YI

003

HS

003

FT

002

FE

002

P-102-1

Pump P-001



TankT-100

LV

005

LY

005

LAHH

005

LALL

005

LAH

005

LIC

005

LAL

005

LE

005.1

LT

005

LE

005.2

P-103-1PRD1

I/P

Leve

l co

ntr

ol l

oo

p b

.ep

s



Figure 1-33Piping & InstrumentDiagram



Figure 1-34Individual control loops





Control Example

We want to control the fluid level of the tank. For this system we have anautomatic inlet valve and a manual outlet valve. Inside the tank for fluidlevel measurement we have a float that is connected to a fulcrum and leversystem that automatically adjusts the inlet valve. Is this a closed loop? Yes,recall for a closed loop we need an input, a decision maker and an output.In this system, the inlet is a closed loop. It is a purely mechanical system.The controller/decision maker is the fulcrum/lever arm. The float is themeasurement. The output or manipulated variable is the inlet valve. Forthe level to stay steady the inlet and outlet must remain the same. Is theaction of this controller direct or reverse? The answer is reverse – recall forreverse, when we decrease the PV, we increase the output.

Figure 1-35Tank level controlwith a mechanicalsystem



Notice the fulcrum has the lever arms that are 20” and 10”, which is a 2:1ratio. With that said, if the input changes 1” the output will change 2”.This is proportional control. Proportional control looks at the input changeand changes the output a proportional magnitude.

Let’s examine a situation of proportional control to determine how well itcontrols this process. Assume we need 150gpm in and 150gpm out tomaintain the setpoint. If we then get a disturbance or load change wherethere’s now 250 gpm in, the output will match the 250 gpm, however thelevel will be much lower and won’t be able to get back to the setpoint. Thedifference between the setpoint and the new steady level is called offset.

Control Technologies

Though control technologies have changed, the control methods are thesame. Controlling the loads on or off or modulating them, by using electrical,pneumatic, electronic, can control a process manually or automatically,locally or remotely, or computer-based control equipment. The followingfigures show examples of different combinations of these variables.

Local Manual – On/Off or Modulated

Figure 1-36Manual controlsystem – on/offor modulated

This is an example of a control technology that was common around the1900s, whose system was controlled manually. One manual valve was forlevel and one manual valve was for temperature. An operator would haveto come out, look at the temperature and adjust the valve. This is openloop. If this system is designed to operate at 100 °F and the actualtemperature is 95 °F, the operator has a choice of opening the inlet valve(TV1) at any percent. He can open it a little or he can fully open it. This ismodulated control.



Remote Electrical – On/Off

To make the system a little more efficient, the system has been upgradedslightly to have remote indication (as shown in the following figure withremote indicators LI2, HS2, HS1, TI1). This type of control was commonaround the 1920s. Notice that the inlet temperature valve and the outletlevel valve are now solenoid-controlled valves. These valves are just on/off(or open/closed) valves.

Figure 1-37Manual controlsystem – on/offremote electric

Local Pneumatic – Modulated

As the system is developed further, notice the control signals are pneu-matic. With the addition of TIC1 we now go from manual control toautomatic control. Pneumatic control was common around the 1940s. Theoperator now just needs to set the setpoint. Since TIC1 (and LIC2) canmodulate the valves the control improves.



Figure 1-38Local pneumaticmodulated system

Remote Analog Electronic – Modulated

Notice for electronics we still have the same principles of control – aninput goes to the controller, the controller makes a decision and the outputis adjusted. The methods of accomplishing this are now done by means ofelectronic analog signals. As the price of electronics went down, (in the1970s to 1980s) this soon became the most cost effective method of control.

Figure 1-39Remote analogelectronicmodulated system



Remote Digital Electronic – Modulated

As the technology improved again the devices changed, but the methodsfor control are still the same. For this situation we have some sort ofcomputer control system that may be a DCS, PLC or something similar.

Figure 1-40Remote digitalelectronic modulatedsystem

We have a control room to which all the inputs from the field discretedevices were wired. If these devices were located 1000 ft. from the controlroom, we had to have long runs of wire to accomplish the terminations.



Figure 1-41Control room

With electronic and communication improvements, a more cost-effectiveway of laying out the system would be to have distributed controllersmounted in the field near the field devices, which would significantlyreduce the wire runs. These distributed controllers would then send theirinformation back to the supervisory controller for reporting and indicatingthe information and measurements. So there would be digital communica-tions between the distributed controllers and the supervisory controller, butanalog signals between the field devices and the distributed controllers.

Figure 1-42Distributed controls



Functionally the PLC system of control is quite similar. For the PLCsystem there is an HMI (human machine interface or operator interface)communicating with the PLC and the PLC remote racks or remote I/Ocommunicating the information from the field devices (sensors and loads)to the PLC’s processor.

Figure 1-43Control systemarchitecture

The big difference between a PLC system and DCS system is how theinformation is processed. The PLC typically processes information as afunction of time, whereas the DCS processes information as the infor-mation changes.

Basic Electrical and Math Concepts

Analog signals where current flows must have a complete circuit.

Host

Computer

Gateway

Node

Controller

PLC

Profibus-FMS

CNC PCS

Profibus-PA

Cell

Level

Information Network TCP/IP

Profibus

Application Range

Profibus-FMSProfibus-DP Profibus-PA

SensorSensor

Field

DeviceI/O Sensor Drive

Trans-mitter

Field

Device



For an electrical circuit to operate, all its properties combine to affect elec-tricity. The relationship between these properties is quantified by Ohm’slaw, which is the most important law in electricity. If you fully understandOhm’s law, you will be able to understand why electricity does what itdoes. Here are the two basic forms of Ohm’s law:

Volts = Amperes x Ohms

Watts = Volts x Amperes

One volt is the force needed to move one ampere through a conductor ofone ohm of resistance. The force of one volt moving one ampere uses onewatt of power. Kilowatt-hours is the measurement of electricity usage. Thisis similar to measuring water usage in cubic feet.The mathematical relationship of Ohm’s law is basically maintained in allelectrical circuits. The following figure shows the relationships betweenvoltage, current and resistance, and voltage, watts and current.

Figure 1-44Ohm’s law

V = Volts = E = Electromotive pressureI = Amps = A = Intensity of flowR = Ohms = Ω = Opposition to flowW = Watts = W = Force of one volt moving one

ampere

You can transform the basic formula V = A x R to get the following:

R = V I = V I R

If resistance is constant and voltage is increased, current will alsoincrease. If voltage is constant and resistance is increased, current willdecrease. For current to remain constant, voltage is decreased along withresistance. For the same amount of power (measured in watts), increasingthe voltage decreases the current needed. In turn, a smaller wire withmore resistance can be used. If you know or measure any two of the threecomponents, you can easily calculate the third (unknown) component ofthe equation.

120V = 10A x 12Ω240V = 20A x 12Ω

V V = EI = AR = Ω I R

OhmsL2_b.eps

WV A



If voltage increases while resistance is constant, current increases.

120V = 10A x 12Ω

120V = 5A x 24Ω

If voltage remains constant and current decreases, resistance increases.

120V = 10A x 12Ω

60V = 10A x 6Ω

If voltage decreases while current is constant, resistance decreases.

2,400W = 120V x 20A

2,400W = 240V x 10A

If power remains constant while current decreases, voltage increases.

Applications to Instruments

There are two main types of transmitters for instruments used in controlloops that regulate loop current:

• Process transmitter, 2-wire, with external DC power supply• Process monitors, 4-wire, with integral DC power supply

For 2-wire transmitters the voltage to supply the device as well as themeasurement signal travel along the 2 wires. 4-wire devices typicallyneed more power than 2-wire devices.

The electrical loads that respond to the range of current produced (andthat are connected to the transmitters) are:

• Indicators• Recorders• Current to Pressure Converters• Signal Resistor(s)

Conductors are the pathways for the electrical signals between devices.The wire used for signals is twisted shielded pair. This wire is specificallydesigned to be resistant to electrical noise and interference.

The following figure demonstrates the differences between 2- and 4-wire transmitters.



Figure 1-452- and 4-wire transmitters

Notice for the 2-wire transmitter circuit, from the positive point of 24VDCpower supply we typically connect the positive terminal of the first device(in this case PT3). From each device after that notice we connect from thenegative of one device to the positive of the next device. If by chance youinadvertently reverse the polarity on one of the devices, the circuit will actlike an open and will not operate properly.

For the 4-wire transmitter, keep in mind that it has an integral powersupply. When connecting these devices be sure that you connect to theproper supply voltage. Nowadays, these are often connected to a 115VACsource, but not always. Be sure to check your manufacturer’s specifica-tions before connecting the power source.

Electrical Principles & Symbols

The electrical relationship between voltage, current, and resistance isaffected by the type of circuit used. Up to this point we have consideredcircuits with only one load and one wire or path. What if there aremultiple loads or paths? This is common in every house, commercial, andindustrial building. There are three basic circuits: series, parallel, andseries-parallel.

Resistance has units of ohms.

Figure 1-46Resistor Symbols



Series Circuits

A series circuit has only one path to and from the power source to all theloads. This one path consists of only two wires, a hot and a neutral, or twohot wires. An example of a series circuit is some Christmas tree lightstrings. In a series circuit the current is constant everywhere in thecircuit. The total resistance is equal to the sum of the individual loadresistances. The required voltage is the sum of all the voltage drops acrossthe individual loads.

Resistors in series: for voltage signals, if the signal is transmitted overlong distances the voltage drops off (because of the resistance in theconductors). For current signals the current in a series circuit is the same.Over long distances the current doesn’t change.

The following figure shows a circuit with a 10VDC voltage power supplywith three resistors in series. The total resistance RT is the sum of theindividual resistances ( R1 + R2 + R3).

Figure 1-47Resistors in series

For series circuits with a current source, the individual loads determinethe voltage. For the circuit in the following figure (and for all seriescircuits) the total voltage ET is equal to the sum of the individual voltagesacross each load ( = E1 +E2 + E3 = IR1 + IR2 + IR3 = 10mA x 100 + 10mAx 250 + 10mA x 1000 = 13.5VDC).



Figure 1-48Resistors in seriesin fixed current loop

Parallel Circuits

In a parallel circuit there are multiple paths or loads. This is how mostloads are arranged. Every receptacle, light and motor in your house or inany building is connected in parallel with each other. The multiple pathsconsist of at least two or more wires, which may be any combination of hotand/or neutral wires.

In a parallel circuit, voltage is constant everywhere. The total resistanceis equal to the inverse of the sum of the inverses of all individual resis-tances. Probably the most important thing to remember for parallelcircuits is that the total resistance will always be less than the lowestindividual resistance. The total resistance is not useful so much in ourcalculations as it is to understanding this aspect of a parallel circuit.Current is the total of all individual load currents. The following figuredemonstrates a parallel circuit.



Figure 1-49Resistors in parallel

Figure 1-50Signal resistor in parallelwith a high resistancevoltage input

Variable Resistors

A 2-wire transmitter acts like a variable resistor. A potentiometer alwayshas a minimum value of zero.



Figure 1-51Potentiometerinternal construction

For variable resistors, the maximum resistance value is always deter-mined by the resistive track.

A rheostat is a two-terminal variable resistor that is put in circuits toadjust current. A potentiometer is a three-terminal variable resistor thatis put in circuits to divide the voltage across it. The following figure showsthe symbols for rheostats and potentiometers.

Figure 1-52Variable resistorsymbols

An example of a rheostat application would be to use a variable resistor inautomobiles to adjust the dash lights. By changing the resistance, itchanges the amount of voltage drop that changes the current. Thefollowing figure illustrates this type of circuit.



Figure 1-53Potentiometer connectedas a rheostat

Voltage divider (potentiometer) is often used for such applications as ananalog input to an adjustable frequency drive to adjust motor speed. Asthe resistance changes it divides the voltage through the two differentparts of the potentiometer.

Figure 1-54Potentiometer used toset an adjustablevoltage

Instrument Math

The most common math used in instrumentation is to determine thepercentages of signal ranges. Notice below that we have ranges of numbers:

0_______ 100______

4_______ 20_______

3_______ 15_______

200_____ 500______



The underlying theme in these is that we have a lower range value (LRV)and an upper range value (URV).

URV – LRV = Span or Delta

To convert any measurement value to a percent you must use thefollowing equation:

Formula 1-2 100 x (Value – LRV)/Delta = %

Once the measurement is converted to a percent, it can then be convertedto an output or applied input value. The percent multiplied by the newdelta plus the zero offset equals the value needed for calibration or validation.

Example: An instrument ranges from 200 to 800 psi. What would be thepercent if the current measurement were 350 psi? Here are the facts:

Value = 350 psiURV = 800 psiLRV = 200 psiSpan = 600 psi

Percent = 100 x (350 – 200)/600 = 25%

The following table is provided to practice conversions:

LRV URV Desired New Value Quantity

4 mA 20 mA 40% mA (4-20mA)

1000 F 2500 F 1200 F mA (4-20mA)

0 psi 500 psi 200 psi psi (3-15 psi)

10 mV 50 mV 33 mV mA (4-20 mA)



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PressureInstrumentation &MeasurementsObjectives

Upon completion of this chapter, the student should be able to:• Recognize basic pressure-measuring principles.• Know the theory and recognize the common pressure-measuring

methods.

Introduction

Mechanical methods for measuring pressure have been used for centuries.Pressure is measured as a force per unit area. Not only do pressuremeasurements provide a means for monitoring and controlling pressure,but also for indirectly measuring other parameters such as level and flow.

Measurement References

One of the basic limitations of all measurements is that the measure-ments are relative. All sensors have a reference point against which thequantity being measured must be compared.

Inaccurate reference pressures as well as inaccurate sensors often causeerrors in pressure measurement. If absolute pressure is to be detected, thereference pressure should be zero or a complete vacuum. Since a referencechamber cannot be evacuated to absolute zero, a nonzero quantity is usedas a zero reference. The higher the reference pressure, the greater theresulting error.

In gauge pressure measurement the reference is atmospheric pressure,

which is variable. Because of the variability of atmospheric pressure,sensor output can change not because there is a change in the processpressure, but because the reference pressure is changing. Barometricpressure can change by as much as an inch of mercury (or 13.6 inches ofwater), which can cause excessive errors.


2

For pressure measurements there are three types of scenarios:• Gauge pressure (psig) – reference is atmospheric pressure• Absolute pressure (psia) – reference is a complete vacuum• Differential pressure – the difference between two pressure levels

The following figure illustrates the differences between the three differentpressure scenarios.

Figure 2-1Pressure measurementscenarios

Units of Measure

Pressure is the ratio between a force acting on a surface and the area ofthat surface. Pressure is measured in units of force divided by area. Thefollowing are some common pressure measurement units:

• Pounds per square inch (psi)• Newtons per square meter, or pascals

Very rarely will you see Psia in process use. Psia is measured in absoluteterms in reference to full vacuum. The following is a short list showingsome quick references of pressure conversions:

1 psi = 27.7

1 atmosphere = 14.7 psia

0.433 psi = 12 in H2O

2.02 in Hg = 1 psi

Variation inAtmospheric Pressure

Gage Pressure

Standard Atmospheric Pressure= 0 psig (0KPag)= 14.7 psia (101 KPaa)

Vacuum Pressure

Absolute Pressure = Gage Pressure + Vacuum Pressure

pressure measurement b.eps



Pressure Measurement Devices

For industrial processes pressure measurement is accomplished by sepa-rating the process from the sensing measuring unit to determine the forcebeing applied.

U-tube Manometer

Among the first pressure indicators were u-tube manometers.Manometers operate on the principle of hydrostatic pressure and the rela-tionship between pressure and the displacement of a column of liquid.This is a very accurate method for measuring low-pressure applications inair or gas pressure applications. The pressure is a differential pressuredetermined by the density times the height.

Figure 2-2U-tube manometer

Because of their size manometers are not well suited for integration intoautomatic control loops. They are typically found in the laboratory or usedfor local indication.

Bourdon Gauge

The Bourdon gauge is a very popular pressure measurement device forlocal indication. There are four types of Bourdon gauges:

• C-shaped• Twist• Spiral• Helixes

Chapter 2: Pressure Instrumentation & Measurements


The principle of the four types of Bourdon gauges are the same. One endis fixed. The surface area on the inside vs. the surface area on the outside;the outside has more surface area. When a force is applied to the inside ofthe tube the outside actually gets the greatest force, causing the tube totry to straighten out.

Figure 2-3Bourdon gauge

For this type of Bourdon gauge the motion is transferred through a gearedsector and pinion that drives the indicating needle on the shaft of a cali-brated range of motion.

The following describes a particular error to watch for when calibratingthese types of meters. If the gauge is 0-30 psig as the pressure goes upthen decreases and the upscale measurement is different than the down-scale measurement, there is a backlash error. This backlash is whatcreates hysteresis. Recall for the Bourdon gauge you’ll often see operatorstapping the meter to get it to go back to the final measurement. Backlashoccurs when the geared sector and pinion are out of calibration.

The calibration procedure demands you do a 5-point check in theupstream direction and a 5-point check in the downstream direction.



Figure 2-4Bourdon tubeMeasurement(Courtesy Wikipedia.org)

Bellows Gauge

The bellows gauge is intended to sense small pressure differences by anindicating needle connected through a small gear train connected to anenclosed and sealed chamber. The mechanical motion is similar to adiaphragm, but has a wider span of movement.





Figure 2-5Bellows gauge

Because of its simple design and principle of operation the bellows gaugepressure sensitivity tends to increase as the size increases. The bellowsstyle is commonly used in pressure gauges and pressure switches.

Piezoelectric

As pressure, force or acceleration is applied to a quartz piezoelectriccrystal, a charge is developed across the crystal that is proportional to theforce applied. This charge can be measured by electronic circuitry andconverted to an output indicative of pressure.

Figure 2-6Piezoelectric pressuregauge

There are two types of piezoelectric crystals: natural crystals andsynthetic crystals. Natural crystals are rugged and withstand shock,however they are sensitive to temperature changes. Synthetic crystalstend to produce higher electrical output.

Pressure Summary

Although there are many other methods and devices for measuring pres-sure, this text concentrates on the methods found in the process industries.

The following figure illustrates a pressure transmitter with instrumentpiping as is typically seen in the process industry.

ElectricalConnector

Shrink TubingGrooves

IC Amplifier5/16 Hex

5/16-24 Thd.

Element Lead

ElementLead

SealSurfacePreloadScrew

QuartzCrystal

ElectrodeEnd Piece

Diaphragm

piezoelectric pressure gauge b.eps



Figure 2-7Pressure transmitterwith instrument piping



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TemperatureInstrumentation &MeasurementsObjectives

Upon completion of this chapter, the student should be able to:• Recognize basic temperature measuring principles.• Become aware of the different temperature measuring instruments

and devices and their applications.

Introduction

Temperature is a difficult property to measure with repeatable and accu-rate results. This is because every temperature measurement we make isan indirect measurement, as there is no direct way to measure tempera-ture. In order to measure temperature we have to infer a change in someother property (such as voltage, pressure, resistance, expansion of liquid,etc.) to assume a temperature change.

The following is a list of different apparatus used to measure temperature:• Thermocouple• Resistance Temperature Detector (RTD)• Thermistor• Bimetallic dial thermometer• Filled capillary & bulb system• Infrared photodiode or phototransistor: pyrometry or temperature gun• Liquid in glass: mercury or alcohol in glass

Of the above methods, the one that is most accurate and repeatable is theliquid-in-glass method. In the past this method was the standard for cali-brating other temperature measuring devices.


3

Measurement Devices and Techniques

For process applications, a characteristic temperature measurementassembly consists of a thermowell, a temperature element and a tempera-ture transmitter.

Bimetallic Temperature Measurement

A bimetallic dial thermometer is primarily used as a local temperaturemeasurement method for industrial use. The element is constructed of twodifferent metals that expand at different rates and at different temperatures.

Figure 3-1Bimetallic dialthermometer

The two metals are physically bonded together and attached at one end.When temperature changes, the movement generated by the expansion ofthe metals drives an indicator on a scale. Thus, we have a mechanicalmovement. This is not a very accurate device, however, it is relativelyinexpensive to manufacture. The bimetallic dial thermometer is verysusceptible to overranging. If you put it in too high of a temperature itcannot withstand the mechanical twisting capabilities. There is only onepoint for calibration, which is accomplished by adjusting a screw. Thetemperature element does not extend the full length of the shaft. Withthat said when placing this device in a measurement application careshould be taken to make sure the sensing element is in contact with whatis being measured.

Rotating Shaft

Free End Attached to Pointer Shaft

Fixed EndBulb



Filled Capillary & Bulb Temperature Measurement

This method depends on the expansion or vaporization of a liquid insidetubing that creates a pressure change that is proportional to a tempera-ture change.

Figure 3-2Filled capillary& bulb measurement

This system is a metallic assembly consisting of a bulb, the capillary tubing,and a Bourdon spring. There is a mechanical connection from the Bourdontube and the indicating lever. The expansion and contraction of the liquid orgas in the bulb is translated into a mechanical motion on the indicator.

NOTE

A rule of thumb for this device is to never disconnect the capillary tube.

This type of measurement system is commonly used for local indication orfor temperature sensing in self-actuated temperature control valves. Dueto the size of the bulb and the frailty of the capillary system (if the filledsystem fails, the whole system must be replaced), the number of theseunits has decreased over the years.

Thermocouple Temperature Measurement

Thermoelectricity is the result of two dissimilar materials being heated orcooled. Copper and iron or nickel and iron are welded together. As heat isapplied to the junction point, the atoms are excited. The copper or nickelatoms repel their electrons into the circuit. In turn, iron electrons flowtowards the other metal.

Bourdon Tube

Volatile Liquid Capilary Tubing

Bulb

Vapor

Chapter 3: Temperature Instrumentation & Measurements


If the junction point is cooled, the iron repels its electrons and the othermetal contributes its electrons to the flow. This results in the circuitreversing the current flow.

Figure 3-3Thermocouples

Thermocouples are very common as safety devices in furnaces and otherappliances. A flame or pilot light provides the heat to the thermocouple. Thethermocouple produces a voltage to activate a coil, which retracts a plunger.The plunger opens an orifice to supply gas to the flame. The plunger alsocompletes the circuit to the main gas valve. If the flame goes out, both theorifice and main valve circuit are disabled, thus no gas will escape.

In 1821, Thomas Johann Seebeck by chance found that a voltage existedbetween two ends of dissimilar metals when a temperature differenceexisted in the metals. The thermoelectric EMF created causes a current toflow in the conductors if they form a complete loop. The voltage created isdependent on the types of metals, however only a few types are usedbecause of their superior response and sensitivity to temperature changes.The thermoelectric EMF measured is in the range of microvolts (µV) per °F.

Main Gas

Thermocouple

Pilot Gas

To Thermostat

Coils

Springs

Thermocouples_b.eps



Figure 3-4Thermocoupleprinciple

There are two contact points in a thermocouple; the measuring junctionand the reference junction. When a heat source is applied to themeasuring junction a thermoelectric EMF is generated proportional totemperature. Since the resistance between the two dissimilar metals istypically too low to effectively measure voltage, typically the extensionwires must be connected to an instrument. The issue with doing this isthat typically the terminals on the instrument are copper, which createsanother thermocouple, which causes a counter EMF. Compensation is theterm that describes the effort to counteract this effect. The meter readingis really the difference between the cold junction and the hot junction, orwith the instrument, the reading between the hot junction and the instru-ment terminals.

The original thermocouples had a reference point that had the two metalsplaced in a bucket of ice water.

Now the devices consist of another temperature sensor to measure thetemperature of the instrument’s terminals. If a meter requires that youhave a specific thermocouple on the input it is hardware compensated.The nice thing about hardware compensation is that it is an immediatecompensation. Software compensation basically takes the thermocoupleinput along with the terminal temperature sensor and does a computationadding the values. This method is a little bit slower, but not significantlyso. This method allows the user to install any thermocouple and selectfrom a menu in the instrument type.

If there is a significant length between the measuring junction and thereference junction, it is important that the extension wires connecting thetwo junctions are of the same metal. Copper should be connected to copper

ALead Wire

Gage

Thermocouple

Tip

TargetSurface

B Vout

+

–

+

–

ReferenceJunctions

Ice Bath(Known ConstantTemperaturefor Reference)

thermocouple junction b.eps



and constantan to constantan. Some manufacturers make terminal blocksspecific to thermocouples.

Figure 3-5Thermocoupleterminal block

Some questions might arise as to whether the extension wires need to bethe same gauge wire as the thermocouple wires. The answer is no. Thinkof the thermocouple as a battery. With an AAA battery we measure 1.5volts. If we have a D cell battery we measure 1.5 volts. It’s not the size ofthe battery that determines the voltage, it is the material inside thebattery that determines the voltage.

There are basically three configurations for thermocouples: grounded,exposed and ungrounded. For the grounded type, the junction is welded tothe sheath. The exposed type has the junction exposed for high-speedresponses. The ungrounded type has the junction insulated from thesheath even at elevated temperatures.

Magnified view showsspecific usage markings

Thermocouple connection block

straddles the bridge between terminals



Figure 3-6Sheathed thermocouple ends

Figure 3-7J-type thermocouple with ceramic insulation

Thermocouples have color codes for the connecting wires. The following isa table for North American thermocouple color codes.

ExposedJunction

UngroundedJunction

GroundedJunction



Figure 3-8Thermocoupledesignations

If you hook up a thermocouple the wrong way the temperature responsewill act in reverse. This will not hurt the thermocouple; just reverse thewires. From Figure 3-8, you can see that there are different temperatureranges, different costs, etc., therefore there must be different types of ther-mocouples.

The following figure describes the advantages and disadvantages associ-ated with thermocouples.

Figure 3-9Advatages anddisadvantages associated withthermocouples

Advantages1. Self-powered (mV output)

2. Simple, rugged (shock resistant) construction

3. Inexpensive (half the cost of RTDs)

4. Wide choice of physical forms & wide temperature range

5. Fast response (if a thermowell is not used)

6. Easy field construction using thermocouple wire

Disadvantages1. Thermocouples need compensation at the reference junc-

tion necessitating corrective software or electronics.

2. Non-linear response



Resistance Temperature Detector Temperature Measurement

Resistance Temperature Detectors (RTDs) are wire-wound and thin-filmdevices that measure temperature because of the physical principle of thepositive temperature coefficient of electrical resistance of metals. In anutshell, they are sensors that measure by correlating the resistance ofthe RTD element with temperature.

Figure 3-10Wheatstonebridge

Most RTD elements consist of a length of fine coiled wire wrapped arounda ceramic or glass core. The element is usually quite fragile, so it is oftenplaced inside a sheathed probe to protect it. The RTD element is madefrom a pure material that has documented and predictable changes inresistance with temperature changes.

RTDs are probably the most accurate and repeatable method for tempera-ture measurement used today in industry. They are also relativelyimmune to electrical noise, which makes them well suited for temperaturemeasurement in industrial environments, especially around motors,generators and other high-voltage equipment.

Platinum RTDs are the most commonly used, then copper, and finallynickel. The common lead wires are joined together near the platinumelement. The third wire is used to nullify the lead resistance. For processcontrol, three-wire RTD is the minimum accepted.

Wheatstone bridge is a well-regulated voltage source.



Figure 3-11RTDs with Wheatstone bridgeexample

Ra =

Rc =

LRa =

LRb =

Rc =

The excitation voltage is going through Ra and Rc as well as through theRTD and Rb. If we put a high resistance meter across the output termi-nals measuring voltage change to equate to the resistance change, thenthere is a voltage drop across the lead resistances LRa and LRb, whichcancel each other out and thus eliminate effects on the reading. If we havea 2-wire RTD we have lead resistance that effects the measurements.

Accuracy and repeatability are the advantages of the RTD over the ther-mocouple, however, the RTD doesn’t cover the wide temperature rangesthat the thermocouple can.

Excitation

Ra Rc

Rb

+LRa

-LRb

-Rc+

OutputRemoved if 2-wire

Connectsif 2-wire

DMM

RTD

w W

heat

ston

e br

idge

b.e

ps



Figure 3-12Advantages &disadvantages associated withRTDs

Thermistor Temperature Measurement

A thermistor is a thermally sensitive resistor that exhibits a change inelectrical resistance with a change in its temperature. The resistance ismeasured by passing a small, measured DC current through it andmeasuring the voltage drop produced. Since the thermistor is basically asemiconductor that performs the function of an RTD, like a semiconductorit has a higher resistance than an RTD (thousands of ohms for thermis-tors vs. hundreds of ohms for RTDs).

The thermistor is not as widely used in the process industry because it has avery non-linear relationship. Thermistors are typically used when there aresmall temperature ranges and the accuracy within that range is critical. Anexample of thermistor sensor use is the digital medical thermometer.Thermistors are also used in electronic equipment to sense temperature.

Figure 3-13Thermistor

Advantages

1. Higher accuracy over a given temperature range

2. Better linearity

3. Reference junction compensation not required

4. Special extension lead wire not required

Disadvantages

1. Maximum temperature limit is under 660 °C (1200 °F)



Thermowell

The following shows a typical RTD assembly.

Figure 3-14Three wire RTD probe

Thermowells are used to provide isolation between a temperature sensorand the environment. A thermowell allows the temperature sensor to beremoved and replaced without compromising either the ambient region orthe process. There are many variations of two basic kinds: low pressureand high pressure. The following figure shows some basic configurationsof thermowells categorized by installation mounting.

Figure 3-15Thermowellconfigurations

Though thermowells can provide a handy and quick means of replace-ment/repair, the following are some disadvantages to think about whenconsidering using them:

(A) Threaded Well–Straight (C) Socket Weld–Design

(B) Threaded Well–Tapered (D) Flanged-Type Well

thermowell configurations.eps



• Increased purchase and installation costs• Slower temporal response to temperature changes• Increased temperature measurement error due mostly to stem

heat loss down the length of the thermowell

Be careful changing out thermowells; be sure to use the same tip diam-eter, profile and length. Other geometries can cause resonant vibrationsthat can cause the thermowell to break off.

Infrared Temperature Measurement

The infrared method is most commonly seen not necessarily in the processindustry but for maintenance and checking for hot spots. In power distrib-ution panels if there is a loose electrical connection or termination, thecurrent will be higher there, thus causing an increase in temperature.

Figure 3-16Infrared temperatureMeasurement (CourtesyOmega.com)

Many industries either test their cabinets or hire outside contractors tocheck their electrical panels for hot spots on a biannual basis. Infraredtechnology is the method used because it is non-contact and provides safetemperature measurement in potential arc flash hazard areas.





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Flow Instrumentation &MeasurementsObjectives

Upon completion of this chapter, the student should be able to:• Recognize basic flow-measuring principles.• Know the theory and recognize the common flow-measuring

methods, instruments and devices.

Introduction

When we look at the big picture of our components of a feedback controlloop (as seen in the following figure), we will be concentrating on the flowdevices involved with the primary element or sensor and the secondaryelement or transmitter.

Figure 4-1Components of afeedback control loop


4

Rangeability is the ability of the meter to be adjusted without major modi-fication (changing out sensors or something of that sort); it is the ratio ofthe maximum span to the minimum span the meter can be adjusted.

Turndown is the ratio of the max value to min value that the meter canhave and still retain accuracy.

Here’s the difference between the two. Let’s say we have two meters. Onehas a 3:1 turndown with a scale of 0-100gpm. The second meter has a 10:1turndown with a range of 0-100gpm. The first meter will maintain thesame accuracy between 33 gpm and 100 gpm. This doesn’t mean it won’tmeasure below 33 gpm, but the measurement’s accuracy will be inferiorbelow 33 gpm. The second meter will maintain accuracy between 10 gpmand 100 gpm. The more turndown there is, the better the meter, and typi-cally the higher the cost. The turndown typically goes with the type ofmeasurement; for instance this turndown might be for orifice platemeasurements, not necessarily a specific manufacturer’s product.

Flow Measurement Methods

There are basically four methods for measuring flow: inferential, velocity,mass, and volumetric.

Inferential measurement means another physical property is measured toinfer a flow measurement. The following are inferential methods formeasuring flow:

• Orifice plate• Venturi tube• Flow nozzle• Elbow taps• Pitot tube & Annubars• Target flowmeters• Weirs & flumes• Variable area rotameter

Velocity measures the speed of the fluid and if the surface area of the pipeis known, the flow rate can be calculated. The following are velocitymeasurement methods:

• Magnetic flowtube• Turbine meter• Vortex meter• Ultrasonic• Radar



With a mass flowmeter, if the mass or weight of the material flowing canbe measured, it can be translated into a flow rate. The following is a massmeasurement method:

• Coriolis meter (one of the most accurate meters)

The volumetric method measures flow by calculating how much time ittakes to fill a specific volume of fluid. The following is a volumetricmeasurement method:

• Nutating disk

In a later section of this chapter each of the flow measuring methods willbe explained in further detail.

Factors that Influence Flow Measurement

Some additional factors to consider when making flow measurements(other than standard factors for most measurements like process condi-tions, measuring range, accuracy) are:

• The type of fluid• The velocity profile• Piping considerations• The line size

Type of Fluid

The type and condition (dirty or clean) of the fluid can present limitationsto flow measurement techniques. For example, on most turbine meterssteam cannot be measured. Some measuring devices such as differentialpressure devices may become plugged or eroded if used where there aredirty or corrosive fluids, whereas magnetic meters would have no problemmeasuring such fluids.

Velocity Profile

Velocity profile is important for determining what measurement methodto apply. For a fluid with laminar flow, the flow of the fluid along the pipesurface is slower than the flow along the center of the pipe.

Chapter 4: Flow Instrumentation & Measurements


Figure 4-2Laminar andturbulent flow

Flow profiles can be classified into three types:• Laminar• Turbulent• Transitional

It is pretty much agreed upon that fluids with a Reynolds number below2000 have laminar profiles. Some texts - depending on the factors consid-ered - list turbulent flows as those with a Reynolds number greater than4000; some are actually greater than 7000.

Figure 4-3Laminar,turbulent andtransitional flow

The Reynolds number is a dimensionless number/factor that considers afluid’s velocity, diameter, density and viscosity to determine flow charac-teristics.



Re = (v D ρ) / μ = Velocity (ft/sec) x Diameter (ft) x Density (lb/ft3)

Viscosity (poises)

Based on the factors that affect the Reynolds number, keep in mind that ifone changes it could have a significant effect on the accuracy of the flowmeasurement. For example, if in your process a fluid with viscous proper-ties like water changes to have viscous properties like thick oil, where theviscosity increases, the Reynolds number will decrease and therefore maymake a particular flow measurement method inaccurate.

Differential Pressure Flow Measurement

A differential pressure transmitter is required as a secondary elementthat converts the range of pressure differences developed across theprimary flow element to 4.0 to 20 mADC.

The differential pressure transmitter’s output signal is non-linear. It isproportional to the square of the flow rate. If a signal that is linear withflow rate is required, a square root extractor may be used to process thesignal of the differential pressure transmitter. This is where you mightrun into a few decisions to make. If the transmitter output will be sent asan input to a PLC or DCS system, you might have the choice of applying asquare root extractor as the output from the transmitter or as the inputinto the PLC/DCS. Make sure it is done at one location. If you apply asquare root at the transmitter output AND the PLC/DCS input, the datawill not represent the flow.

Be sure to be consistent if you can for all transmitters and inputs to yourPLC/DCS.

Figure 4-4Differentialpressure transmitterused with pressure- producing flow elements



The following figure shows an example of the instrument piping for aprimary flow element connected to a differential flow transmitter.

Figure 4-5Instrument piping for a primary flow elementconnected to a differentialpressure transmitter

Orifice plate is probably the most common device used for a differentialpressure method of measuring flow. While it is simple to install, there is acorrect way and an incorrect way to install; it’s not just a plate with holesin it. The sharp edge must face the inlet. Most orifice plates have theinformation engraved on the inlet side.

Piping Considerations

Some factors to consider within the piping that may adversely affect flowmeasurements are variances in the inside piping diameter upstream anddownstream from the measuring device. For flow-measuring loops the flowvalves are typically located downstream of the flow measuring element.

Also, many flow-measuring devices drop some of the line pressure, whichin some cases is not desirable. Differential pressure devices drop pressure

PDT

OrificePlate

ImpulseLines

3 ValveManifoldwith TestPorts

Root ValveRoot Valve

Drain Pot Drain Pot

Drain Valve Drain Valve

4 to 20 mADC

SteamFlow

instrument piping for primary flow element b.eps



to low or moderate levels, whereas Pitot tubes have a low-pressure dropand elbow taps have an insignificant pressure drop.

Some applications, such as with orifice plates, require a straight pipe runof up to 10 to 20 upstream diameters. For these types of measuringdevices most manufacturers have tables and guides for determining therecommended upstream and downstream diameter suggestions.

Line Size

When considering flow-measuring devices, be aware that not all measuringdevices cover all line sizes. Make sure the desired flow device can handlethe line size (and required flow). As an example of some restrictions in linesize, the maximum size of most vortex meters is eight inches.

NOTE

In order to have an output signal from a differential pressure transmitterthat is linear with flow, a square root extractor must process its output. For aproperly calibrated square root extractor, the following relationships shouldexist between the input current values and the output current values:

• OUT mA = 4.0 + 16.0 x SQR (Test Pt. % Differential / 100%Differential)

• For 1% of maximum differential, (10% of maximum flow rate),the input current of 4.16 mADC should cause an output currentof 5.6 mADC.

• For 25% of maximum differential, (50% of maximum flow rate),the input current of 8.0 mADC should cause an output currentof 12.0 mADC.

• For 50% of maximum differential, (70.7% of maximum flow rate),the input current of 12.0 mADC should cause an output current of15.314 mADC.

• For 75% of maximum differential, (86.6% of maximum flow rate),the input current of 16.0 mADC should cause an output current of17.856 mADC.

• For 100% of maximum differential, (100% of maximum flow rate),the input current of 20.0 mADC should cause an output current of20.0 mADC.



Inferential Flow Measurements

Differential-pressure flow measurements actually measure differentialhead pressure through a primary element, which can be for example anorifice plate and a secondary element, such as a differential pressuretransmitter. The relationship between flow rate and pressure is exempli-fied in the following equation based on the Bernoulli principle:

Flow rate = constant x √ (differential pressure/density)

Since we are looking for the flow rate given the differential pressure asquare root extractor function is typically required.

It is usually recommended that the secondary element be mounted abovethe primary element for gas measurement so that condensables do notinfluence the differential pressure. They should be mounted below theprimary element for liquid, condensables and steam so that vapors andgas bubbles flow back to the process.

The four most common methods for measuring flow via differential pres-sure are as follows:

• Orifice plate• Venturi tube• Elbow taps• Pitot tube

The following sections describe the use and advantages/disadvantages ofeach method.

Orifice Plates

How does the orifice plate develop a differential pressure? With a full pipeof fluid, as the velocity increases through the hole in the orifice plate, thepressure decreases.



Figure 4-6Orifice plate

Pipe InsideDiameter

Beta = hole diam./pipe diam.

Beta = d/D

Information Engravedon the Handle

Inferential Flowmeter

Turndown = 3:1

Relatively LowPressure Recovery

D

d

Handle

Inlet

Orif

ice

Pla

te b

.eps



The point of highest velocity (narrowest diameter of flow) is not at theorifice, but downstream of the plate known as the vena contracta point.Past the vena contracta the physical forces for velocity change and thevelocity slows down. Referring to the following figure, point A is the fullpressure in the pipe, and B is the pressure at the vena contracta point,which is the lowest pressure point. Notice the recovery point C is lowerthan A, meaning there is a pressure loss due to the orifice plate. This canbe a disadvantage depending on your application. Unfortunately, this is apermanent loss.

Figure 4-7Orifice platepressure curve

If the orifice plate is mounted backwards, you can induce errors of up to20%. Solids in the fluids or corrosives can etch away at the orifice, whichcan lead to inaccuracies over time. The following figure illustrates anorifice plate application.

A100

80604020

0

100806040200

C

B

P1 P2

Flow

Vena Contracta PointOrifice Plate(Concentric)

Orifice Plate InformationEngraved on Handle

Orifice Plate



Figure 4-7Orifice plateapplication

Running water, air bubbles and/or solids may cause issues with horizontalpipe, so they must be considered and accounted for. The air will congre-gate near the top of the plate and eventually affect the flowmeasurements. To overcome this you can get orifice plates with weepholes. For gas applications the opposite is true – i.e., we don’t want fluidbuild-up on the bottom. The following figure shows an orifice plate withweep holes.

Figure 4-8Orifice plate

Pipe InsideDiameter

Weep Hole: to PassEntrained Gas inLiquid

Weep Hole: to PassEntrained Gas inLiquid

Beta = hole diam./pipe diam.

Beta = d/D

Information Engravedon the Handle

Inferential Flowmeter

Turndown = 3:1

Relatively LowPressure Recovery

D

d

Orif

ice

Pla

te 2

b.e

ps



This is an eccentric orifice plate. Notice that the hole should really be nearthe bottom. Most eccentric orifice plates use older technology.

Figure 4-9Eccentric orifice plate

Pipe Tap Arrangements

In order to measure the differential pressure when using an orifice platewe need to consider tap placement where the pressure measurements willbe taken.

For best accuracy (which can change based on the Beta ratio) it is recom-mended to have 20 diameters of straight pipe upstream and 10 diametersdownstream. So, for example, for a 4-inch pipe almost 6 feet of straight pipeupstream is needed and almost 4 feet of straight pipe downstream is needed.

Figure 4-10Vena contracta taps

If you don’t have the luxury of 20 upstream and 10 downstream diame-ters, you might consider using straightening tubes, which are placedinside the pipe and cause the flow to straighten by the time it reaches theorifice plate.

Liquid Flow

1 Dia. Calculated

Taps on top for gas flow,but on the bottom for liquid

High Pressure Low Pressurevena contracta taps b.eps

Orifice Plate InformationEngraved on Handle

Orifice Plate(Eccentric)

eccentric orifice plate b.eps



Vena contracta tap to the orifice plate. For gas flow the taps should be onthe top with the transmitter above the line. For liquids, the taps should beon the bottom with the transmitter below the line. As an alternative, fordirty fluids, taps can be located on the middle.

The most accurate flow tap location method specifies 2 ½ diametersupstream and 8 diameters downstream, because there the pressure hasstabilized. However, calculations must be made and pipe taps fittedaccordingly, which can add labor expenses.

Figure 4-11Most accurate taplocations

The most convenient and inexpensive tap method is flange taps, in which theflange manufacturer provides threaded tap points on each side of the flange.

Figure 4-12Flange tap locations

Gas Flow


Flange top locations b.eps

Liquid Flow

2-1/2 Dia. 8 Dia.


High Pressure Low Pressure

most accurate tap locations b.eps



Venturi Tube

Venturi tubes are more expensive than an orifice plate, however, theorifice plate causes more permanent energy losses and is less accurate.

The Venturi effect follows the same principle as the orifice plate(Bernoulli’s principle), in which if there is a constriction in the pipe, theflow must increase, while the pressure must decrease (and vice versa).

Figure 4-13Venturi tube

To avoid drag in the venturi tube the entry cone is typically 30 degreesand the exit cone is 5 degrees.

The nice thing about venturi tubes is if there’s a dirty fluid, you don’t geta buildup of solids.

Flow Nozzle

The flow nozzle is similar to the venturi tube except that there is norecovery cone. The flow nozzle is more accurate than an orifice plate, butless accurate than the venturi tube. Its costs are also between an orificeplate and a venturi tube. One thing to consider when using a flow nozzleis that in order to remove the flow nozzle, you not only have to unbolt theflange, you have to spread the pipe. To overcome this problem you can boltit to a school piece and lift the whole thing out.

Figure 4-14Flow nozzle

A100

80604020

0

100806040200

C

B

P1P2

PressureCurve

Flow

% P

ress

ure

venturi tube velocity profile b.eps



Elbow Taps

As fluid travels through an elbow, centrifugal force is exerted upon theouter edge (relative to the direction of flow). Pressure taps are placed onthe outer and inner bends of the elbow. Though these are very economicaland easy to install, accuracy is poor.

The flow through an elbow can be expressed by the following equation:Equation 4-1

Flow = constant x √ (R x H x D3 x Density)

where, R = elbow’s centerline radiusH = ΔPD = elbow or pipe diameter

In pumping systems you need to know if the pump is pumping. This wouldbe an inexpensive application downstream of a pump by putting a differ-ential pressure switch to verify there is flow.

Figure 4-15Elbow taps

Pitot Tube

The Pitot tube flow measurement device consists of a probe comprised of twoparts. One part measures the impact pressure (high side) the other measuresthe static pressure. The averaging Pitot tube has four or more pressure tapslocated at calculated locations to measure the impact pressure.

This device does not require as turbulent a condition as an orifice platerequires. The convenient thing about the Pitot tube is that it can beremoved during production (for cleaning/inspection, etc.).

This is a very popular method for air or gas, and it is used on aircrafts todetermine speed.

Pipe Elbow

DifferentialPressure

Transmitter

D

Low PressureR

Flow

High Pressure



Figure 4-16Averaging Pitot tube

Parshall Flume

To measure large capacity flow, a Parshall flume might be considered. Achange in level in this device indicates a change in flow.

Figure 4-17Typical Parshall flume



Velocity Flow Measurements

For measuring flow, several methods have been proven to measure aparticular property’s change in proportion to flow. The following describesome methods of measuring these properties that vary with flow:

• Volts or millivolts vary with flow rate• AC voltage’s frequency or beat frequency varies with flow rate• The phase between two AC voltages vary with flow rate• Resistance varies with flow rate• Delay time between sending an RFI pulse and receiving the echo

Magnetic Flowmeter (Mag Meter)

The magnetic flowmeter is a device specifically for electrically conductiveliquids or slurries. A magnetic field is applied to the metering tube. Recallfrom basic electricity that voltage induced across a conductor as it movesperpendicularly through a magnetic field is proportional to the velocity ofthe conductor. The magnetic coils in the flowmeter generate a magneticfield applied to the metering tube that is perpendicular to the direction offlow, which results in a potential difference proportional to the flowvelocity that is perpendicular to the magnetic flux lines. The voltageproduced is proportional to the average velocity of the fluid’s volumetricflow rate.

To operate, this flowmeter requires a conducting fluid such as water thatcontains ions, and an electrical insulating pipe surface.

Figure 4-18Magnetic flowmeter

The metering portion (as well as the electrode) of the mag flowmeter typi-cally has some sort of Teflon or similar coating. The fluid being measuredwill determine the coating to be used. In applications such as slurrieswhere the fluid can be very abrasive, over time the abrasiveness can wear





away the electrode, which will result in no readings. The following figureshows a magnetic flowmeter.

Figure 4-19Magnetic flowmeterflowtube

The following are some advantages of the magnetic flowmeter:• Measures very low flow rates (theoretically down to zero, but typi-

cally 3ft/s)• No moving parts• Unaffected by changes in density, viscosity and pressure

The following are some disadvantages of the magnetic flowmeter:• Above average cost• Large size/weight• Needs a minimum conductivity of 5-20 micromhos/cm or µS/cm• The line must be full with no air bubbles

If using glass or plastic pipe upstream and downstream of the magflowmeter (where the conductivity of the fluid can cause the pipe togenerate static electricity) you will need to ensure proper grounding.

Since the mag flowmeter is a primary element it will typically generatemillivolts. If there are long distances between the mag flowmeter and thefacility’s PLC or DCS system, a transmitter that can send the signallonger distances will be needed. The following figure illustrates a magflow transmitter.

Figure 4-20Magnetic flowmetertransmitter

Vortex Shedding Meter

Vortex shedding meters have proven to be accurate, reliable, repeatableflowmeters for liquid, steam and gas applications. They can provide turn-down ratios as high as 30:1.

The bluff body typically shaped like a square, rectangle, T or trapezoid, isimmersed across the pipe perpendicular to the fluid flow. As fluid passesthe bluff body, vortices are generated downstream at a frequency propor-tional to the volumetric flow rate. The frequencies or vibrations can bedetected by different technologies such as piezoelectric, differential pres-sure, capacitance, etc. and converted to flow.



Figure 4-21Vortex shedding meter

The vortex meter has no moving parts and it can be installed in virtu-ally any position. For liquids, however, the line should be kept fullwith no gas bubbles.

Turbine Meters

Turbine meters have a spinning rotor with propeller-like blades mountedon bearings in a housing. The rotor spins as fluid passes over it. The flowrate is proportional to the rotational speed of the rotor. To detect the rotorspeed, manufacturers use a variety of methods including mechanicalshafts and electronic sensors. The turbine meter in the following figureuses a magnetic pickup coil that sends the magnetic field into the pipe. Asthe rotor spins it generates a magnetic pulse in the coil. The frequency ofthe pulses is proportional to the fluid velocity.



Figure 4-22Turbine meter

Turbine meters can be used with liquids and gases, though it is not recom-mended for measuring steam flow. Since turbine meters are sensitive tothe presence of swirls, they require either a straight run prior to the meteror straightening vanes to ensure a uniform velocity profile. Turbine metersare sensitive to viscosity; low viscosity fluids are best. If turbine meters areuses in fluids with contaminants, strainers may be necessary to minimizedamage to the meter. Care should be taken to prevent hammer as theturbine meter with its moving parts can be damaged.

Target Flowmeter

Target flowmeters sense and measure forces caused by liquid impactingon a target suspended in the liquid stream. The flow rate is achieved bymeasuring the force exerted upon the target.

Figure 4-23Target flowmeter



In the simplest form, the meter consists only of a hinged, swinging platethat moves outward along the liquid stream and serves as a flow indicator.

A more sophisticated type uses a precise low-level force transducer as thesensing element. The force of the target caused by fluid flow is sensed by astrain gauge. The output signal is proportional to the flow rate.

Target flowmeters are useful for measuring flows of dirty or corrosive liquids.

Ultrasonic Flowmeter

There are two types of ultrasonic flowmeters: Doppler and time-of-travel(or transit) meters.

The transit meter has transducers mounted on each side of the pipe. Theconfiguration as seen in the following figure has sound waves travelingfrom transmitter to receiver at an angle that is 45 degrees to the directionof flow.

Figure 4-24Ultrasonic flowmeter

The speed of the signal traveling between the transmitter and receiverincreases or decreases with the transmission of the velocity of liquid beingmeasured. Since the time of travel for the ultrasonic signal is sensitive tomaterial in the fluid, the liquid must be relatively free of gas or solids tominimize signal scattering and/or absorption.

The Doppler meters measure the frequency shifts caused by the fluid flow.Two transducers are mounted in a case attached to one side of the pipe. Asignal of known frequency is sent into the liquid. As the liquid is moving,solids, bubbles or any discontinuity in the liquid cause the pulse to bereflected and the frequency is shifted to the receiver. The frequency shiftis proportional to the liquid’s velocity.



Figure 4-25Doppler ultrasonic flowmeter

The Doppler style ultrasonic flowmeter does not work very well withclean liquids.

Some advantages of ultrasonic flowmeters are that they are non-intru-sive, have low installation costs, and portable units can be obtained.However, they are not as accurate as a mag flowmeter.

Variable Area Rotameter

Variable area meters, often called rotameters, consist of a tapered tubeand a free moving suspended float. Though they are classified as differen-tial pressure units, in reality they are constant differential pressuredevices. The float’s movement up and down the tube is related to flow andcan produce a linear signal with flow.

Figure 4-26Variable area flowmeter

The rotameter is affected by fluid density. It will not perform very well inhigh-viscosity fluids. Some advantages of the rotameter are: it performswell with low flow rates, it is inexpensive, self-cleaning, provides directindication, and is simple to install. Some disadvantages to the rotameterare that it can only be mounted vertically (unless a spring-loaded model isused), and it cannot be used with erosive, crystallizing or opaque fluids.

Diagnal Mode Reflect Mode

Flow Flow

doppler ultrasonic flowmeter b.eps



The rotameter needs no power to operate unless accessories arepurchased to enable the rotameter to transmit data.

Mass Flow Measurements

A mass flowmeter is a device that measures how much liquid is flowingthrough a tube. It does not measure the volume of the fluid; it measuresthe amount of fluid mass passing through the device.

Coriolis Meters

The Coriolis meter consists of one or two tubes that are forced to oscillateat their natural frequencies perpendicular to the flow direction. There aretwo common Coriolis effect tube types: straight and curved.

With straight tubes, as fluid is pumped through the mass flowmeter andthere is mass flow, the tubes twist slightly.

Figure 4-27Coriolis effectwithin a straight tubeCoriolis mass flowmeter(Courtesy of Micro Motion)

The straight tube design requires less space, can be easily cleaned, andhas little pressure loss. Unfortunately, the straight tube must be perfectlyaligned with the pipe. When fluid flows through the tubes the Corioliseffect causes a phase shift in the electrically generated magnetic coils(which act as sensors). The time lag between the two sensors (one on eachtube) is proportional to the amount of mass flowing through the tubes.

PickoffCoil

PickoffCoil

FlowTube

ReferenceTube

Case TransitionBracket

Case TransitionBracket

Drive Coilstraight tu

be b.eps



Figure 4-28Coriolis effectwithin a straight tubeCoriolis mass flowmeter(Courtesy of Micro Motion)

In the curved tube design, when fluid is flowing it is led through twoparallel tubes. An actuator causes the tubes to vibrate. When no fluid isflowing the vibration of the two tubes is symmetrical.

Figure 4-29Coriolis effectwithin a curved tubeCoriolis mass flowmeter (Courtesy of Micro Motion)

When there is mass flow, the symmetrical vibration of the two tubes isdisrupted. The mass flow exerts a force on the arm’s angular rotationmomentum on the upstream and downstream portions of the arms. Thiscauses the two tubes to vibrate at a frequency that are shifted in phasewith respect to each other. The degree of the phase shift is a measure forthe amount of mass passing through the tubes.

Figure 4-30Coriolis effectwithin a curved tubeCoriolis mass flowmeter (Courtesy of Micro Motion)



Along with flow rate, the Coriolis meter can also provide fluid density. Forthis type of meter the pipe must be full and remain full to avoid trappingair inside the tube. When considering Coriolis meters, also remember thatthere is a great deal of pressure loss due to the small tube diameters.

Volumetric Flow Measurements

With positive displacement volumetric meters, the units operate by sepa-rating liquids into accurately measured increments and moving them on.A connecting register counts each segment. Since every segment repre-sents a discrete volume, the positive displacement units are popular forbatch and accounting applications.

These units tend to be good candidates for measuring flows of viscousfluids or where simple mechanical metering is needed.

Nutating Disk Positive Displacement Flowmeter

The nutating disk meter has a moveable disk mounted on a concentricsphere located in a spherical side-walled chamber. The pressure of theliquid passing through the measuring chamber causes the disk to move ina circulating path without rotating about its own axis. The only movingpart is the measuring chamber.

Figure 4-31Nutating disk positivedisplacement flowmeter

A pin extending perpendicularly from the ball is connected to a mechan-ical counter (or transducer) that monitors the disk’s motion. Each cycle isproportional to a specific quantity (volume) of flow.

Because of their many moving parts, these units are susceptible tomechanical wear, especially in dirty fluids. They do not perform well forsteam and are only for forward flow applications.



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5Level Instrumentation &MeasurementsObjectives

Upon completion of this chapter, the student should be able to:• Recognize basic level measuring principles.• Know the theory and recognize the common level measuring methods.• Understand the variables that can affect the decision of what type

of level sensor to use.

Introduction

Level is an interface between two material phases. These phases couldbe a gas and a liquid, a gas and a solid, two solids or an interfacebetween two liquids. A more practical definition of level is: how muchstuff is in the container.

Level Measurement Types

When deciding upon which type of level sensor to use for a particularapplication (or to replace one that has not performed well in a given appli-cation), answering the following questions can help narrow the list ofpossible candidates:

• Does the sensor need to detect level continuously or will a pointsensor suffice?

• Are there any disadvantages (and to what extent) if the sensor isin contact with the product level to be measured?

• Can the level be measured directly or indirectly (hydrostatic head– responds to both level and density)?

• Can the process be shut down for sensor removal or maintenance?

Once those questions are answered, if there are still a number of possiblelevel sensor solutions, consider the traditions or preferences of the partic-ular plant or particular process. User familiarity and spare partavailability can also influence the decision.

Other issues to consider are if there is agitation, will the agitation causethe level signal to cycle? Also be aware that the relationship between leveland tank volume is a function of the cross-sectional shape of the tank.With vertical tanks the relationship is linear, whereas with horizontal orspherical tanks, the relationship can be non-linear.

Other things to consider are do you need local indication or remote indica-tion? If remote, what type of signal will be needed?

Sight Glass

Sight glasses are very good for local indication. With a closed tank weneed to tap into the tank. Often the liquid in the sight glass can becomecloudy. To overcome the cloudiness, a specially designed magnetic floatwith flags can be used. The following figure shows an example of a sightglass application.

Figure 5-1Sight glasslevel measurement

Differential Pressure Level Measurement

The high side is connected at the minimum level. If it is an enclosed tank,there is the possibility of pressure forced on the liquid that must bemeasured. For an open tank, the low side is left open. For an enclosedtank the low side is connected above the maximum level. The differentialpressure is equal to the level of inches times the specific gravity as thefollowing formula shows:

Equation 5-1

ΔP=h x SG where, ΔP = differential pressure

h = head

SG = specific gravity



There are two conditions that one might encounter. If there’s a closed tankwhere the liquid doesn’t become a vapor we don’t have to worry aboutcondensate getting on the line. In this case this is a dry leg application,i.e., the low side is always absent of condensate.

Figure 5-2Differential pressurelevel measurement

If the liquid is vaporizing and vapor gets into the line and condenses, itcan cause measurement errors. This is known as a wet leg application. Tocombat this phenomenon fill the low side up to the maximum level of thepipe with a wet fluid or some compatible fluid to that being measured.Then the transmitter must be set to have an elevated zero.

The following illustration shows a flange mount level transmitter thatwould be connected to the high side as attached to a tank. Keep in mindthat these are differential pressure transmitters.

Figure 5-3Flange mountlevel transmitter

Chapter 5: Level Instrumentation & Measurements


To avoid the wet leg/dry leg issue, a remote seal differential transmittercan be used. With this device the diaphragms are extended, and in thecapillary tubes they are filled with some sort of silicon that puts a headpressure on it.

The following figure shows a remote seal differential transmitter.

Figure 5-4Remote sealdifferential transmitter

NOTE

A rule of thumb for this device is to never disconnect the capillary tube. Ifit is inadvertently disconnected it cannot be re-calibrated unless it is sentback to the manufacturer.



Figure 5-5Pressure transmitterto measure level

Either method (wet or dry) assures a constant reference leg for the differ-ential pressure sensor, which guarantees that the only variable will be thelevel in the tank.

If the specific gravity changes, this is not the best level measurementmethod to recommend.

Bubbler

Bubblers provide a simple, inexpensive, but less accurate level measure-ment for corrosive or slurry solutions or applications where the tank isimmersed or underground. With the bubbler method, compressed air or aninert gas is introduced through a flow regulator and a pressure regulator.The rule of thumb is you try to get a flow rate of about 1 bubble per second.



Figure 5-6Bubblerlevel measurement

A differential pressure regulator maintains constant flow, while the tanklevel determines the backpressure. As the level drops, backpressure isproportionally reduced and is read on a pressure gauge calibrated inpercent level.

The pressure transmitter would be located downstream of the pressureregulator and flow regulator.

The best flow regulator would be one that is a constant flow regulator sothat as the pressure changes, it can still provide a constant flow/bubble.

Displacer Level Sensor

While a float usually follows the level of a liquid, the displacer remainspartially or completely immersed. The weight of the displacer is reducedas more liquid is introduced. The force goes down as the liquid level goesup. This is a force transducer that is proportional to the amount of fluiddisplaced by the buoyancy force. The weight of the displacer causes anangular displacement on the force tube, which consists of a torsion springand a frictionless pressure seal. The displacement is linearly proportionalto the displacer’s weight. The top of the level is the top of the displacer.



Figure 5-7Displacer level sensor

Other sensors, such as springs or force balance instruments, can alsodetect the buoyant force. Note that when the force measurement isdetected with a spring there is some slight movement, whereas with aforce balance instrument the displacer remains in one position while onlythe liquid level varies.

Float Level Sensors

When evaluating the various float sensor designs for continuous levelmeasurement, the float and tape level sensor is the oldest and arguablythe most accurate. A tape or cable connects from the float inside the tankto a gauge board or take-up reel mounted on the outside of the tank. Thefloat is guided up and down the tank by guide wires or can travel inside astilling well.



Figure 5-8Float and tapelevel sensor

Float and tape level sensors are typically used in remote, unattended,stand-alone applications, or they can be interfaced with data transmissionelectronics to be integrated into a plant-wide control system.

Figure 5-9Float and tapelevel measurement

As a troubleshooting guide be sure to maintain the guide wires undertension and clean. If not properly maintained, the float can become stuckon the guide wires or the tape can get stuck to the pipe.



Another float type of level sensor is the weight and cable. The floatremains on the surface but is guided up and down

Figure 5-10Weight and cablelevel sensor

Capacitance Level Sensors

Two conductors separated by an insulator form a capacitor. A capacitor oran object with capacitance stores a charge. The quantity of charge storedis measured in Coulombs. Capacitance is measured in Coulombs per Voltor Farads. Capacitance increases with the area of the conductors, A, andthe dielectric constant of the insulator, ?. Capacitance decreases with thedistance between the conductors, d.

Figure 5-11Capacitance

When the two conductors are at different voltages the system is capable ofstoring an electric charge. If the area, the distance between the conduc-tors, or the dielectric constant is changed, the capacitance changes andcan be related to level.

The following figure illustrates a vertical probe that can be extended bysolid rods up to a length of 4 to 5 ft, or with a cable with a weight, whichcan be used up to 50 ft.





Figure 5-12Capacitancelevel measurement

For a conductive liquid (left side Figure 5-12) the probe has an insulatedsheath to provide the capacitance. As the liquid level changes height, thedistance between the conductors decreases and thus changes the capaci-tance. It can also be said that the dielectric constant is changed because it’sgoing from a dielectric constant for the liquid to a dielectric constant for air.

For a non-conductive liquid (right side of Figure 5-12), the dielectric prop-erty of the insulator is changing, which changes capacitance.

Conductive liquids can cause a short circuit between a bare probe and thevessel wall. A special insulator coating on the conductive probe surfacecan be specified in many cases.

Figure 5-13Capacitance

Suggested Baffle Types

Pipe Section Steel Plates

175 mm7”

D95mm(3.75”)

1-1/4” NPT

A) Horizontal B) Horizontal C) Low Profile

1-1/4 NPT

D

FlexibleCable

CableWeight 200 mm

(7.8”)

115 mm(4.5”)

15 mm (50”)Max CustomerSpecifiedLength

25 mm(1”)

3/4” NPT

50 mm(2”)

Flange

VesselWall

capa

cita

nce

leve

l sen

sor b

.eps

Radiation-Based Level Sensors

A material’s tendency to reflect or absorb radiation can be measuredand translated into level. The most common types of continuous levelsensors are those that operate using radar/microwave, ultrasonic, andnuclear radiation.

The source sends out radiation to a detector on the opposite side of thecontainer. As the radiation is transmitted through the liquid, a portion of itis absorbed by the liquid, thus decreasing the amount received at thedetector. The output of the receiver is highest when the level is lowest. Thesystem is calibrated to read 0% level when the detector current output ishighest. 100% level is set to match the lowest value of output current.

Figure 5-14Nuclear levelmeasurement

The main benefits to radiation-type level sensors is that they do notmake physical contact with the process fluid and there are no movingparts. Radiation (nuclear) sensors are usually considered whennothing else will work. With nuclear sensors the Nuclear RegulatoryCommission (NRC) requires a license specifying design proceduresthat guarantee safe installation.



Radar and Ultrasonic Level Sensors

The operating principle for radar sensors is that a burst of energy istransmitted in radar or microwave form and the time it takes for thereflection energy to return is proportional to the level inside the tank.

Since radar beams penetrate plastic and fiberglass, the non-contact radargauges can be isolated from the process vapors by a seal.

Figure 5-15Radar levelmeasurement

Radar is good for applications in which there is foaming or bubbling of theliquid that is invisible to the radar, whereas it would not be invisible toultrasonic transmissions; however, it typically comes at a higher cost thanultrasonic sensors. The following figure illustrates a good application ofradar level measurement.



Figure 5-16Radar levelmeasurementapplication

The velocity of an ultrasonic pulse varies with both the substance throughwhich it travels and the temperature of that substance. At room tempera-ture the speed of sound in atmospheric air is 762 mph while it is 3,353mph through water.

Figure 5-17Ultrasonic levelmeasurementapplication

Ultrasonic level sensor assemblies can consist of a separate transmitterand receiver elements. More often, a single transducer is cycled on and off

A) Returned Echo Timing B) Signal Absorption

C) Contact

CasingBrine Pipe

Hydrocarbon

Interface

Brine

Cavity

Transducer

Ground Level

ultr

ason

ic le

vel a

pplic

atio

ns b

.eps



at regular intervals to listen for a reflected echo. The time transpiredbetween the transmitted sound wave and the received echo wave (that is‘bounced’ off the interface layer between the air and the process fluid) isproportional to the amount of fluid in the tank. Unagitated, stagnantliquids and solids consisting of large hard particles are good reflectors andmake good candidates for ultrasonic applications. Foam, fluff, dust, mist,or humidity in the vapor tend to absorb the ultrasonic pulse, which limitsthe application of ultrasonic sensors.

Most modern ultrasonic sensors have temperature compensation features,filters and self-calibration.



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Density, Specific Gravity & Analysis

Objectives

Upon completion of this chapter, the student should:• Understand density and specific gravity measurement techniques

and applications.• Know the theory and recognize common analyzing and monitoring

techniques for pH, chemical concentration and gas concentration.

Density & Specific Gravity

Density is the weight of unit volume of material measured in:• Pounds per cubic foot – lb/ft

3

• Grams per cubic centimeter – g/cm3

Specific gravity is the relative density of a material compared to water fora fluid and compared to air for a gas. Specific gravity does not have a unitof measure; it is a constant.

Continuous measurement of the density of liquids or bulk material isoften seen in the following applications:

• Concentration measurement of acid, alkali, saline solutions• Crystallization monitoring• Monitoring solid content in slurries, aluminum production, mining

Measurement Types & Principles

The following are some methods for measuring density:• Differential pressure – double bubbler system• Coriolis mass flow meter (see page 4-24)• Nuclear radiation• Displacer – strain gauge


6

Double Bubbler System

The double bubbler system has two submerged tubes through whichbubbles are forced. The differential pressure is measured between the twolines, which corresponds to specific gravity of the material. If the specificgravity changes from 1.1 to 1.2 it reflects a 5-inch water height change.

Figure 6-1Double bubbler systemfor measuring density

The differential pressure transmitter indicates the weight of a 50-inchcolumn of fluid. The weight will vary with the density of the fluid.

Nuclear Radiation

For this method, density measurement operates according to the physicallaw of attenuation of gamma radiation, which is dependent upon the radi-ation energy, the measuring path length and the density of the product.

The energy and the measuring path length are kept constant, whichmeans the attenuation is only dependent upon the density of themeasured product. The detector measures the radiation intensity by ascintillation detector and is then converted to a density or concentrationmeasurement.

The following figure illustrates a typical arrangement for nuclear radia-tion density measurement.



Figure 6-2Nuclear density measurement

(the highlighted items should go with the previous graphic)1. Sealed and shielded source that will be mounted lateral to the pipeline 2. Scintillation detector mounted opposite to the source container 3. Clamping device or measuring path two-wire standard cable 4. Evaluation unit with digital display and 0/4-20 mA analogue

output of the density or concentration and alarm outputs

Strain Gauge

A strain gauge is a conductive ribbon on an elastic substrate whose resis-tance increases as the ribbon is stretched by an external force. The straingauge can be bonded to the supporting structure of a tank to measure thetotal weight of the tank and its contents. Piping should not be attached tothe tank.

Figure 6-3Strain gauge

1

3

2

Chapter 6: Density, Specific Gravity & Analysis


There is a network of strain gauges in a load cell. This method can beused on the tank by measuring the amount of fluid entering the tank. Itmust be a free-floating device. This can also be used to measure density.

Monitoring & Analyzing Techniques

Process analyzers provide robust measurements to help manufacturersbetter control their process through timely measurements of critical prop-erties, leading to increased efficiency and quality. The following analyzingtechniques/devices will be explained in this chapter:

• Conductivity• pH

Conductivity

Conductivity describes the ability, or how well an object conducts elec-tricity. It is the inverse of resistance. Units of conductivity are expressedin mhos, which is 1/?, or what is currently expressed in siemens.

Conductivity measures the amount of ions flowing through a material. Itis basically a resistance meter. Scaling will affect conductivity, which iswhy the conductive probes should be monitored for scaling. Also, forconductivity measurements the sensor isn’t typically introduced into theprocess. A sample is taken out of the process for measurement. A typicalapplication is to measure conductivity in boilers or cooling towers. Theconductivity in the liquid is representative of solids in the system, whichcan cause scaling within the system.

pH

pH stands for the potential of hydrogen ions in a liquid. The pH probe is avery sensitive device that is specifically designed to only allow hydrogenions to pass. Each pH bulb is made by hand. pH is very sensitive totemperature. Most pH instrument measurement manufacturers haveproducts with temperature compensation features.



Figure 6-4pH bulb andreference electrodes

The pH measurement loop is made up of three components: the pH sensor(which includes a measuring electrode), a reference electrode, and a temper-ature sensor. The pH measurement loop is essentially a battery in whichthe positive terminal is the measuring electrode and the negative terminalis the reference electrode. The measuring electrode, which is sensitive to thehydrogen ion, develops a voltage directly related to the hydrogen ionconcentration of the fluid. The reference electrode just provides a stablepotential against which the measuring electrode can be compared.

+–

+–

+–

+–

+–

+–

+ –+ –

+ –

+–

+–

+–

+–

+–

+–

+–+–

Wire Connection Point

MeasurementElectrode

GlassBody

Seal

Bulb Filled withPotassium Chloride

‘Buffer’ Solution

SilverWire

SilverChloride Tip

Voltage ProducedAcross Thickness of

Glass Membrane

Very Thin Glass BulbChemically ‘Doped’ withLithium Ions So As To Reactwith Hydrogen Ions Outsidethe Bulb



Figure 6-5pH measurementcomponents

Note that pH is a logarithmic measurement of the number of moles ofhydrogen ions (H+) per liter of solution.

Figure 6-6pH cell potential

Though the design and operational theory of pH electrodes is a verycomplex subject, the important point to understand is that these two elec-trodes generate a voltage that is directly proportional to the pH of the



solution. At a pH of 7 (neutral), the electrodes produce zero volts betweenthem. At a low pH (acid) a voltage of one polarity develops, and at a highpH (caustic), a voltage of the opposite polarity develops.

If you have spare pH probes be sure to keep them in a solution. If they godry they have a tendency to not operate properly.



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Manipulating TheProcess

Objectives

Upon completion of this chapter, the student should:• Be able to examine globe valve characteristics.• Be able to examine fail-safe modes.• Understand how a valve positioner operates.• Review the advantages of variable frequency drives in process control.

Final Control Element

Recall from the components of a feedback control loop as shown in thefollowing figure, where the final control element fits into the system.

Figure 7-1Components ofa feedback control loop

The final control element (control valve in this example) is the device thatwill manipulate the variable that directly affects the process. The goal is that


7

as the final control element is operated, there is zero error or the processvariable remains at setpoint. Control valves allow continuous varying of theorifice in a fluid flow line that results in a changing rate of flow.

Globe ValveA globe valve regulates flow by adjusting the stem that causes the plug toextend or retract. Manual operation is achieved by turning a handwheel.Typically, automated valves use a sliding stem and are opened and closedby an actuator assembly. Extending the plug into the seat closes the valvepreventing flow through the valve. By retracting the plug an orifice isopened, allowing fluid to pass through the orifice.

Figure 7-2Throttlingglobe valve

As fluid flows through the valve, velocity increases and pressuredecreases. If the discharge pressure is higher than the fluid’s vapor pres-sure, cavitation will occur. According to the Bernoulli effect, in fluiddynamics, when the fluid’s velocity increases, the pressure decreases.When fluid passes through a restriction, such as a valve, and the fluidvelocity reaches the point where the fluid begins to vaporize, bubbles arecreated in the fluid. If the fluid velocity lowers drastically, the vaporbubbles implode (the vapor changes back to liquid and the bubblescollapse into the fluid). This condition is cavitation. The main effects ofcavitation are mechanical damage typically to the trim assembly in theform of eroded holes. As cavitation is starting, obvious indications are ahissing noise downstream of the valve. When cavitation becomes fullydeveloped, the indications turn into loud vibrations and the sound is likerocks passing through the valve.

Typical problems that lead to cavitation are insufficient backpressuredownstream of the valve, or the valve is sized too large for the application.Over time, the plug becomes destroyed and thus when the valve issupposed to be closed fluid still passes through it.



Valves are sized based on the Cv and is determined by the manufacturerthrough testing. The Cv is the number of U.S. gal/min of 60°F water whenthere is a 1-psi differential pressure drop across the valve. The followingequation illustrates the relationship.

Equation 7-1

Cv = Q (gal/min) x √ dp(psi)/specific gravity

Flow CharacteristicsThe flow characteristics of the globe valve are basically determined by theshape of the plug. If the manufacturer changes the shape, the flow charac-teristics will change.

The three characteristics to be aware of are as follows:• Linear trim: a proportional flow change for a proportional valve

opening.• Quick opening: for low flows (up to around 40% valve open) there

is an essentially linear effect; 0-40% valve movement correspondsto a 0 – 70% flowrate.

• Equal percentage: (large differential pressure applications) if thevalve movement is from 20-30% the change in flow in percentage isthe same as changing the valve movement from 70–80%.

Figure 7-3Control valvetrim characteristics

Chapter 7: Manipulating The Process


Actuators

The actuator is what drives the control valve. The actuator can be of twotypes: diaphragm or piston. Of the two types the diaphragm type requireslower operating pressure (usually 3-15 psi) due to its larger surface area.

Figure 7-4Actuators

Diaphragm valves operate by forcing a flexible diaphragm against abridge or weir to stop the flow. Within the diaphragm valve category thereis a weir-type design and a straight-through design (also known as apinch valve). The straight-through valve has a lower differential pressurerating than the weir design.

The diaphragm valve is great for slurries and for liquids that containsolids. The diaphragm valves are low-cost valves requiring relativelysimple maintenance (though they tend to require more maintenance),however they have poor flow characteristics and are not very accurate formodulation.

Valve Failure Modes

The place where the pneumatics are connected does not necessarilydictate the valve failure mode operation.



Figure 7-5Control valvefailure modes

Let’s first define some of the terms:• Air-to-close: means that the valve requires pneumatic force to close

the valve. Upon failure (loss of pneumatic force), the valve opens.• Air-to-open: means that the valve requires pneumatic force to open

the valve. Upon failure (loss of pneumatic force), the valve closes.With steam, the failure mode is typically failed closed. With chilled water,the failure mode is failed open.

NOTE

Though these are general statements, each valve’s failure mode will haveto be determined individually, dependent on the valve’s application.

Valve Positioner

The valve positioner is the device that provides the motor force to theactuator. A valve positioner is a closed loop control system that continuesto change the air pressure to a valve diaphragm until the valve stemstrokes to a position proportional to the 4-20 mADC signal it receivesfrom the controller.



Figure 7-6Valve positioner

The output of the valve positioner is the air. The valve positioner has tohave a linkage to the valve (as feedback) so it knows how much movementoccurred in the valve. Keep in mind that the operation of an I/P isdifferent than a valve positioner.

Pneumatic Device Operation – I/P

The following is a simplified diagram of an I/P. Basically, for any pneu-matic device to work we need an air supply.

Figure 7-7Pneumatic deviceoperation 4-20 µA DC

Exhaust

Air Supply

Outputto Valve

pneu

mat

ic d

evic

e op

erat

ion

b.ep

s



The air supply is divided into two sections; the major artery goes to asmall valve (which opens and closes upon need), the second artery goes toan orifice that leads to a small chamber. The nozzle will bleed out air andis regulated by the orifice and the size of the nozzle. The pressure in thechamber is determined by the position of the flapper. If a magnetic coiladjusts the flapper, as 4-20 mADC is applied, and mADC goes up theflapper comes closer, which increases the pressure in the chamber, whichwill open the valve and let air go to the output of the diaphragm. Whenthe pressure of the output equals the pressure in the chamber, the valvewill close. The biggest issue with pneumatics is air is constantly beingconsumed. Also, the orifice is very tiny and must be protected from dirtand moisture. The air supply must be instrument air not plant air.

Split-Ranged Control Valves

For large valves in applications that have widely varying conditions(where there might be cavitation and/or flashing), a technique known assplit-ranged control valves is used.This technique is used to increase the turndown of the flowrate that iscontrolled without requiring a large control valve to operate near its fullclosed position, which would cause wire drawing of the valve’s seat.Basically there are two valves in parallel, while the two I/Ps are in series.

In the following example, I/P converter #1 is set to deliver 3-15 psig tocontrol valve #1 for a signal range of 4-12 mA (for adding 0 to 25 gpm). I/Pconverter #2 is set to deliver 3-15 psig to control valve #2 for a signalrange of 12-20 mA (for adding 25 to 500 gpm).

Figure 7-8Split-rangedcontrol valves

TankT-201

LV

204A

LT

204

LV

204B

2”-MW-2022-CST

6”-MW-2021-CST

LIC

204

split-ranged control valve b.eps





Butterfly Valves

The butterfly valve is light, easy to use and relatively inexpensive. Theyare not the optimal control devices because they do not provide linearperformance when the valve is in the near-closed position.

Figure 7-9Butterfly valve

The butterfly valve is not used in cavitation or noise applications or forslurries or dirty solid-bearing fluids.

Ball Valves

Ball valves are nice for shut-off type with no restrictions when fullyopened, but they are not necessarily suggested for throttling or controlapplications.

Figure 7-10Ball valve

A variation of the ball valve is the plug valve. Like ball valves, plug

valves have linear or equal-percentage flow characteristics withranges of 1:10 to 1:100.

Figure 7-11Rotary plug valve

Variable Speed Drive & Motor

Probably one of the most recent applications that has had a huge impacton the process control industry is that of using a variable speed drive andmotor as the final element in a flow control loop.

Figure 7-12Variable speed driveand motor

In the old days a valve was placed downstream of the pump. When thevalve was fully closed, the pump was working hard – wasting energy. Thetypical setup uses an A.C. adjustable frequency drive (AFD or variablefrequency drive – VFD) which by adjusting the frequency of a three-phaseA.C. motor allows the motor to operate from zero to one hundred percent ofits nameplate speed (and possibly faster depending on the motor’s manu-facturer ratings). In this case the manipulated variable is the pump speed.



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TroubleshootingObjectives

Upon completion of this chapter, the student should:• Understand how to troubleshoot a two-wire circuit.• Be familiar with general troubleshooting techniques.

Introduction

When troubleshooting electrical circuits, the problems can be narroweddown to two possibilities: open circuits or short circuits.

An open circuit is a circuit condition in which some portion of thecomplete electrical path has been disrupted. There could be many causesfor the open circuit. Most likely it is due to a loose wire or connection(termination) point, but it could also be due to a faulty device. Forexample, if a fuse blows, internally the linkage inside the fuse melts,preventing an electrical path through the fuse. The obvious symptom ofopen circuits is that the load does not function at all. Recall that for acircuit to operate there must be a complete electrical path for current toflow from the power source to the neutral (which is typically grounded).

A short circuit is a circuit condition in which an alternate (not in the orig-inal design) electrical path has been introduced to the circuit. Obvioussymptoms of circuits with shorts in them are loads becoming energizedwhen they are not supposed to be. For instance, a light is on when it is notsupposed to be on.

Testing for Open Circuits

In the following circuit Ohm’s law is used to determine the current flowthrough the simple series circuit. Recall from Chapter 1 that Ohm’s lawstates that V = IR. The voltage in the circuit (from the power supply) is20VDC. If all the resistances are summed (8KW + 2KW + 4KW + 6KW),the total resistance for the circuit equals 20KW. The total current flowingthrough the circuit – I = V/R, or 20VDC/20KW = 1 mA.


8

Figure 8-1Testing for anopen circuit

The circuit can be analyzed at each component now knowing the current,or it can be referenced with a meter by putting the black (neutral) lead ata ground point, then moving the red lead throughout the different pointsin the circuit. The following figure illustrates the voltage levels one shouldsee at the different points, if the circuit is operating properly.

Figure 8-2Testing for anopen circuit,voltage test points

If there is an open in the circuit, when using a meter to measure thevoltage across the devices that do not have an open, zero voltage will beshown across them. The device where the open circuit is occurring will



show a voltage drop on the meter when measuring across it (in thefollowing figure, VR3).

Figure 8-3Testing for anopen circuit, voltageacross device

Again, another way to test for open circuits would be to place the meter’sblack lead on the ground point, then move the red lead to different testpoints on the circuit. As seen in the following figure notice test points A andB show no voltage drop while points C and D show a voltage drop of 20VDC.

Figure 8-4Testing for anopen circuit, voltagetest points

Chapter 8: Troubleshooting


Testing for Short Circuits

As with the case of open circuits, we are going to use Ohm’s law to eval-uate normal circuit conditions as well as short circuit conditions.Reviewing the circuit in the following figure illustrates that the currentthrough the circuit should be 0.5 mA.

Figure 8-5Testing for ashort circuit ata fixed voltage

If a short circuit exists in this simple circuit, as shown in the followingfigure, the current increases (because resistance is lessened, in thisexample). Notice that when the current is calculated using Ohm’s law itequals 1mA.



Figure 8-6Testing for ashort circuit ata fixed voltage

This method is relatively simple since the devices are just resistors. Whenthe devices are 2-wire devices like those shown in the following figure thesystem becomes more complicated. The first item to note is that thedevices are polarity sensitive. From the power supply starting at the posi-tive source, the first device (PT3 in this example) must have its positiveterminal connected to the power supply’s positive terminal. From theother side of PT3, its negative terminal gets connected to the positiveterminal of the next device (PI3). This scenario (positive to negative)continues until the last device. where its negative terminal connects tothe power supply’s negative terminal.



Figure 8-7Converting aprocess variable toa current variable

In this example, the pressure transmitter adjusts its internal resistanceso that the current flowing through it is proportional to the pressurevalue at its port. Converting this application into an electrical circuitwould produce the circuit shown in the following figure. Notice how PT3acts like a variable resistor.

Figure 8-8Testing for ashort circuits ina transmitter loop

In this example, if there is pressure of 50 psig, that should relate to PT3setting the loop current to 12.0 mADC. Since the current is the samethrough the circuit, and knowing that the voltage across PT3 (at 12.0



mADC) is 18.0 VDC, VR2 and VR3 must be equal (since they are of equalresistance).

Now if there is, for example, a short across PR3 (as shown in the followingfigure), while PT3 still maintains 12.0 mADC, notice that Rt varies to1750 ? and causes the voltage across PT3 to increase to 21.0 VDC.

Figure 8-9Testing for short circuits ina transmitter loop

In this example notice that there was no indication on the pressure indi-cator PI3 that anything had changed. This is because the pressuretransmitter PT3 did its job by adjusting its resistance.

In a circuit where there is an improper installation as illustrated in thefollowing figure, where there is an additional (improper) ground pointnotice that there is a voltage drop across PT3 and PR3, but not across PI3.





Figure 8-10Short circuit in a transmitter loop withan installation error

Troubleshooting Suggestions

The following are some troubleshooting suggestions that can be used tohelp diagnose common problems incurred in control loops.

1. Have the operator show you the problem, rather than tell youabout it.

2. Check with the operator and the log book to see if this is a repeatproblem. Does the problem have a pattern; does it only occurduring a specific mode of operation, such as Auto, or at a specificstep in the sequence of operation, or at a specific time of each day,or when a specific operation is in progress?

3. Compare the process variable values on the loop instruments withthe same values shown on the process-mounted instruments; i.e.,the level on the sight glass with that shown on the level controller’sreadout. If different, check instrument piping and drain pots.

4. Operate the control loop in Manual mode. If the control valve orother final control element can be manipulated, then the problemmust be in the sensors, transmitters, or the controller or its utili-ties or circuitry.

5. Eliminate the constant changes and dynamics by opening the loop;remove the input sensor and replace it with the calibrator signalset to a mid-range value of the process variable. Then note the looptuning parameters and change proportional to 100% and integraltime to infinity and derivative time to zero. Now set the setpoint tothe same midrange value and track the resulting signals throughthe instruments and devices within the loop. If you find that adevice has all the signals, power and air it needs and doesn’t outputa proper signal, you have found the defective device in the loop.

6. Range the calibrator over the instrument’s full input range, effec-tively providing a 5-point check of the calibration. If the calibrationis “OFF”, then re-calibration should correct the problem.

7. Before changing out an instrument, remember to check for all theoperational requirements of the instrument; i.e., electrical powersupply, air supply, pilot contact, potentiometer, sensor, communica-tion cable(s), signal pairs, and configuration.

The following figure demonstrates the introduction of a calibrator in acontrol loop to troubleshoot problems (as in step #6 of the troubleshootingsteps).

Figure 8-11Tracking a calibratorsignal through anopened control loop



In addition to the seven troubleshooting steps for a control loops, thefollowing points are suggestions/questions to react to when maintaininginstrument piping.

Pneumatic components:

1. Are all pressure regulators set correctly? Check the pressure gaugesthat are mounted on them. Are all air supply shutoff valves open?

2. Has moisture been drained from the filter-lubricator-regulator?

Other accessories:

1. Are the “wet” legs fully filled?2. Are the “dry” legs empty; i.e., are condensate drain pots drained?3. Check pulsation dampeners for being plugged.4. Are all the three-valve manifold bypass (equalizer) valves fully closed?



Installation Example Drawings







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Controllers

Objectives

Upon completion of this chapter, the student should:• Understand three-mode controllers and how the Proportional,

Integral and Derivative functions affect the system.• Understand different control methods (ratio, cascade, feedforward).

Introduction

Some of the different control techniques that can be used to control aprocess were presented in Chapter 1, but a more detailed examinationwill be presented in this chapter. The following lists the different controltechniques:• Feedback

— On/Off: this type of control has no proportional, integral or deriva-tive functions.

— Time Proportional On/Off: this is seen primarily in temperatureloops using electric heaters where temperature needs to becontrolled at a particular setpoint. The output is essentially On/Offcontrol, but over a proportion of time.

— Analog output: as the input signal deviates from setpoint, thecontroller adjusts the output by way of proportional, integral andderivative functions.

• Ratio• Cascade• Feedforward


9

Control Modes

When reviewing the components of a feedback control loop, keep in mindthat the controller’s function is to take the high-level signal from thesecondary element or transmitter (typically as a 4-20 mADC signal),compare that with the setpoint and adjust the output signal accordinglyto try to maintain the process variable near the setpoint.

Figure 9-1Closed loop offeedback control loop

Final ControlElement, i.e.Control Valve

3 to 15 psig

ProcessDisturbances

Low LevelSignal

SignalConverter

or I/P

SecondaryElement orTransmitter

4 to 20 mADC

High LevelSignal

Controller 4 to 20 mADC

High LevelSignal

Setpoint

Closed Loop

ProcessManipulated

Variable

Measuredor Process

Variable

PrimaryElement

or Sensor

Recorder,Indicator, Alarms



Proportional Control

As an example, we’ll examine the performance of a tank level proportionalcontrol system controlled by mechanical means. This system is illustratedin the following figure.

Figure 9-2Tank level proportionalcontrol with a mechanicalcontrol system

This is a proportional control system. The adjustment that can be made isto move the fulcrum along the rod (left or right). The distances of the twolever arms represent the proportional control. Notice that the lever armconnected to the incoming valve is 20 inches in length whereas the leverarm connected to the float is 10 inches in length. In this system, the ratioof the two arms is 2:1. If the float lowers 2 inches, the lever arm connectedto the influent valve will rise 4 inches.

If we graph the different control scenarios (i.e. positions of the fulcrum),we will then be able to see the outcome on the system.

LEVER ARMOPEN

OUTLETEFFLUENT

Level Set pointchanged by Adjustingheight of vertical rod

Proportional GainAdjustment - Slide

Fulcrum along Rod

LEVER ARM

FULCRUM

20” 10”INLET

INFLUENT

ACTUAL LEVEL

FLOAT

Chapter 9: Controllers


Figure 9-3Proportional control scenarios

Keep in mind that the proportional band in this system is the amount oftank level change in percent that will cause the control valve to change tobe 100%. The gain of the control system is the amount of output (controlvalve change) divided by the corresponding amount of input (tank levelchange). It is also equal to the inverse of the proportional band: Gain =100% / (proportional band in percent).

In the graph above, the proportional band or gain would be related to theslope of the two lines.In the above graph, the proportional band of the solid line is ______%.In the above graph, the proportional band of the dashed line is ______%.In the above graph, the gain of the solid line is ______.In the above graph, the gain of the dashed line is ______.

The following figure illustrates another way to look at the differencebetween proportional band and gain.

Figure 9-4Proportional vs. gain

The proportional band is related to the difference between the processvariable and the setpoint (error). If we have a 25% change (or error), weget a zero percent output; with a 25% error above setpoint, we get a 100%change in output.

10075

50

250

025

50

75100

% Input % Output

Gain =

MoreEffect

LessEffect

100PB

Proportional Band

Gain

50%

2.0 1.0

100%

gain vs proportional band 1 b.eps

CONTROLVALVE

POSITIONin %

50%

0%

100%

0% 50% 100%25% 75%

SETPOINT= 50 %



The following figure illustrates a condition of less effect, i.e. for a 100%change in input, there’s only a 50% change in output.

Figure 9-5Proportional vs. gain

As a general statement about proportional band, a larger number forproportional band results in a smaller effect on the system; a smallernumber for proportional band results in a larger effect on the system.Similarly with gain, a larger number for gain results in a larger effect onthe system; a smaller number for gain results in a smaller effect on thesystem.

If we look at how the proportional band affects the system over time wesee some interesting characteristics of this control. The following figureillustrates the setpoint, PV, and the output. Assume a reverse-acting openloop control with a gain of 1.0.

Figure 9-6Open loop proportional controlover time

Time

PV

ASetpoint

Output

open loop proportional control over time b.eps

10075

50

250

025

50

75100

% Input % Output

Gain =

MoreEffect

LessEffect

100PB

Proportional Band

Gain4.0 0.5

50%25% 200%

2.0 1.0 0.667

100% 150%

gain vs proportional band 2 b.eps



Initially, when the PV is running at or near setpoint, the output remainsconstant. When a disturbance occurs, causing the input to increase (pointA on the graph), the output reacts by changing 100%. Once the outputchanges, the input won’t change any further, but notice the input is nownot at the setpoint. Proportional band with this open loop control will notbe able to get the input back to the setpoint.If we assume the same scenario but use a closed loop system, we will seethe outcome operate over time as illustrated in the following figure.

Figure 9-7Closed loopproportional controlover time

Even though we have a smoother control, notice proportional control stillcannot get the PV back to the setpoint without a manual reset. Offset isthe difference between the PV and the setpoint. The amount of outputchange is only dependent on the amount of input change. The integralfunction of a PID, as will be described in the next section, is what gets thePV back to the setpoint.

Integral Control

For most control systems, proportional-only control will not be acceptablebecause it cannot get the PV back to setpoint. The two terms for this func-tion are integral and reset. Integral is quantified in minutes/repeat, whilethe reset is quantified in repeats/minute.

Using the same example that we examined with proportional-only control(reverse action, 100% proportional band), we’ll observe the differencesbetween integral and reset. Reset = 1/I, I = 1/Reset, where I = integral.

Time

PV

Setpoint Offset

Output

closed loop proportional control over time b.eps

Closed Loop Proportional Control Over Time



Figure 9-8Open loopproportional and integral controlover time

Integral will repeat the proportional action in the time period (in thisexample once per minute).

Reset windup is the condition in which the reset continues to drive theoutput beyond the limit of the output. Anti-reset windup is the pointwhere the integral is turned off (where the output cannot affect thecontrol).

Derivative Control

For the last mode of a three-mode controller, derivative is a quantity ormode that tries to anticipate what changes need to be made for upcomingevents. For example, as you are driving on a flat road and see a hillcoming up, when you are approaching the hill, you’ll depress the acceler-ator in hopes that by doing this you’ll be able to maintain your currentspeed as the load on the vehicle increases caused by the hill.There are two terms associated with the derivative function – derivativeor rate. Both of these terms are quantified in minutes.

Derivative is not necessary for all control applications. Most control appli-cations will do just fine with proportional and integral.Once again using the same example that we examined with proportionaland integral (reverse action, 100% proportional band), we’ll observe howthe derivative function affects the input and output.

Time

PV

ASetpoint

Output

open loop proportional integral control over time b.eps

Time0 1 2 3 4 5 6

PV

Setpoint Offset

Output

Open Loop Integral

Open Loop Proportional



Figure 9-9Derivativecontrolover time

When the input changes, the proportional portion of the control willadjust the output so that it mimics what the input is doing (seen as pointP on the previous figure). The derivative will calculate the rate of changeand anticipate what the outcome will be at one minute. Derivative tries toreduce the overshoot of a P and I control.

Control Modes Example

Shown in the following figure is an electronic modulated closed-loop feed-back control system controlling temperature and level.

Figure 9-10Feedbackcontrol system

Before getting into the details of this system, the following is a review ofthe feedback controller modes.

TT1

HEATING FEED IN

FEEDOUT

HEATING WATER IN

LY2

A/S

A/S

TY1

SETPOINT

TV1 LV2

SETPOINT

TE1

HEATING COIL

PROCESSTANK

CLOSED LOOP

LT2

TIC1TIC1

LIC2LIC2

WATER OUT

Time

PV

PIDSetpoint

Output

derivative control over time b.eps

PI



Proportional: responds to the magnitude of the error. The change of theoutput signal is proportional to the error. The proportionality constant isthe gain or 100%/PB%.

Integral: responds to the duration of the error. Used to remove offset(error) and drive the input back to the setpoint. Adds an additional compo-nent to the output signal that ramps up over a selected time interval.Anti-reset windup: a feature for a controller with integral mode thatcancels the integral component of the output when the output exceeds aspecific value. This prevents the integral component from driving theoutput to saturation, in a situation in which the output signal is notmanipulating the process (i.e. there is no fluid to flow through a controlvalve). Integral then continues to ramp up again from zero.

Derivative: adds an additional short-term component to the output that isproportional to the rate at which the error is changing. This anticipates acorrection for a measurement whose response to the process is slow.Responds to how fast the error is changing.

Looking at the temperature portion of the feedback system shown inFigure 9-11, the following figure illustrates that we see a temperaturedisturbance of 20°F.

Figure 9-11Temperaturedisturbance

% of Input

Range

0 %

100 %

50 %

Time

60 %

50º F

250º F

150º F170º F

Out % = 50% While Process Temperature is at Setpoint

Setpoint

LOOP PROCESS UPSET



For proportional-only control, the following figure illustrates how thesystem will react.

Figure 9-12Proportional onlycontrol

Notice in this mode of control that the offset remains. Though the distur-bances can be controlled, the PV still cannot reach the setpoint and thusthere is an offset. This is not the desired result.

Figure 9-13Proportional and integral control

For integral control, there is more output change for every interval of timeand the end result eliminates the offset.

Out % = 50% + Gain x Error% (Proportional + Integral)

Out % = 50% + (100% / PB%) x Error% + Integral

Out % = 50% + (100% / 100%) x 10% + Integral = 60% + added 10% every Ti

Input Range

0 %

100 %

50 %

Time

60 %

50º F

250º F

150º F170º F

Offset

Input Range

0 %

100 %

50 %

Time

60 %

50º F

250º F

150º F170º F

Out % = 50% + Gain x Error% (Proportional Only)

Out % = 50% + (100% / PB%) x Error%

Out % = 50% + (100% / 100%) x 10% = 60% (PB = 100%)

Out % = 50% + (100% / 20%) x 10% = 100% (PB = 20%)

Offset



Tuning Feedback Controllers

The goal of tuning is to have the measured variable quickly return to itssetpoint following a disturbance. For effective tuning the values of propor-tional band (PB), reset time (Ti), and derivative time (Td) are adjusted sothat there is a minimum of the process variable from the setpoint, andthere is no limit to cycling the control valve.

¼ Decay Ratio

One common control method is called ¼ Decay Ratio. With this methodthe controller is adjusted so that the system’s response curve has a decayratio of one-quarter when given a setpoint bump or temporary distur-bance. The following figure illustrates the response curve.

Figure 9-14¼ Decay Ratiocontrol

For this control method, the height of the first peak is reduced to one-quarter as seen in the height of the following peak.

Ziegler-Nichols

The Ziegler-Nichols tuning method is a tuning method developed in 1942.The procedure is to first record the tuning parameters and then removeintegral and derivative by setting the integral time to infinity and thederivative time to zero. This method is used to test the process in order toset the parameters.

“Good Control”-1/4 Decay Ratio



The gain is then increased and the setpoint changed and returned to itsoriginal value while watching the output device oscillate. If the oscilla-tions die out, add more gain. If the oscillations increase in amplitude,reduce the gain. When the gain setting produces equal amplitude oscilla-tions that continue for one minute or longer without reaching full limits,you have set the ultimate gain, Ku. The proportional band PB inpercentage is equal to 100/Ku. The ultimate period, Pu, is then measuredwith a stopwatch. The best gain is one-half of Ku or

PB = 2 x PBu (for proportional only).

PB = 2.2 x PBu , where Ti = Pu/1.2 (proportional plus integral)

PB = 1.65 x PBu, where Ti = Pu/2, and Td = Pu/8.0 (P + I + D)

Disadvantages for this method are that many processes cannot toleratethe period of overshoot, and the time needed to accomplish this tuningmethod may not be acceptable to production managers.

Damped Oscillation

The damped oscillation tuning method is very similar to the Ziegler-Nichols method. It is used when sustained oscillations are not allowedand the ultimate method cannot be used.

The integral and derivative are removed by setting the integral time toinfinity and the derivative time to zero. The gain is adjusted from a verylow value until a quarter-wave damped oscillation is observed when asetpoint bump is applied.

Only the period, P, is measured with a stopwatch. The gain is thenlowered to a very small value and then integral and derivative (ifrequired) are set according to the following equations:

Ti = Pu/1/5 , and Td = Pu/6.0

The gain is then increased until quarter wave damping is again observedin response to a setpoint bump.

Examples

Advanced controls methods are used to reduce error in control schemesand automatically adjust for changing conditions. Three methods will bediscussed here, which are Ratio, Cascade and Feedforward. The goal ofthese advanced methods is to enhance the goal of any control loop whichis to keep the process variable at setpoint.



The following figures show examples of a ratio controller, a cascadecontroller and a feedforward and feedback control systems.

Ratio Control

The concept of ratio control is that of blending, or mixing two quantitiestogether to obtain a final product that keeps a specific proportional rela-tionship. A ratio controller has an additional input signal that will be usedto calculate the setpoint for the controller based on the ratio given or setin the controller. This setpoint value will be used in the PID portion of theratio controller to change the primary input as needed via output changesas in a normal PID controller.

Figure 9-15Ratio controllerfor proper flow ratio

The first ratio control example has an uncontrolled or “wild” flow beingmeasured by a flow instrument; this flow measurement is sent to a ratiocontroller as the basis for calculating the setpoint of a second controlledflow. The second flow is measured and the measurement is sent to theratio controller to be manipulated by the ratio controller to obtain thedesired setpoint. As an illustration of this, the second flow is desired tohave a ratio of 0.5 to the “wild” flow. If the “wild” flow were 200 gpm thanthe ratio controller would calculate a setpoint of 100 gpm for the secondcontrolled flow to be obtained. The ratio controller would use the PIDsettings to change the output and achieve this setpoint. If the “wild” flowchanges to 300 gpm than a new setpoint of 150 gpm would be calculatedand the ratio controller would change the output to obtain this value.

MULT

I / P2

A/S

CONTROLLED FLOW

CHEMICAL#1

FV2

Setpoint

FT2

FT1

FE1

FE2

FIC2

UNCONTROLLED (WILD) FLOW

Ratio Relay

CHEMCAL#2



Figure 9-16Ratio controllerfor proper combustion

The second ratio control example has two controlled flows that need to beblended together to obtain good efficient combustion for a furnace. Thefirst or primary flow is the air which has a setpoint that is changed by ahuman and the controller is a PID controller able to keep the PV atsetpoint. The air flow measurement is also sent to a second controller as asecond input for Ratio control. The second controller uses the air with aratio value that is set by a human to calculate and change the setpoint forthe amount of fuel flow necessary to obtain the ratio that is set. It isimportant to realize in this example that both controllers have PID thatwill control their respective flows, the second controller receives thesecond input to achieve the ratio desired.

Cascade Control

The principal for cascade control is to reduce error in a controlled process.Cascade control involves two loops that have an interaction; in otherwords one loop changes the other. A primary or master process is affectedor disturbed by a secondary or process, for cascade control to be effectiveand worthwhile the secondary loop needs to have a fast process character-istic while the primary loop has a slow process characteristic. Both loopsare controlled by PID controllers, the primary controller output is used bythe secondary controller as a second input that changes the setpoint whenthe controller is in the cascade mode. Another view of Cascade is that onefeedback loop is put inside another feedback loop, the slave loop inside themaster loop.

FUEL

MULT

I / P2

A/S

TO FURNACEAIR

FCV2

Setpoint

FT2

FT1

FE1

FE2

FIC2

Ratio Relay

FCV1

I / P1

A/SFIC1

MIXING TEESetpoint



Figure 9-17Cascade controller

In our example the furnace temperature is a slow process, the tempera-ture controller will therefore be the primary or master controller with theoutput going to the secondary controller as a second input. The fuel flow isa fast process, which changes or disturbs the furnace temperature process,therefore within the cascade control is the secondary or slave control loop.The fuel flow controller will minimize the effect of fuel flow changes dueto pressure fluctuations before the temperature process could be affected.This is the advantage of having cascade control over the temperaturecontrol directly changing the fuel valve. If the primary process and thesecondary process have the same response characteristic then cascadecontrol is not worthwhile. So the disturbing process needs to have a fasterresponse than the primary process to be an control advantage while alsobeing cost effective.

To tune the cascade control, put the secondary controller in automaticmode and tune this controller until the desired response is obtained.When the secondary controller is tuned, put it in cascade mode and theprimary controller in automatic mode, then tune the primary controllerfor the desired response.

Feedforward Control

An analysis of feedback control, using a PID controller as a decisionmaker, will reveal that the feedback control requires the process to haveerror to work. The reason for this is that disturbances cause error in theprocess and the controller reacts only to the error. With feedback controlwe have a “feedback penalty” in that we will have error with this methodof control. Since the goal of any control is to not have error, then anythingwe can do to prevent or anticipate error can be a benefit to the success ofthe control scheme.

TT1

OUTPUT

FCV2

A/S

SP#2

FT2

FE2

FUEL

TE1

FURNACE

FIC2

I / P2

TIC1

INPUT

SP#1



Feedforward control is a control scheme that will monitor major distur-bances to a process control loop and make changes to the manipulatedvalue before the process variable changes; thus preventing error.Feedforward control is using methods that a human operator will use incontrolling any loop. If a human has an indication that shows a distur-bance coming to a process control that the operator is monitoring than theoperator will make a change in manual in anticipation of the disturbancechange to prevent or reduce error. Feedforward control is not obtained byinstalling a feedforward controller but is an engineered control that isdesigned for each individual process control to be used, so it is a customcontrol that can be incorporated in most PLC, DCS or computer controlsystems. Feedforward control does have a problem. Since it is a form ofopen loop control, it will react to the major process disturbance it is moni-toring; however, most processes have multiple disturbances that willchange the process variable. So, while feedforward control reduces oreliminates the error from the major disturbance over the long term, feed-forward control can not do anything about error from other minordisturbances. When feedforward control is installed, to achieve theprimary goal of control, a feedback control will need to be used in conjunc-tion with the feedforward control.

Figure 9-18Feedforward andfeedback control system

TIC1

LT2 LIC2

HEATING WATER OUT

FEED IN

FEEDOUT

HEATING WATER IN

TV1 LV2

LY2

A/S

A/S

TY1

HEATING COIL

Setpoint

SUM

FT3

FE3

MULT

Setpoint

TT1



In our first example, controlling the temperature of a continuous processis the goal of the installed control scheme. The process temperature has amajor disturbance in the form of feed flow changes which will affect theprocess temperature. This example shows both feedforward and feedbackcontrol being used to effectively minimize error in this control. The feed-forward control portion is a flow measurement on the incoming flow to thetank with a calculation being performed on the flow to determine themanipulated value that will achieve the desired temperature. As the flowchanges to the tank, this disturbance will be anticipated and changesmade to the manipulated value before the temperature would change(thus preventing error). However, the feedforward control will not react tochanges to environmental temperature changes around the tank orchanges to the manipulated variable such as pressure or temperaturethat will cause error in the tank temperature over a long period of time.To correct for these disturbances a feedback controller will need to be usedin conjunction with the feedforward control. With both feedforward andfeedback controls, the error is reduced greatly over just feedback control.

Figure 9-19Feedforward andfeedback control system

With the second example, feedforward and feedback control schemes areagain being used to minimize the error in the pH control for the tank’sdischarge. In this example the feedforward scheme is to monitor twomajor disturbances to the process, in the form of feed flow and feed pH.The values obtained is used in calculating the amount of acid needed tocorrectly achieve the desired pH in the output stream. However, the feed-forward control can not detect any changes to the acid concentration usedand error will result over a long time period. To correct for small distur-bances, a feedback control is added to the feedforward control for goodresults.

INFEED

Setpoint

SUM

FT3

MULT

FE3

ACID

CALC

PhE5

pHIC5

I / P5

A/S

OUTFEED

MIX

FCV5

pHE4



Notes

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Control Systems

Objectives

Upon completion of this chapter, the student should:• Understand the similarities and differences between PLCs, DCSs,

SCADA systems and HMIs.• Become familiar with current process control system technology

trends.

Introduction

Many questions arise concerning the differences between programmablelogic controllers (PLC) and distributed control systems (DCS). In the past(over 10 years ago), the PLC was the primary controller used for machinecontrol, while the DCS dominated in process control. If your facilityproduces widgets, you’ll probably be using a PLC, whereas if you producechemicals, you’ll probably be using a DCS, and 10 years later, this is stillcommon but becoming more blurred. That still doesn’t define the differ-ences.


10

Figure 10-1DCS process systems

Originally the DCS performed the control functions of the analog panelinstruments it replaced. Its interface, which is basically an HMI (humanmachine interface), mimicked the displays of the analog panel instru-ments. Over time, the DCS systems were improved allowing them tocontrol sequence logic (to control batch processes) as well as continuousprocesses. A typical DCS system performed hundreds of analog measure-ments and controlled dozens of analog outputs using multi-variableProportional, Integral and Derivative (PID) control.

While the DCS was being used in the process industry, the PLC was usingthe same 8-bit microprocessor technology to implement Boolean logiccontrol to replace conventional relay and solid-state logic for starting/stop-ping motors, energizing/de-energizing solenoids, etc.

The big change with the DCS was its move from proprietary hardware topersonal computer and standard LAN technologies. The trend in the DCSsystem is in its network capabilities to which “smart” devices areconnected with no I/O hardware modules of its own.

Host

Computer

Gateway

Node

Controller

PLC

Profibus-FMS

CNC PCS

Profibus-PA

Cell

Level

Information Network TCP/IP

Profibus

Application Range

Profibus-FMSProfibus-DP Profibus-PA

SensorSensor

Field

DeviceI/O Sensor Drive

Trans-mitter

Field

Device



One of the biggest differences between the PLC and DCS is how vendorsmarket them. DCS vendors typically sell a complete, working, integratedsystem, and tries to be your sole control supplier. The PLC vendors typi-cally market the PLC as more of a do-it-yourself device, which issometimes simpler to execute.

Figure 10-2PLC I/O network system

The PLC operates in the following fashon: reads the inputs, executes thelogic, and determines the state of the outputs. The logic is typically ladderlogic. To make a PLC look like a DCS system, an HMI is added which canbe tricky when managing the databases of each system due to manufac-turer differences, communication drivers, etc. vs. a package that is allinclusive for the DCS.

Since the components of a PLC and DCS system (processor, input/outputmodules, communication to devices, operator interfaces) can be verysimilar, the following section will illustrate an overview of one of thesesystems.

Node 1 Node 2 Node 3 Node 4 Node 5 Node 6 Node 64

Smart

Photoelectric

Sensor

Proximity

Switches

(nonintelligent)

Photoelectric

Sensors

(nonintelligent)

CAN Chip

Inside I/O

Port

Smart

Push Button

Station

Smart

Operator Interface

(Multiple Inputs)

Smart Valve

Manifold

(16 Outlets)

To Nonintelligent I/O Devices

(Max of 128 I/O per Node

Using Up to 8 Addresses)

High-Density

I/O Concentrator

Smart

Servo

Drive

no

de

s a

nd

co

nn

ectio

ns b

.ep

s

Channel 1

(64 Nodes)

Channel 2

(64 Nodes)

SDS Host

Controller Interface

Chapter 10: Control Systems


Control System Overview

The following block diagram shows that a PLC system has this basicarchitecture: a central processing unit (CPU), which is basically acomputer, input hardware, output hardware, a power supply and anexternal means of programming the PLC (hand-held terminal, laptop withsoftware, etc.). Also, although not part of the PLC, are the devicesconnected to the inputs and the outputs, such as pushbuttons, limitswitches, proximity switches, analog signals, solenoids, motor starters,lights, etc.).

Figure 10-3Detailed block diagram of relation of modules

PowerSupply

Logix [my program]

TOR

INPUT

STATUS

OUTPUT

SCANNERPro

cessor

INPUT

Start

Stop

LS1

LS2

NEG

24 VDC

IN 1

IN 2

IN 3

IN 4

IN 5

IN 6

IN 7

IN 8

OUTPUT

VAC

OUT 1

OUT 2

OUT 3

OUT 4

OUT 5

OUT 6

OUT 7

OUT 8

Communication

Cable

Monitor

Programming

Terminal

Input/Output

Modules

Inputs

(Located on Machine)

Input Module Output Module

L1

M1

M2

L1

L3

L2

L2

Outputs

(Located

on Machine)

syste

m p

iece

s b

.ep

s



Each PLC system consists of the following basic electronic hardwarecomponents:

• Central Processing Unit (CPU)• Programming Interface Device• Power Supply• Input section• Output section

The following sections of this chapter will present the details of each ofthe PLC components listed above.

Central Processing Unit

The central processing unit (CPU) is the brain of the PLC. It makes allthe decisions about what outputs to turn on or off.

Figure 10-4Processor, orcentral processingunit

The set of user instructions the microprocessor processes is called theprogram. The program is written by the user, and after being written isstored in program memory (often called ladder memory). As theprogram is user definable, there are some restrictions regarding syntaxand instruction placement, and each PLC manufacturer may havedifferent restrictions.

When the CPU is in Run mode, it will execute the instructions containedin the user program in a sequential order. While executing the instruc-tions, decisions are being made and the outcomes of the decisions arestored in the CPU’s data memory.

Most PLCs contain two memory types. The first type is called read onlymemory (ROM), which is nonvolatile memory. This memory contains theprocessor’s machine language to process the program’s instructions. Thesecond type of memory is random access memory (RAM). RAM isvolatile memory, meaning when the power is off, the memory in RAM willbe lost. Thus, as the RAM is where the user program is stored, after a



power outage the PLC will not be able to run the machine or processbecause there is not a program in the memory. Since this can be devas-tating to production, most PLC manufacturers give the user the option fora battery backup or EEPROM backup.

Figure 10-5Battery backupon the processormodule

It is highly recommended that the date be recorded in a maintenance logwhen a battery is replaced. These batteries last approximately 2 ½ to 3years if they are installed properly in the processor and kept cool in thecabinet by some sort of cabinet cooling system. While EEPROM backup isa good form of backup, any time a program change is made, the EEPROMchip must be burned (written to), which is done by taking the processoroffline (put in program mode). A typical problem users have withEEPROM chips is that once their machine is up and running and theEEPROM chip has been burned, over the next few months they may havemade program modifications or altered their process somewhat, but theyforgot to burn in the new changes. Later, after a power outage, when thesystem is re-powered the EEPROM loads the old program into memory(over the new changes) and the new changes that were made will be gone.

Another issue to consider is that since the machine’s sensors and loads(about which the processor is making decisions) are typically higher-voltage devices, they must be isolated and translated into signals theprocessor can safely process. This isolation and signal translation isaccomplished through the electronics built into the I/O modules and willbe explained in further detail later in this chapter.

Note also that most PLC manufacturers have important diagnostics builtinto the hardware of the processor module.



Figure 10-6Processor diagnostics

PLC manufacturers typically include following indicator lights on thefront of their processors, which allow the user to get a quick indication ofsome major issues that can affect the PLC system without having to getout the programming terminal:

• Run mode indicator• Processor fault indicator• Communication indicator(s)• Low battery indicator• Forces indicator

Programming Interface Device

The programming interface device is the device that allows the user tocreate the program that the PLC processor will run. This is the devicethat probably has undergone the most significant changes over the last 20years. Starting from a video display terminal, we now have laptops withoperating systems. Although there are several methods and devices dedi-cated to program the PLC, the most widely used device is the computer.

Communication

Indicators

Run, Force, Fault,

& Battery Indicators

500processor status b.eps



Figure 10-7Laptop with PLCprogramming software

Each PLC manufacturer requires that the user use their programmingsoftware to program their PLC. A manufacturer may have several PLCprogramming software packages that may look and operate similarly, butare designed for different PLC models. Be aware that these software pack-ages are typically sold with a price tag of several thousand dollars for onelicense.

Communications

Typically the programming software only allows the user to create theprogram that the PLC will process. There is usually another softwarepackage (communication server software) running on the programminginterface device that allows the ladder logic programming software tocommunicate through one of the computer’s ports to a port on the PLC toallow the ladder logic program to be uploaded/downloaded, edited andmonitored.

The following figure shows an example of the software packages and thecabling needed to actually do the programming.



Figure 10-8Example of the softwareand hardware needed fora laptop to PLC programmingoption

The communication options are different for each PLC manufacturerranging from inexpensive RS-232 methods to more complicated methodsthat may require special (expensive) computer boards and cables. Thecommunication server software will need to be configured for the type ofcommunication (protocol, speed, and hardware) between the computerwith the ladder logic programming software and the PLC.

Power Supply

The power supply is used to supply the voltage and current demands ofthe CPU and the input and output (I/O) portions that are to operate. Thepower supply connects to a modular PLC through a connector andsupplies the rack with the power needed for sending signals between theCPU and the I/O modules.

Linx

RS232

PLCPC

Logix [my program]

INPUT

STATUS

OUTPUT

SCANNER

Pro

ce

sso

r

PowerSupply

Comm Ports

Communications

Cable

Programming

Terminal

PLC Programming

Software

(Ladder Logic)

Communications

Server Software

(Communications:

Setup Protocol

and Pathways)

so

ftw

_h

ard

w b

.ep

s



Figure 10-9Power supply andits function



Most power supply modules have a hold time (the time the system isoperational during a brief power loss) that is typically between 20milliseconds and 3 seconds. Thus, during a brief power loss, the PLCshould still remain operational.

Figure 10-10Power supply andinternal electronics

The power supply has an electronic board inside which, due to the capaci-tors on it, is the module most likely to fail. Like any electronic module,heat is the main problem. Make sure the PLC system is properly installedaccording to the manufacturer’s recommended spacing within enclosuresto allow for convection cooling.

Machine devices such as switches, photoeyes, temperature switches, push-buttons, motor starters, or solenoids that get wired to the input andoutput modules must be supplied with external power sources provided bythe user. This will be explained further in the next section of this chapter.

Input and Output (I/O) Sections

The input and output modules are the means by which the programmablecontroller senses and controls the system. Input modules are the eyesand ears and feel of the machine sensing the status of the field inputdevices (switches, sensors, etc.). Output modules are the muscle of themachine, controlling the machine by operating valves, motor starters,heaters, lights, i.e. that do the work.

Discrete digital modules process signals as either an “on” or an “off.” Thesewill be compared to analog modules that process signals over a range orpercentage of the maximum signal in Chapter 5.

Fixed I/O PLC

The fixed I/O PLC consists of fixed or built-in input and output sections, apower supply and the processor. The fixed I/O PLC has limited expand-ability, if any at all. Typical I/O arrangements might have 8 inputs and 8outputs, or 12 inputs and 8 outputs.

PWRSPY b.eps



Figure 10-11Fixed I/O PLC

This is usually manufacturer dependent. This type of PLC usually hasless processor memory and fewer programming capabilities, which is alsowhy they typically cost much less.

Figure 10-12Fixed I/O PLCwith expandability

Many PLC manufacturers that make the fixed I/O PLC are also makingthem modular. The base system may have the processor and a few inputsand outputs, but it still offers expandability to add more input and outputmodules.

Modular I/O PLC

The other type of PLC is the modular I/O PLC. This type of PLC consistsof the input and output sections on removable plug-in type units calledmodules. These modules plug into slots in a rack where the power supplyand CPU reside. The placement of the modules is manufacturer depen-dent. Some manufacturers require that the processor reside in theleftmost rack slot while others require that the processor reside in therightmost slot. There are also newer PLCs that allow more than oneprocessor in a single rack and in any slot location.

OUT0

OUT1

OUT2

OUT3

IN0

IN1

IN2

IN3

Inputs

Start

Stop

L1 L2

LS1

FloatSwitch

PLC

Horn

LightR

Sol A

M1

MotorStarter OL

Outputs

IOCPLCb.eps



Figure 10-13Modular PLC systems

The modular I/O PLC also has another level of modularity. Along withhaving “modules” reside in a rack, there can be several racks that areconnected via a communications link to one processor in a main rack, orseveral processors that are linked together.

Figure 10-14InterconnectingPLCs

As we continue with this seminar, we will find that with advancednetwork systems now available there are even more levels of modularityand expandability for I/O. These will be discussed in detail later in thistext, and will involve remotely connected I/O systems.



Figure 10-15Input modules withinterchangeable terminal strips

Field devices are wired to a terminal strip on the input section. Mostmanufacturers have their input modules so that the terminal strip isreplaceable, so in the event of a module failure, the terminal strip isunscrewed from the faulty module and plugged into the replacementmodule.

Each field device is wired to a separate terminal on the input module. Theinputs are addressed so both the user and the processor can know wherethe field device’s signal will be stored in the processor’s memory.



Input Section

Inputs are field devices that inform the PLC of the status of themachine/process, or that provide control information about the environ-mental condition of the process. Input sensors can provide machineinformation about (but not limited to) temperature, pressure, flow, level,location, etc.

Digital Input Modules

The main purpose of digital input modules is to convert voltage levelsfrom field devices into “1s” and “0s” stored in an assigned address locationin the processor’s data memory that corresponds to the specific modulelocation in the PLC system. The location is rack dependent, meaning theprocessor needs to know in what rack the module is residing (if more thanone rack is in the system) and where within the rack the module islocated (which slot).

Capacity of Inputs

Most PLC manufacturers offer digital input modules that have from 4 to32 input terminals per module. Probably the most widely used module isthe 16-point module.

Figure 10-1616-point digitalinput module

Voltages

Like the multiple voltage levels available for wiring a machine’s sensors,digital input modules are designed so the electronics can manipulate andprocess specific voltage ranges for the field devices they are connected to.Most PLC manufacturers have digital input modules designed to handlethe following AC and DC voltage levels:

• 120 VAC• 240 VAC• 24 VDC



• 5 VDC• 48 VDC• 125 VDC

Although an input module may be designed to handle 120 VAC, whathappens if the incoming voltage is only 83 VAC? The processor never seesthe actual line voltage that is applied to the field devices and inputmodule. In order to protect the PLC system from the typically higher linevoltages used for field devices, the input modules perform the followingfunctions through their electronics:

• Termination• Isolation• Translation• Threshold detection• Indication

Termination

Termination means that the module provides a terminal connection pointfor the wire connecting itself and the field device.

The following figure shows a typical wiring diagram for a 120VAC, 8-pointdigital input module.



Figure 10-17Digital inputand field devicetermination diagram

Isolation

A common method used to provide the module with protection from thehigher voltages of the field devices is optic isolation. Optic isolation workslike this: the terminal points are connected electrically to small IC chipscalled optic isolators.

1100

1110

1120

1130

1140

1150

1160

1170

1180

1190

1200

I:1/0

I:1/1

I:1/2

I:1/3

I:1/4

I:1/5

I:1/6

I:1/7

AC

Com

X3

PB

LS1

FS2

TS3

PS4

FS5

MO

PB7

X4Slot 1

Inp

ut W

ire

b.e

ps



Figure 10-18Optic isolationin input modules

A. Threshold detection

B. Isolation

C.& D. Translation

If voltage is present on an input terminal, it causes a light-emitting diode(LED) to illuminate inside the chip. The beam of light shines down a smallphysical channel in the chip. At the other end of the channel is a photo-transistor (a transistor activated by light), which becomes energized whenlight hits it. Only the phototransistor side of the chip has access to theinternal organs of the PLC. The light channel provides a physical air gapas protection between the field devices and the PLC. If a serious over-voltage is present in the field, the usual result is that the LED burns out,but no damaging voltage reaches the inside of the PLC.

IPTMDb.eps

LowVDC

1

N

H

InputDataTable

120 VAC

C

D

A B



Translation

Translation is the process by which voltages in the real world are trans-formed into the voltage levels that the PLC sees as “1s” and “0s.” Voltagetranslation is done by circuitry associated with the phototransistor. Thistransistor is in a low-voltage DC circuit, and the voltage levels here corre-spond to what the PLC uses as “1s” and “0s.” When the transistor isenergized by a beam of light, a voltage level is generated by the circuitrythat the PLC treats as a “1.” When the transistor is turned off, the voltagelevel corresponding to “0” is generated.

Threshold Detection

If the voltage on the input is exactly 120 VAC, then the input section willconvert it to the voltage level for a “1,” to be used internally by the PLC.Unfortunately, voltages in the real world tend to fluctuate up and down inan unpredictable manner. If the voltage on an input drops to 110 VAC, wewould all probably agree that this should also be treated as a “1” by thePLC. What if the voltage drops lower? When does the PLC stop treatingvoltage on an input as a “1” and start treating it as a “0?”

On an AC input board, each terminal point is connected to a full waverectifier. The rectifier converts the AC signal to a DC signal. The DCvoltage is then compared to a set DC voltage (called the thresholdvoltage), and if the level is above this threshold, it is counted as a “1,” or ifit is below this threshold, it is counted as a “0.” Threshold detection isusually done with a Zener diode circuit.

Threshold detection is important because it allows the voltages connectedto field devices to vary without disrupting the operation of the PLC.Threshold detection also provides noise immunity for the input section.This means that if a switch is open in the real world, and there is a greatdeal of electrical noise in your facility, it may result in low voltage beingpresent on the input. This is especially true if the wires to the inputboards are particularly long. Even though there may be some voltagepresent on the input, the PLC will reject it unless the voltage is above thepreset threshold for the board. This allows PLCs to operate in electricallynoisy environments that would cause a normal computer to shut down.



Indication

We can see the result of one function performed by the input section. It iscalled an indication. There is a light for each terminal point on an inputboard. If voltage is present on a particular input (and within the range ofthe threshold detection), the light illuminates. If no voltage is present, thelight is turned off. These lights are affectionately called idiot lights.Practically every input board being used today gives you a visual indica-tion of the status of the inputs. These are extremely useful to you whenyou want to troubleshoot or find out if a particular input sensor is sendingits signal to the module.

Output Section

Outputs are field devices that the PLC controls. These consume powerand are also known as loads. The loads can be (but are not limited to)motor starters, solenoids, lights, etc.

Figure 10-19Output module

Like the input module, the output module has the loads wired to indi-vidual terminals on the module. The outputs are addressed so both theuser and the processor can know where the field device’s signal will bestored in the processor’s memory.

Digital Output Modules

The primary purpose of digital output modules is to take the “1s” and “0s”sent from the processor and convert them to voltages that the loadsconnected to the digital output module need in order to energize or de-energize the loads.

Out 0

Out 3 Out 2 Out 1 Out 0

0 0 1 0

0 VAC

Out 1 120 VAC

Out 2 0 VAC

Out 3 0 VAC

L1 L2

M

R

G

ou

tpu

t u

pd

ate

b.e

ps



Capacity of Outputs

Most PLC manufacturers offer digital output modules that have from 4 to32 input terminals per module. Probably the most widely used module isthe 16-point module.

Voltages

Like the multiple voltage levels available for wiring a machine’s loads,digital output modules are designed so the electronics can manipulate andprocess specific voltage ranges for the field devices they are connected to.Most PLC manufacturers have digital output modules designed to handlethe following AC and DC voltage levels:

• 120 VAC• 240 VAC• 24 VDC• 5 VDC• 48 VDC• 125 VDC

The output modules perform the following functions through their built-inelectronics:

• Termination• Isolation• Translation• Indication

Termination

Termination means that the module provides a terminal connection pointfor the wire connecting itself and the field device.

The following figure shows a typical wiring diagram for a 120VAC, 8-pointdigital output module.



Figure 10-20Digital outputand field devicetermination diagram

Isolation

Since the PLC uses low-level DC signals internally, it cannot directlyprovide power to 120 VAC outputs. Therefore, the PLC must be isolatedfrom the external world. As with the input boards, the most common elec-tronic method for providing isolation is with optic isolators.

Figure 10-21Optic isolationin output modules

1210

1220 1160

2110

2120

1230

1240

1250

1260

1270

1280

1290

Slot 2X5 X6

O:2/0

O:2/1

O:2/2

O:2/3

O:2/4

O:2/5

O:2/6

O:2/7

VAC1

MO

M1

R

G

A

M7

SOL5

SOL6

Ou

tpu

t W

ire

b.e

ps



Translation

When the CPU sends a “1” to an output board, a transistor on the outputboard is energized. On one type of output board called a triac, the tran-sistor turns on a firing circuit for the individual triacs connected to eachoutput. A triac is a solid state AC switch. When it is energized (fired), itallows the AC power for the load to pass. When it is turned off, no power isdelivered to the load. In another type of output board, called a relay output

board, the transistor on the board energizes the coil of a small relay. Thiscauses the contacts of the relay to close. One side of the contacts isconnected to a power source (L1), and the other side is connected to theload. When the contacts close, power is delivered to the load. Most outputboards are fused in the event of a short circuit in the field.

Indication

Like the input board, the output board gives a visual indication to theuser of the status of each output. This is normally done with an LED. Ifthe LED is on, it indicates that voltage is present on an output. If theLED is off, no voltage is present on the output.

Figure 10-22Indicator lights on adigital output module

Output Status

Indicator Lights



A blown fuse indicator light is also available on some output boards if theoutputs are fused. As with the input boards, indications can be a valuabletool for troubleshooting.

Addressing

In order for a PLC programmer to actually do the programming, theaddressing of the modules and terminals must first be defined either bythe PLC manufacturer or the user.

Module Location-Dependent Addressing

Most PLC manufacturers have a module location-dependent (I/O)addressing system. This means that when programming the processor tomake decisions about instructions in the program that are addressed to aparticular input or output, the address syntax is dependent on the datatype, module slot location in the rack, and terminal number on themodule.

Figure 10-23Module-dependentaddressing

PowerSupply

INPUT

PROCESSOR

OUTPUT

OUTPUTPro

ce

sso

r

OUTPUT

INPUT

01

23

45

6

Processor

Inputs (16)

Outputs (16)

Outputs (16)

Outputs (16)

Inputs (16)

Blank

I:1

O:2

O:3

O:4

I:5

Data Memory Table

Slot #

Rack Number = 0

Data Memory Table

Addressing Format

Data

Table

Type

Data

Table

Number : Rack

Number

Slot

(or Word)

Number

Bit

Number

ad

dre

ssin

g s

ch

em

e 1

b.e

ps



The U.S. postal system has defined the following format for addressingletters that users must follow in order to get a letter delivered from onelocation to another:

Person’s NameStreet AddressCity, State Zip Code

Figure 10-24Mail delivery requirescorrect addressing format

Each PLC manufacturer does the same thing within a specific PLC model.In order for the user to write a program that has the processor make adecision about the status of a signal from a sensor, the sensor must firstbe wired to one specific terminal on a specific input module that is locatedin a particular slot of a particular rack. The programmer, when writingthe program, must provide the information (address) of the sensor to beprocessed, or the processor will have no clue of how to process the instruc-tion in order to make a decision about it.

The type of instruction the user programs the PLC to process will deter-mine the addressing format, i.e. whether the instruction should beaddressed to the bit level or the word level.

NameStreet AddressCity, State Zip code

USPS

posta

l syste

m b

.eps



Processor Scan Routine

The uninformed sometimes think that a PLC is a continuous system(continuous systems have wired connections between the inputs and theoutputs). A button is pushed and immediately a result is seen. This is anillusion due to the speed with which a PLC operates. The steps that a PLCfollows are:

1. Update the input data table.2. Execute the program.3. Update the data tables based on the decisions made in program

execution.4. Process communication requests.

The following figures show the details of what happens for each of thesteps in the processor scan routine.

1. Update the input data table.

Figure 10-25Update inputdata table

2. Execute the program.

Processor

The processor then updatesthe data table accordingly.

IN0

IN1

IN2

IN3

L2

L1

IN3 IN2 IN1 IN0

0 1 1 0

The processor polls the input module tosee if there's voltage on input terminals.

?

Data Memory Input b.eps



Figure 10-26Executing theprogram

3. Update the data tables based on the decisions made in program execution.

Figure 10-27Update data tablesafter program execution

After the processor updates the output data table based on the conditionsof the ladder logic, it then sends the newly updated information of theoutput data tables to the output modules where the output modules thenallow or prevent the voltage from being sent to the devices connected tothem. This can be seen in the following figure.

Processor

OUT3 OUT2 OUT1 OUT0

IN2 IN1 OUT1

IN2 IN1 OUT2

IN3

0 0 1 0

The processor updates the output data table based on conditions of the ladder logic.

Processor Updates b

.eps

Processor

IN3 IN2 IN1 IN0

IN2 IN1 OUT1

IN2 IN1 OUT2

IN3

0 1 1 0

The processor executes the ladder logic based on the data tables.(Instructions highlighted are logically TRUE.)

Program

Execute

b.eps



Figure 10-28Outputs updatedaccording to data processedduring program execution

4. Process communications requests.

Figure 10-29Process communicationrequests.

After completing these steps, the PLC does the same thing over and overuntil it is commanded to stop by switching the processor out of RUN mode.

Updating the inputs and turning on the outputs of a PLC is called scan-ning. The time it takes the programmable controller to do the stepsmentioned is called the scan time. Scan time is variable. It depends uponthe number of inputs and outputs (I/O) and upon the length of theprogram. The more I/O installed, or the longer the program that the PLCmust execute, the longer the scan time will be. While it may seem that itwill take a PLC a long time to do anything, in reality the scan time is veryshort for most PLC applications. This is because the PLC is extremelyfast. Typical scan times range from 5 to 50 milliseconds. One millisecondis one-thousandth of a second.

Processor

The processor communicateswith external devices (remoteI/O, touchscreens, fieldbuss,scada systems, etc.).

Processing

Commun

ication

Re

quests

b.eps

Out 0

Out 3 Out 2 Out 1 Out 0

0 0 1 0

0 VAC

Out 1 120 VAC

Out 2 0 VAC

Out 3 0 VAC

L1 L2

M

R

G

ou

tpu

t u

pd

ate

b.e

ps



While PLC manufacturers know how much I/O can be used, they have noway of knowing how long your program will be. Thus, they generally tellyou how fast the scan time will be by specifying how many milliseconds ittakes to execute the instructions contained in a certain amount ofprogram memory. Program memory is counted in “Ks,” which is short forKILO. KILO means 1000 to the average person. In PLCs, however, 1K =1024 words of program memory. One word of memory is generally morethan enough room to hold “1” simple instruction that looks at a bit in datamemory. Complicated instructions may require more than “1” word. A PLCmanufacturer might tell you that the scan time required for a program is10m sec/K of memory. This means that for each 1K of program memoryused, it will increase the scan time by 10m sec.

Summary of Scan Routine

The first thing a PLC does during its scan is update the inputs. It looks ateach terminal screw on the input boards and, if voltage is present, itmakes the proper bit in memory a “1.” If no voltage is present, it makesthe proper bit in memory a “0.” Usually a PLC’s data memory is organizedin rows called registers. If the CPU uses an 8-bit microprocessor, theregisters are 8 bits wide. If the CPU uses a 16-bit microprocessor, theregisters are 16 bits wide. Generally, the number of inputs on a boardexactly matches the number of bits in a register. This results in the firstinput board’s bits being stored in the first input register in data memory,the second input board’s bits being stored in the second register, etc. Asthe PLC scans the inputs, all the bits in the input section of data memoryare updated.

The following illustration summarizes the scan process.

Figure 10-30Cyclical processorscan routine

ScanProcess_b.eps

BeginEnd

Communications& Housekeeping

Execute TheProgram

(Math, Mov, ect.)

Update TheOutput Image

Input ImageIs Updated



NOTES

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