Module I-6 Temperature Instruments

7/29/2019 Module I-6 Temperature Instruments

1/48

TASK DETAILING MANUAL

Module I-6 www.arfanali.webs.com Page 1

Practical Demonstration of

TEMPERATURE INSTRUMENTS

MODULE NO. : I-6

MODULE SUBJ.: Temperature Instruments
http://www.arfanali.webs.com/http://www.arfanali.webs.com/http://www.arfanali.webs.com/


2/48



Tasks:

I-6.1 Perform calibration on bimetallic dial type temperature gauge.

I-6.2 Perform calibration on filled system temperature indicator.

I-6.3 Perform service and calibration of a temperature switch..

I-6.4 Check the accuracy of:

a) Thermocouple output, and b) RTD output.

I-6.5 Perform servicing and calibration of a pneumatic temp. transmitter.

I-6.6 Perform servicing and calibration of an electronic temperature

transmitter with RTD input.

I-6.7 What are self regulating temperature valves? Function & application.

I-6.8 Service and calibrate a pneumatic temperature controller.

I-6.9 Perform calibration of an electronic transmitter using thermocouple

input.

MODULE No.: I-6 Temperature Instruments

TASK No.: I-6.1

Perform calibration on bimetallic dial type temperature

gauge.

Reference: None


3/48



Materials: 1. Cleaning Rags, and2. Solvent.

Equipment & Tools: 1. Tool Box, and2. Standard temperature bath with reference

thermometer..

Conditions: To be performed in instrument workshop..

Requirements By Trainee:

To study the task and familiarise himself, Be able to select proper tools to perform this task, Describe different types of temperature gauges, Understand the concept of bimetallic temperature elements (flat and helical), Understand the purpose of using thermowells for temperature elements, Be able to perform calibration adjustments of dial temperature gauges, Describe an understanding to his trainer, and Write observations and procedures in his workbook.

TASK No.: I-6.1 Continue

Details:

Bimetallic Thermometers

A bimetallic thermometer is a device, which senses temperature with a helical or

spiral bimetallic strip and displays the temperature on a dial for visual observation.

The bimetallic temperature element expand when the temperature increases and

contract when the temperature decreases. The increase length per unit length per

degree of temperature increase is called the coefficient of thermal expansion for

that material.


4/48



A bimetallic element can be formed in spiral or helix to increase the amount of

motion available for a given temperature change.

The spiral form of bimetallic element is convenient for housing in a circular flat

case and is typically used is dial thermometers that measure ambient temperature.

The helical form is well suited for housing in a narrow tube (stem) for insertion

into a fluid directly or housing within a thermowell with a small bore.

TASK No.: I-6.1Continue

Figure I-6.1A, Typical Bimetallic

Dial Thermometers Types

Figure I-6.1B,Exploded andSectional View ofa Bimetallic DialThermometer


5/48



Figure I-6.1A shows a typical bimetallic dial thermometer, one end of the helical

element is welded to the bottom of the stem and the other end is welded to theshaft. The pointer is attached to the end of the shaft. The sensing portion of the

thermometer is that portion of the stem that contains the element.

The standard calibration accuracy for bimetallic thermometers is +/- 1% of the

full-scale reading. Some thermometers allow single point calibration, which is

accomplished by rotating the scale behind the pointer, either by rotating the case

or by adjusting a gear from a screwdriver slot protruding through the case. Others

allow no adjustment.


Figure i-6.1B illustrates exploded and sectional views of bimetallic dial

thermometer, where the service is limited to replacement of the dial window glass

if is broken, adjust or replacement of the pointer. An external adjustment screw is

usually provided so that the thermometer can be calibrated at a single point, but

there is usually no adjustment for span.


6/48



Bimetallic thermometers are available in convenient range increments for

measurement between50 to 500 degree C.


TASK No.: I-6.2

Perform calibration on filled system temperatureindicator

Reference: None

Materials: 3. Cleaning Rags, and4. Solvent.


7/48



Equipment & Tools: 3. Tool Box, and4. Standard temperature bath with reference

thermometer..

Conditions: To be performed in instrument workshop..


To study the task and familiarise himself,

Be able to select proper tools to perform this task, Understand the concept of filled system temperature elements, Understand the purpose of using thermowells for temperature elements, Be able to perform calibration adjustments of filled system temperature gauges, Describe an understanding to his trainer, and Write observations and procedures in his workbook.


Details:

Thermometers

Filled system thermometers have a bulb filled with an expanding substance,

(usually an inert gas) and a dial, which is controlled by a bourbon tube. The bulb

is connected by a capillary tube, which can be up to about 50 feet (15 meters)

long, to the dial mechanism. Their accuracy is about the same as a bimetallic

thermometer and they are much more expensive. Therefore, filled systemthermometers are not usually unless remote installation of the gauge is desired.


8/48




Figure I-6.2A, Filled System Devices, Types of Compensating


9/48



The indicating temperature of filled system thermometer are subject to significant

error from elevation changes in the capillary and are relatively difficult to

compensate and The reading may change somewhat with changes in ambient

temperature. The errors thus induced can be minimised by a compensating

mechanism, which senses the ambient temperature and automatically adjusts the

temperature reading. Figure I-6.2A illustrates types of compensation of gas field

system.

Figure I-6.2B, FilledSystem Thermometers


10/48




As shown in figure I-6.2C an industrial duratemp thermometer components and

replacement parts.

Calibration:

To calibrate filled system thermometer; arrange calibration set-up using reference

accurate thermometer with heating bath to compare between them. Consult your

trainer for details.


TASK No.: I-6.3

Figure I-6.2C, Filled System Thermometer Components


11/48



Perform service and calibration of a temperature

switch

Reference: None

Materials: 1. Cleaning rags.

Equipment & Tools: 1. Tool Box, and2. Digital Multimeter / Digital Voltmeter (DVM).

Conditions: To be performed at workshop.


To study the task and familiarise himself, Be able to describe the operation principle of temperature switches, Be able to identify the main parts of a mechanical temperature switch, Understand deadband error of a mechanical temperature switch, Understand the advantages of a solid-state temperature switch, Perform periodic and corrective maintenance of temperature switches, Draw / Sketch calibration set-up of the solid-state temperature switch, Discuss an understanding with his trainer, and Write observation and procedures in his workbook.

TASK No.: I-6.3Continue

Details:

Electric Temperature Switches

An electric temperature switch is a device, which causes a contact to open or close

with a change in temperature. Most switches can be used as either high


12/48



temperature or low temperature sensors, depending on how they are calibrated and

electrically connected.

Most mechanically operated temperature switches use a vapour-filled system or a

liquid-filled system to operate pressure switch. Gas-filled systems generally do not

develop enough power for switch use. Figure I-6.3A shows vapour filled systemmechanically operated temperature switch.

Filled system switches are available for both local and remote mounting. The local

mounting type has the bulb rigidly attached to the switch mechanism and housing.

The assembly has a threaded connection so that it can be screwed into and be

supported by a thermowell. The remote mounting type has the bulb connection to

the switch mechanism by a capillary tube from 6 feet (2 meters) to 25 feet (8

meters) or more. The switch cannot be separated from the bulb in the field.

One disadvantage of mechanically actuated temperature switches is significantdeadband. When temperature switch trips on raising or falling temperature, it does

not reset at exactly the same temperature as it tripped. The difference is called the

deadband or reset. Sometimes the required trip point is very near the normal

operating temperature. In these cases, a solid-state temperature switch should be

used.

Solid-state temperature switches use a resistance temperature detector (RTD), a

thermocouple, or a thermistor to detect the temperature and contain the required

amplifiers and other electronic circuitry to activate a relay or solid-state output

device at the set temperature. Figure I-6.3B shows a solid-state switch for controlroom mounting.

The output from the temperature switch is usually either a microswitch or a relay.

The contact configuration is most often either single-pole, double threw (SPDT) or

double-pole, double-threw (DPDT).



13/48




Figure I-6.3A, Mechanically Operated Temperature Switch

Figure I-6.3B,Typical Solid State

Temperature Switch


14/48



A double-pole type switch or snap-disk mechanism is needed if two circuits, such

as a shutdown and alarm that annunciates that shutdown are to be actuated by the

same sensor. Multiple temperature set adjustments are if one temperature switch

needs to be actuated at more than one set-point. Switch housing must be suitable

for hazardous environment. Figure I-6.3C shows temperature switches with

various types of housing.


Figure I-6.3C, Temperature Switch Housing for Various Installations


15/48



Pneumatic Temperature Switches

A pneumatic temperature switch is a device, which senses temperature and

actuates a valve to supply or vent a gas or air signal. Pneumatic switches are not as

abundant as electric switches, but they are still available from several sources.

Some manufacturers will substitute an air relay for the microswitch in a

mechanically actuated temperature switch.

The pneumatic temperature switches, which are essentially converted electrical

switches use the same principles of operation as do their electrical counterparts.

The ones designed for the pneumatic system are often called temperature valves.

They are manufactured with both two-way valves and three-way valves. The two-

way type, are designed so that a vent port opens when the temperature exceeds the

set point, the three-way valve type either connects the receiver device to the

pressure source or vents it, depending on the temperature. These units use the

bimetallic principle. A typical temperature valve is shown in figure I-6.3D.


TASK No.: I-6.4

Check the accuracy of:

a) Thermocouple output, and b) RTD output

Figure I-6.3D, Typical Temperature Valve


16/48



Reference: None

Materials: Thermocouple element and RTD element

Equipment & Tools: 1. Tool Box,2. Bath heater with reference temperature

thermometer, and

3. Digital Multimeter.

Conditions: To be performed at workshop.


To study the task and familiarise himself, Be able to select proper tools to perform this task, Understand the concept of thermocouple and RTD as temperature sensors, Be able to recognise using of the reference temperature tables for

thermocouples and RTDs.

Demonstrate the accuracy procedure of thermocouple and RTD, Discuss an understanding with his trainer, and Write observation and procedures in his workbook.


Details:

Thermocouples

A thermocouple is a junction of dissimilar metals used to measure temperature.

When two different metals come in contact with each other, thermal energy is

converted into electrical energy. Any two metals can be used and the amount of

electrical energy created is a direct function of the absolute temperature except in

circumstances. Also the amount of energy converted depends on the metals


17/48



selected. Certain combinations of metals have been identified which create enough

energy in a sufficiently linear manner so that they can be used to measure

temperature with a high degree of accuracy.

Thermocouple wires of the selected metals, are joined together to make electrical

contact. They can be twisted, welded, soldered or even wrapped under the samescrew. The electrical limitations are that the junction, including any third metal,

must be at the temperature to be measured, the wires must be insulated from each

other from the junction to be receiver, and if the junction is grounded, there must

be no other ground. The only physical limitation is that the wires must be able to

stand the environment to which they are subjected.

Connection of thermocouples does present some difficulty because when the

selected metals are connected to any other metal, such as copper wire, another

thermocouple is created and the temperature of this connection will affect the

measurement as much as the temperature of the primary junction.


Figure I-6.4A shows a thermocouple constructed from copper (Cu) wire and a

copper-nickel alloy wire named constantan (c) connected to a voltmeter made of

copper. The constant an wire must be connected an wire must be connected to

copper somewhere in addition to the thermocouple to complete the circuit, but this

will form another thermocouple. This connection is made so that the temperature

can be held constant at a known temperature and is called the cold junction or

reference junction (J2). A temperature that is easy to create and duplicate is that of

a bath formed by pure water and the ice that water, 32F (0C) by holding the

reference junction at the ice point, the temperature of the primary junction (J1) can

be found by measuring the voltage it creates in reference to the voltage created by

the reference junction.

Figure I-6.4A,

Copper-Constantanthermocouplecircuit with anexternal reference

junction


18/48



Tables of the voltage created at temperature versus the ice point are published by

the Untied National Bureau of Standards (NBS) and are used worldwide. By

measuring the thermocouple voltage, the temperature can be found from the table.

If the reference junction is located where the temperature is known or can be

measured accurately, then the junction voltage for this temperature can be addedto the measured voltage to find the temperature of the primary junction. All of the

connections and the measurement are made to a thermally conductive, but

electrical insulating material known as the isothermal block. This block is usually

in the instrument case, but in large installations is sometimes done elsewhere by

minimising the thermocouple wire. If the temperature is computed circuit it is

known as software compensation. If an electronic circuit is used to correct the

reading, it is known as hardware compensation or an electronic ice point.

The thermocouple is connected to the isothermal block by wire made from thesame metals as the thermocouple, called thermocouple extension wire. A

thermocouple extension wire is usually a shielded, twisted pair with the shield

grounded at the instrument to minimise interference pickup. Terminal strips

constructed of thermocouple material are available and should be used if

intermediate connections are required. Only a few mili-volts are produced by a

thermocouple, so careful attention to proper wiring and shielding is essential to

good measurement.


Figure I-6.4B,HardwarecompensatedThermocouple

Assembly.


19/48



Some thermocouple assemblies are manufactured so that the thermocouple makes

electrical contact with the sheath (called ground junction), and some are

manufactured where the thermocouple is electrically insulated from the sheath

(called ungrounded junction). A third option, is where the thermocouple extends

slightly beyond the sheath (called exposed junction). Exposed junction offer the

fastest response, but are not used in oil and gas processing because they are

subject to physical damage.


They would need to be installed without a thermowell to take advantage of this

faster response. Grounded junction offer faster response than ungrounded

junctions because the contact area which provides the electrical connection also

provides good thermal conduction. Also, grounding at thermocouple provides the

most nearly symmetrical circuit, which reduced interference picked up by the

wires to a minimum. Grounded thermocouples should be selected unless other

components of the circuit require that, the ground be at some other point, or the

process fluid and piping are not at ground potential.grounding any measurement

loop at more than one point will usually cause measurement errors because ofpotential difference in the grounding system.

These errors are more acute with low voltage signals such as generated by

thermocouples. These statements do not preclude grounding the extension wire

shield at the receiver, which is recommended.

The most common and least expensive thermocouple is iron versus constant an

(ISA type J). Type can be used for measurement from -320F (-195C) to1400F

Figure I-6.4C,Three Basic Typesof ThermocoupleAssembly


20/48



(760C), but is normally limited to 32 to 1000F (0-500C). Type J is usually

furnished when no specific type is specified.

Chromal versus alumal thermocouples (ISA type K) offer better corrosion

resistance. Type K can be used for-310F (-190C) to 2500F (1370C) but

usually limited to 32 to 2000F (0 to 1000C) Type K does not produce as muchout put as type

Copper versus constantan thermocouples (ISA type T), are usually used when

temperatures below zero are to be measured. While the usable range for type T,

310 to 750 F (- 190 to 400 C), is the same for the lower limit and less for the

higher limit than for types J and K, the recommended range is -290 to 700F (-180

to 370C). The materials used in type T behave more predictably at low

temperatures than those used for types J and K.

Chromel versus constant an thermocouples (ISA type E) provide the largest

voltage change per temperature change for standard thermocouples. An output of

40 millivolts at 1000F can be compared to 30 mv for type J and 22 mv for type

K. type E can be used for 320 to 1830F (-195 to 1000() and is recommended

for32 to 1600F (0 to 870F).


Some sources extend this range downward to -100F (-73 C), but type T is

generally considered a better choice for below freezing temperature. Type E has

more tendency to change characteristics with time than types J, K and T.

These four types of thermocouples comprise the base metal thermocouples.


21/48



Figure I-6.4D, Output versus temperature curves for the four types ofbase metal thermocouples. (Types J, K, T and E)

Other thermocouple types, called the noble metal types are available for

measurements where the base metal types are not suitable. They are made from

expensive metals such as platinum, rhodium, iridium and tungsten thus are moreexpensive. Also, they do not provide as much output as the base metal types.

These noble metal thermocouples are used in laboratories, for molten metals and

other applications, but are rarely used in production facilities.


Figure I-6.4E,TypicalInstallation ofThermocouple orRTD Element

into Thermowell


22/48



Thermocouple Accuracy Check:

Figure I-6.4F shows thermocouple accuracy check set-up. Every six month

thermocouple sensor must be checked against the standard conversion table.

Thermocouple has no repair or maintenance procedure.



23/48



Check procedure

1. Unscrew the thermocouple head cover and disconnect the electrical wires totransmitter.

2. Using V adjustable wrench to remove the thermocouple element from thethennowell.

3. Immerse the element in a regulated thermal bath with a reference temp.indicator.

4. Using accurate digital multivolt meter to record measured mv values across theleads wires of the thermocouple at different temp. values (5 points at least).

5. Compare the record data with the standard data table for the specifiedthermocouple type.


Resistance Temperature Detectors (RTDs)

The resistance of a conductor usually increase as the temperature increase. If the

properties of that conductor are known, the temperature can be calculated from the

measured resistance. A resistance temperature detector (RTD) is a conductor of

known characteristics constructed for insertion into the medium for temperature

measurement. Any conductor can be used to construct an RTD, but a few have

Figure I-6.4F,Typical BenchSet-up forThermocouple

Accuracy Check.


24/48



been identified as having more described characteristics than others. The

characteristics, which are desired, include.

1. Stability in the temperature range to be measured. The material must not melt,corrode, embrittle or change electrical characteristics when subjected to the

environment in which it will operate.2. Linearity. The resistance change with temperature should be as liner as

possible over the range of interest to simplify readout.

3. High resistively. Less material is needed to manufacture an RTD with aspecified resistance when the material has a high characteristic resistively.

4. Workability. The material must be suitable for configuring for insertion intothe media.

The materials which, have been identified as having acceptable characteristics are

copper, nickel, tungsten and platinum.

Copper has good linearity, workability, and is able up to 250F (120C), but has

low resistively, thus either a long conductor or one with a very small cross-

sectional area is required for a reasonable resistance. Nickel and nickel alloys have

high resistively, good stability and good workability, but have poor linearity.

Tungsten is brittle and difficult to work with. Platinum has been accepted as the

material which best fits all the criteria and has been generally accepted for

industrial measurement between -300 and 1200 F (-150 and 650C).

The effect of resistance inherent in the lead wires of the RTD circuit on the

temperature measurement can be minimised by increasing the resistance of the

sensor; however, the size of the sensor will also be increased. RTDS are

commercially available, with resistance from 50 to 1000 ohms at 32F (0C) and

increase resistance 0.385 ohms for every C of temperature raise.


Chemically pure platinum has a resistance of.392 ohms per C for a 100 ohm RTD

in accordance with the American (A) standard.

When the resistance of the RTD is found by measurement, the temperature can be

calculated:

C = (Ohms reading -100)/0.385

The accuracy of this calculation is determined primarily by the accuracy of the


25/48



reading. Modem instruments can measure resistance very accurately and the

temperature can be determined precisely if the resistance of the connecting circuit

is insignificant or is known. Unfortunately, this resistance usually not negligible or

known for most practical circuits. The wire that usually used (16 AWG stranded

copper) has a resistance of approximately 4 ohms per 100 feet (305 m). If it is

assumed that the RTD is connected to the instrument by a 625- foot cable as

shown in Figure, the total resistance will be 5 ohms larger than the RTDresistance, which will cause a 23.4 F (13C) error. Furthermore, copper wire has

a temperature coefficient of about 0.0039 ohms /C/ so the reading will vary about

a degree for every 20 change in ambient temperature. These errors can be

compensated by measuring the resistance of every loop and keeping track of the

ambient temperature. A compromise connection method for RTDS that uses three

wires and a balanced bridge circuit is shown above. For this circuit, Rl and R2 are

selected to be the same resistance so that the voltage at the negative terminal of

the voltmeter is one half o the supply voltage. R3 is selected to be the same

resistance as the RTD at the base temperature, 100 ohms if 0C is used as the base.

For this circuit, it is important that wire A and wire B have the same resistance.The usual practice is to run the three wires as a shielded raid, thus they will all be

the same length and the same resistance within manufacturing tolerance.


Figure I-6.4H, Three-wire RTD circuit with a balanced bridge

Figure I-6.4G,Two-wire RTD

Circuit


26/48



At the base condition, the positive terminal will also see one-half of the supply

voltage and the reading will be zero. If 5.2 volts is used to power the bridge, the

voltage will be 2.6 at each terminal of the voltmeter at the base temperature. When

the temperature of the RTD is raised one degree C, the voltage reading will

increase to one millivolt. The symmetry will be upset as the reading moves away

from the base temperature and the one millivolt per degree will not continue to beexact, but various schemes of completion are available to give an acceptable

reading.

The proceeding paragraphs are intended to explain the basis of two, three and four

wire RTD connections. The selection of resistors and compensation schemes are

left to the manufacturer of the instrument, but the facilities engineer selects which

of the connection methods to use. The three wire method is the proper selection

for virtually all production facility applications.

Resistance temperature detectors (RTDS) are the most frequently used electronictemperature sensors for production facilities.

The industry has standardised on RTDS that are calibrated to Din standard 43760

which is also known as the European standard RTDS which meet this standard

measure 100 ohms at 0C, are made of platinum and exhibit a resistance increase

of 0.385 ohms per C temperature increase. Another standard, called the American

Standard, is available but is not in wide use.


RTDS are usually purchased as a probe assembly consisting of the RTD sensor

installed in a type 304 stainless steel sheath. The sheath is held in the thermowell

by a fitting which is threaded on both ends for attachment to the thermowell and

the head so that the tip of the sheath touches or is very near the end of the well.

The preferred method of attachment of the sheath to the fitting is with a spring

assembly which allows the fitting to be screwed into the thermowell as the spring

is compressed. The spring holds the sheath firmly against the bottom of the well

for good heat transfer. Another method is to sliver solder the sheath into the fitting

which makes a good firm assembly, but requires a small clearance from the

bottom of the well. The third popular method is with a comparison fitting so that

the sheath can be pushed against the bottom of the well after the fitting is screwed

into the well. The compression nut is then tightened to hold the sheath. The

compression fitting

allows use of a universal probe in different lengths of thermowells.


27/48



The head of the assembly is a chamber where the leads from the RTD and the

leads to the receiver instrument can be terminated and connected to each other.

RTD Accuracy Check

Figure I-6.4I shows RTD accuracy check set-up. Every six month RTD sensormust be checked against the standard conversion table. RTD element has no repair

or maintenance procedure.

Accuracy Check Procedure:

1. Adjust the multimeter to OHM position.2. Measure the resistance value of the RTD at workshop temperature, suppose it

is 20 c.

3. From reference table find the RTD value (107.79 ohms) and check themultimeter reading to be the same value.

4. Prepare the temperature bath maintained at 100 c.TASK No.: I-6.4 Continue

5. Immerse the sensing part of the RTD in the bath, when the multimeter readingis stabilised, note the reading.

6. Check the value of RTD at 100 c. from the table and the multimeter reading tobe the same (138.5 ohms).

7. Increase the temperature of the temperature bath gradually to 200 c.8. When the multimeter reading is stabilised, note the reading.9. Check the value of the RTD at 200 c. from the table, and the multimeter

reading to be the same (175.84 ohms).


28/48




TASK No.: I-6.5

Perform servicing and calibration of a pneumatic

temperature. transmitter

Reference: OJT Instructor to arrange reference catalogue / Servicemanual for pneumatic temperature transmitter model

relevant to each working area.

Materials: 1. Cleaning rags, and2. Solvent.

Equipment & Tools: 1. Tool Box,2. Stirred heated control bath,3. Standard pneumatic calibrator, and

Figure I-6.4I, Typical Bench Set-upfor RTD Accuracy Check.


29/48



4. Standard output gauge.Conditions: Work permit.


To study the task and familiarise himself, Be able to describe the main parts of pneumatic temperature. transmitter, Understand the principle of operation of pneumatic transmitter, Describe wiring connections of pneumatic temperature transmitter, Describe the procedure for calibrating a pneumatic transmitter,

Perform periodic maintenance and troubleshooting of a pneumatic transmitter, Discuss an understanding to his trainer, and Write observations and procedures in his workbook.


Details:

Temperature Transmitters

Temperature transmitters are used when it is necessary to convert the signal from

a temperature sensor to one of the standard signals for transmission over a long

distance or interface with other instruments. The signal is usually 4 to 20 mA. for

electronic transmission and 3 to 15 psig for pneumatic if a transmitter is used.

Other signals can be used if required by the receiver, but these are the most

common and should be used if possible.

It is also possible to bring a temperature measurement into a control room without

using a transmitter, a thermocouple / RTD can be wired directly to an instrument

in the control room and this is acceptable practice. Figure I-6.5A illustratesFoxboro 12A series pneumatic temperature transmitter, which is a force-balance

instrument, that continuously measures temperature and transmits it as a

proportional 3 to 15 psi air pressure output signal.


30/48




Principle of operation

Figure I-6.5A, Pneumatic Temperature

Transmitter, Foxboro 12A Series

Figure I-6.5B, Pneumatic TemperatureTransmitter Principle of Operation


31/48



As illustrated in figure I-6.5B, any change in the sensor temperature of the gas-

filled thermal element causes a change in the gas pressure and, therefore, a change

in the force being applied to the bottom of the force bar. The force bar pivots

about a cross-flexure, and any motion of the force bar causes a change in the

clearance between the nozzle and the top of the force bar. This produces a changein the output pressure from the relay to the feedback bellows, until the force

exerted by the bellows balances the force exerted by the thermal system. The

output pressure, which establishes the force balance, is the transmitted pneumatic

signal and, therefore, is proportional to the measured temperature. The signal is

transmitted to a pneumatic receiver to record, indicate, and/or control.


Calibration

Figure I-6.5C illustrates bench calibration set-up of a pneumatic temperature

transmitter, follow the detailed calibration procedure listed in the reference

catalogue or service manual. Consult your trainer.

Maintenance and Servicing

Maintenance and servicing of pneumatic temperature transmitter are limited to

clean or replace its parts, such as:

Figure I-6.5C, Pneumatic TemperatureTransmitter Calibration Set-up


32/48



1. Supply Air Filter blow out at least once a day,2. Replace Screen Filter of the process inlet,3. Clean Nozzle Assembly,4. Clean booster relay Restrictor,5. Replace thermal element,6. Replace compensating bellows,TASK No.: I-6.5 Continue

7. Replace Booster Relay,8. Install or Adjust derivative unit,9. Change Range Bar, and10.Adjusting Flexure Cap Screw.Disassembly;

Normal servicing of the transmitter does not require the removal of any parts other

than those already mentioned. Further disassembly is not recommended because

of possible loss of accuracy or damage to the transmitter, detailed servicing

procedures are mentioned in maintenance section of the selected transmitter

model. Consult your trainer.


33/48




TASK No.: I-6.6

Perform servicing and calibration of an electronic

temperature transmitter with RTD input.

Reference: OJT Instructor to arrange reference catalogue / servicemanual for electronic temperature transmitter model

relevant to each working area.

Materials:

Equipment & Tools: 1. Tool Box,2. Standard dc power supply 24 Vdc at 35 mA,3. Resistance decade box, and4. Digital multimeter.

Conditions: Work permit.


To study the task and familiarise himself, Be able to describe the main parts of RTD input electronic temp. transmitter, Understand the principle of operation of RTD input transmitter, Describe wiring connections of RTD input transmitter, Describe the procedure for calibrating an RTD transmitter,

Perform periodic maintenance and troubleshooting of an RTD transmitter, Discuss an understanding to his trainer, and Write observations and procedures in his workbook.


34/48




Details:

RTD input Electronic Temperature Transmitter

Temperature transmitters for new installations are predominantly electronic with 4

to 20 mA. Outputs and inputs from thermocouples or RTDS. These transmitters

can be mounted in the field and on the thermowell or in the field on a support and

connected to the sensor by a cable.

Temperature transmitter mounted in the field must be protected from the elements

by an appropriate housing. A weatherproof (INEMA 4) housing is adequate for m

most applications, even in Division 2 hazardous area because there are no arching

contacts in a typical temperature transmitter. An explosion proof (NEMA 7)

housing is required for Division I area unless the installation is certifiedintrinsically safe. The energy level required in temperature transmitters is such

that they can be used in intrinsically safe installations if isolated from the power

supply and receiver by approved barriers.


Figure I-6.6A, Electronic Temperature Transmitter with RTD Input


35/48



Figure I-6.6A, illustrates the main parts of an RTD temperature transmitter, which

are:

RTD sensor and thermowell,

Transmitter electronic housing includes; Range board, Amplifier board and Output

board.

Figure I-6.6B, illustrates field-wiring connections of 3-wire RTD input

temperature transmitter. The dc power supply to be regulated at 24 Vdc grounded

at negative side terminal. The transmitter will operate with current signal loop 4-

20 mA proportional to the calibrated temperature range.

Theory of Operation

Figure I-6.6C, illustrates RTD input electronic temperature transmitter functional

block diagram The specific operation of the different functional blocks described

below:


RTD: Is a platinum temperature sensor, 100 Ohms at zero degree C.

Resistance Bridge: This bridge converts the resistance versus temperature

relationship of the sensor to a millivolt versus resistance signal.

Modulation, ac Amplification, Demodulation: The differential signal from the

bridge is converted to an ac signal. This signal is then amplified and converted

Figure I-6.6B, Electronic TemperatureTransmitter Typical Wiring Connection


36/48



back to a dc signal, and this to ensure that the signal will not be affected by

ambient temperature changes.

Dc Amplification, Current Control: The dc signal is further amplified to drive a

transistor that controls a current signal that proportional to sensor temperature.

Voltage Regulation: A voltage regulator circuit provides a stable voltage to

ensure the signal is independent of supply voltage and load resistance variations.

Calibration

The transmitter is calibrated at the specified range on the nameplate, figure I-6.6D

illustrates calibration set-up of RTD input electronic temperature transmitter. To

re-calibrate this transmitter, reference catalogue or service manual has the detailed

procedure for calibrating an RTD transmitter. Consult your trainer.


Figure I-6.6C, Electronic Temperature

Transmitter Functional Block Diagram


37/48



Maintenance

Repair:

In case of transmitters failure, the first step is to determine whether the fault lies

with the sensors or the transmitters electronics, to repair or replace the faulty

device.

RTD Test:A platinum RTD with an ice-point resistance of 100 Ohms should read

approximately as shown in reference R vs. T table. Consult your trainer.

Electronics Assembly:

The transmitter is designed for easy replacement of its plug-in, modular circuit

boards. A malfunction can be most easily isolated by substituting boards one at a

time until the unit functions properly.

Detailed procedures for disassembly and reassemble of transmitters electronics

are listed in the reference service manual. Consult your trainer.


TASK No.: I-6.7

What are self-regulating temperature valves? Function

& application

Reference:

Figure I-6.6D, Electronic Temperature

Transmitter Calibration Set-up


38/48



Materials: 1.Cleaning rags, and

2. Solvent.

Equipment & Tools: 1. Tool Box, and2. Digital Multimeter orstandard output gauge.


Requirements by Trainee:

To study the task and familiarise himself, Understand, zero value of a level transmitter (pneumatic / electronic), Demonstrate zero adjustment of a level transmitter, Be able to perform field zero check of a level transmitter, Discuss an understanding with his trainer, and Write observation in his workbook.


Details:


39/48




TASK No.: I-6.8

Service and calibrate a pneumatic temperature

controller

Reference: OJT Instructor to arrange reference catalogue / Servicemanual for pneumatic temperature indicating

controller model relevant to each working area.

Materials: 1. Cleaning Rags.Equipment & Tools: 1. Pneumatic calibrator,

2. Standard test gauges,3. Service/ Repair Kit, and4. Tool Box.


40/48



Conditions: Work permit

Requirements by Trainee:

To study the task and familiarise himself, Understand principle of operation of a pneumatic temperature controller, Describe the effect of the controllers modes (PID) on the output signal, Be able to perform periodic adjustments / calibration of a pneumatic indicating

controller,

To perform P.M, service, parts replacement of an indicating controller, Draw/ Sketch calibration set-up in his workbook, Discuss an understanding to his trainer, and Write observations and procedures in his workbook.


Details:


41/48



Temperature Controllers

A temperature controller is a device which senses temperature and manipulates an

end device to control that temperature. The sensor is one of temperature sensors

and the end device can be control valve to control any process variable or other

device.


Pneumatic temperature controller supplied by 20 psi regulated air pressure and the

output signal is 3 to 15 psi. Full featured, temperature controllers offer more

precise control than the simple type. The integral and differential control modes in

addition to the proportional mode will allow stable operation in fast processes

where simple controller would oscillate between no output and full output. Figure

Figure I-6.8A, PID Pneumatic TemperatureController Components Location


42/48



I-6.8A illustrates a full featured, pneumatic temperature controller, in which use

an external temperature sensor.

Principle of Operation

As shown in figure I-6.8B A PID pneumatic temperature controller. This

temperature controller operate on the motion balance principle; motion from a

pneumatic feedback unit balances the motion from a process measuring element.

When the temperature at the sensing element increases, the bourdon spring uncoils

and moves the process pointer to the right and the baffle-actuating pin to the left.

The movement of the pin lowers the baffle to decrease the nozzle-baffle gap and

increase the nozzle-back pressure. This pressure is fed to chamber A of the output

relay.

As the pressure in chamber A increases, the diaphragm assembly moves the relay

stem downward closing the vent port and opening the air supply port to increasethe output. The output increases until it balances the downward force on the

diaphragm assembly.

By proportional response, the output pressure is fed to the follow-up bellows and

raises the baffle-actuating pin and baffle and the output change is proportional to a

change in process measurement.

Reset response, automatically returns the process variable to the set-point after a

sustained load change. This is accomplished by opposing the action of the follow-

up bellows with a reset bellows.

Pre-act response, reduces the offset caused by a process disturbance as well as

reduce the recovery time following the disturbance. This is accomplished by

feeding the output pressure to the follow-up bellows through a needle valve.

The controller can be set for either direct or reverse action by positioning the gain

dial.



43/48




Calibration

Calibration adjustments of pneumatic temperature controller are limited to

perform the following:

Figure I-6.8B, Schematic Diagram of PID

Pneumatic Temperature Controller


44/48



1. Process pointer calibration; zero adjustment, span adjustment and linearityadjustment.

2. Pneumatic-set pointer calibration; zero adjustment and span adjustment.3.

Controller alignment; fixed high controller, differential-gap controller,

proportional controller, proportional plus reset controller, proportional plus

pre-act controller and proportional plus reset plus pre-act controller.

4.Nozzle-height adjustment.Detailed procedures to perform the above adjustments are mentioned in the

reference service manual of the relevant pneumatic temperature controller model.

Consult your trainer.

Periodic Servicing

If the air supply is clean and dry the instrument should be serviced once a year. If

the air supply is dirty or oily, more frequent servicing may be required. Servicing

the pneumatic controller is limited to clean / repair / replace its parts; such as:

1. Clean nozzle-tip and baffle-surface,2. Clean or replace the output relay,3. Clean the manual regulator,4. Clean reset and ore-act restrictors, and5. Replace internal O-Ring seal of auto-manual switch.Detailed procedures to perform the above services are mentioned in the reference

service manual of the relevant pneumatic temperature controller model. Consult

your trainer.


Troubleshooting:

Reference catalogue or service manual of the pneumatic temperature controller

model used has the details of possible causes ofcontrollers problems and actions

to be taken to overcome these problems. Consult your trainer.


45/48




TASK No.: I-6.9

Perform calibration of an electronic transmitter using

thermocouple input.

Reference: OJT Instructor to arrange reference catalogue / service

manual for Thermocouple input electronic temperaturetransmitter model relevant to each working area.

Materials: None

Equipment & Tools: 1. Tool Box,2. Standard dc power supply 24 Vdc at 35 mA,


46/48



3. A stirred ice bath,4. Thermocouple reference junction, and5. Digital multimeter.



To study the task and familiarise himself, Be able to describe the main parts of thermocouple input temp. transmitter, Understand the principle of operation of thermocouple input transmitter, Describe wiring connections of thermocouple input transmitter,

Describe the procedure for calibrating a thermocouple input transmitter, Perform periodic maintenance and troubleshooting of a thermocouple

transmitter,

Discuss an understanding to his trainer, and Write observations and procedures in his workbook.


Details:

Figure I-6.9A Elect. Temp. Transmitter withThermocouple Input Functional Block Diagram


47/48



Theory of Operation

Figure I-6.9A, illustrates thermocouple input electronic temperature transmitterfunctional block diagram The specific operation of the different functional blocks

described below:

Thermocouple: A thermocouple consists of two specific dissimilar metals joined

at the measurement site, the thermocouple produces a millivolt-level signal

proportional to process temperature.

Bridge: Cold-junction compensation for the thermocouple is provided by two

compensation resistors, the output from these resistors and the temperature-

sensitive simulates that of a thermocouple temperature.

Modulation, ac Amplification, Demodulation, dc Amplification, Current

Control: Identical to that of RTD input transmitters.


Isolation: Isolated power to run the unit, is provided by a dc-to-ac converter,

which feeds a transformer. On the secondary side of this transformer, diodes

rectify that ac to provide dc power to the circuitry.

Voltage Regulation: Regulates the voltage across the dc/ac converter, to protecttransmitters electronics against damage due to reverse power hook up.


48/48


Maintenance

Repair:

In case of transmitters failure, the first step is to determine whether the fault lies

with the sensors or the transmitters electronics, to repair or replace the faulty

device.


Thermocouple Test:

A thermocouple could be tested with a reference junction relevant THE

thermocouple type and should read approximately as shown in reference mV vs. T

table. Consult your trainer.

Electronics Assembly:

The transmitter is designed for easy replacement of its plug-in, modular circuit

boards. A malfunction can be most easily isolated by substituting boards one at a

time until the unit functions properly.

Detailed procedures for disassembly and reassemble of transmitters electronics

are listed in the reference service manual. Consult your trainer.

Figure I-6.9B, Temperature Transmitter

Thermocouple Input Calibration Set-up

Date post:	03-Apr-2018
Category:	Documents
Upload:	nabil160874
View:	219 times
Download:	0 times

Module I-6 Temperature Instruments

Documents