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Energy Systems Engineering Technology

Temperature Module Page 1

College of Technology

Instrumentation and Control

Module # 7 Temperature Measurement

Document Intent:

The intent of this document is to provide an example of how a subject matter expert might teach

Temperature Measurement. This approach is what Idaho State University College of

Technology is using to teach its Energy Systems Instrumentation and Control curriculum for

Temperature Measurement. The approach is based on a Systematic Approach to Training where

training is developed and delivered in a two step process. This document depicts the two step

approach with knowledge objectives being presented first followed by skill objectives. Step one

teaches essential knowledge objectives to prepare students for the application of that knowledge.

Step two is to let students apply what they have learned with actual hands on experiences in a

controlled laboratory setting.

Examples used are equivalent to equipment and resources available to instructional staff

members at Idaho State University College of Technology.

Temperature Measurement Introduction:

This module covers aspects of temperature measurement as used in process instrumentation and

control. Temperature measurement addresses essential knowledge and skill elements associated

with measuring temperature. Students will be taught the fundamentals of temperature

measurement using classroom instruction, demonstration, and laboratory exercises to

demonstrate knowledge and skill mastery of measuring temperature. Completion of this module

will allow students to demonstrate mastery of knowledge and skill objectives by completing a

series of tasks using calibration/test equipment, temperature indicating, and temperature

transmitting devices.



References

This document includes knowledge and skill sections with objectives, information, and examples

of how temperature measurement could be taught in a vocational or industry setting. This

document has been developed by Idaho State University’s College of Technology. Reference

material used includes information from:

1. American Technical Publication – Instrumentation, Fourth Edition, by Franklyn W. Kirk,

Thomas A Weedon, and Philip Kirk, ISBN 979-0-8269-3423-9, Chapter 2

2. Department of Energy Fundamentals Handbook, Instrumentation and Control, DOE-

HDBK-1013/1-92 JUNE 1992, Re-Distributed by http://www.tpub.com



STEP ONE

Temperature Measurement Course Knowledge Objectives

Knowledge Terminal Objective (KTO):

KTO 1. Given examples, EVALUATE temperature measurement fundamentals as they

apply to measuring temperature in process control variables to determine

advantages and disadvantages associated with different types of devices used to

indicate, measure, and transmit temperature.

Knowledge Enabling Objectives (KEO):

KEO 1.1. DEFINE Temperature and its importance as a process variable

KEO 1.2. DEFINE Heat and how it is measured in the United States

KEO 1.3. DEFINE Specific Heat as it applies to thermal energy

KEO 1.4. DEFINE Energy as it applies to temperature

KEO 1.5. List Six important elements of Temperature, Heat, and Energy

KEO 1.6. DEFINE Absolute Zero Temperature

KEO 1.7. DESCRIBE Four commonly used temperature scales, compare their ranges,

applications, and where these scales are used

a. Fahrenheit ( 0F )

b. Rankine ( 0R )

c. Celsius ( 0C )

d. Kelvin ( 0K )

KEO 1.8. CONVERT Temperature readings between Fahrenheit, Rankine, Celsius, and

Kelvin temperature scales



KEO 1.9. EXPLAIN the need for Reference Temperatures as applicable to industrial

processes and why boiling and freezing temperatures are inadequate to define a

temperature scale

KEO 1.10. DESCRIBE Heat Transfer as it applies to Thermal Equilibrium

KEO 1.11. DESCRIBE Heat Conduction

KEO 1.12. DESCRIBE Heat Convection

KEO 1.13. DESCRIBE Heat Radiation

KEO 1.14. DESCRIBE Heat Capacity

KEO 1.15. DESCRIBE Temperature Response Time

KEO 1.16. EXPLAIN The Principle of Differential Thermal Expansion

KEO 1.17. DESCRIBE How Thermal Expansion Thermometers work

KEO 1.18. EXPLAIN How Bimetallic Thermometers work

KEO 1.19. EXPLAIN Pressure-Spring Thermometers work

KEO 1.20. DESCRIBE Temperature Bulb Location considerations Vapor – Pressure Bulbs

KEO 1.21. DESCRIBE The Response Time considerations for Pressure-Spring

Thermometers

KEO 1.22. DESCRIBE What an Electrical Thermometer is

KEO 1.23. DESCRIBE What a Thermocouple is and how it is used

KEO 1.24. DESCRIBE The Seebeck Effect as it pertains to a Thermocouple:

KEO 1.25. STATE The Law of Intermediate Temperatures

KEO 1.26. STATE The Law of Intermediate Metals



KEO 1.27. LIST The Standard Color Code, Wire Type, Polarity, Maximum Temperature

Range, and uses for the following types of Thermocouples in the United States &

Canada:

a. “J”

b. “K”

c. “T”

d. “E”

e. “N”

f. “R”

g. “S”

h. “B”

KEO 1.28. DESCRIBE A brief description of the following type of Thermocouple

Measurement Circuits:

a. Difference Thermocouples

b. Thermopiles

c. Averaging Thermocouples

d. Pyrometers

KEO 1.29. DESCRIBE What a Resistance Temperature Detector is and how it is used

KEO 1.30. DESCRIBE How the Wheatstone Bridge Circuit us used to measure the

resistance change of an RTD

KEO 1.31. DESCRIBE A basic overview of a Thermistor and its application

KEO 1.32. DESCRIBE How a Thermistor can be used as a temperature switch

KEO 1.33. DESCRIBE The principle of operation of a Semiconductor Thermometer

KEO 1.34. COMPARE advantages and disadvantages of Thermocouples, Resistance

Temperature Detectors, Thermistors, and Integrated Circuit Sensors.

KEO 1.35. DESCRIBE The principle of operation of an Infrared Radiation Thermometer



KEO 1.36. DESCRIBE Calibration Considerations for Temperature Measuring Instruments

using the following:

a. Dry Well and Mircobath Calibrators

b. Blackbody Calibrators

c. Electronic Calibrators

DOE FUNDAMENTALS OBJECTIVES

KEO 1.37. STATE three basic functions of temperature detectors

KEO 1.38. DESCRIBE the two alternate methods of determining temperature when the

normal temperature detection sensing devices are inoperable

KEO 1.39. STATE two environmental concerns which can affect the accuracy and reliability

of temperature detection instrumentation

KEO 1.40. Given a simplified schematic diagram of a basic bridge circuit, STATE the

purpose of the following components:

a. R1 and R2

b. Rx

c. Adjustable Resistor

d. Sensitive Ammeter

KEO 1.41. DESCRIBE the bridge circuit conditions that create a balanced bridge

KEO 1.42. Given a block diagram of a basic temperature instrument detection and control

system, STATE the purpose of the following blocks:

a. RTD

b. Bridge Circuit

c. DC-AC Converter

d. Amplifier

e. Balanced Motor/Mechanical Linkage



KEO 1.43. DESCRIBE the temperature instrument indication(s) for the following RTD

circuit faults:

a. Short Circuit

b. Open Circuit

KEO 1.44. EXPLAIN the three methods of bridge circuit compensation for changes in

ambient temperature



TEMPERATURE MEASURMENT

KEO 1.1. DEFINE Temperature and its importance as a process variable

The word Temperature indicates the hotness or coldness of a body with reference to some

standard value. The measurement of temperature is probably the most widely measured and

controlled industrial variable. Temperature of a substance is simply a number that tells you how

hot or cold a substance is.

If two bodies are placed in contact with each other, the one that has the higher temperature will

transfer heat to the other. To have meaning, temperature must be measured on a definite scale.

Hotness or coldness is expressed in the units (degrees) of that specific scale.

There are changes in the physical or chemical state of most substances when they are heated or

cooled. This is why temperature is one of the most important of the measured variables

encountered in the industrial environment and the most often measured of all process variables.

Many temperature measurements are involved in heat transfer, boiler operation, Heating

Ventilation Air Condition (HVAC) systems, welding and a host of many other industrial

processes.

KEO 1.2. DEFINE Heat and how it is measured in the United States

HEAT is energy that flows to a body, causing it to increase in temperature, melt, boil, expand, or

undergo other changes.

When heat flows to a body, the body’s thermal energy increases; this together with pressure

determines the body’s temperature and physical state. For example, assume that heat is added to

water in a container at atmospheric pressure, increasing the water’s thermal energy. The

temperature of the water increases until it boils and evaporates to steam, thus changing its state.

If the thermal energy of the same liquid decreases, the temperature drops. If enough thermal

energy is removed, the water changes state again, becoming a solid (ice).

The traditional unit for measuring heat in the United States is the British Thermal Unit (BTU). A

BTU is the amount of thermal energy required to raise the temperature of 1 pound of water 1

degree Fahrenheit. The metric unit for measuring heat is the joule (J).



Adding a fixed quantity of heat raises the temperature of a body or material by a fixed amount as

long as there is no change in the state. In general, you can say that adding heat to a body

increases its thermal energy, resulting in a raise in its temperature.

Temperature change is not only the indication of a change in thermal energy. The nature of the

body or material is also important. Different substances (water and aluminum, for example)

require different quantities of heat to change temperature. A pound of water requires 1 BTU to

change 1 degree Fahrenheit in temperature. A pound of aluminum requires only 0.22 BTU to

change 1 degree Fahrenheit in temperature. Although the temperature of the water and aluminum

change by the same amount, the thermal energy required for each change differs.

SUMMARY

Temperature indicates the hotness or coldness of a body with reference to a standard

value.

Temperature measurement is important to many process variables measured

throughout industry because of changes in the physical or chemical state of most

substances when they are heated or cooled.

Heat is energy that flows to a body, causing it to increase in temperature, melt, boil,

expand, or undergo other changes.

The traditional unit of measuring heat in the United states is the British Thermal Unit

(BTU).

KEO 1.3. DEFINE Specific Heat as it applies to thermal energy

SPECIFIC HEAT is defined as the ratio of heat required to raise the temperature of a certain

weight of substance 1 degree Fahrenheit (measured under constant pressure). The specific heat of

aluminum is approximately 0.22 as previously mentioned. Every substance has a specific heat

that differs for that of other substances. Thus, at the same temperature, different substances

contain different amounts of thermal energy.

KEO 1.4. DEFINE Energy as it applies to temperature

Temperature is the degree of intensity of heat measured on a definite scale. Temperature is an

indirect measurement of the heat energy contained in molecules. When molecules have a low

level of energy they are cold, and as energy increases they get warmer. This energy is in the

form of molecular movement or vibration of the molecules.



KEO 1.5. List Six important elements of Temperature, Heat, and Energy

a. Temperature Scales

b. Reference Temperatures

c. Heat Transfer

d. Heat Capacity

e. Response Time

f. Thermal Expansion

KEO 1.6. DEFINE Absolute Zero Temperature

ABSOLUTE ZERO is the lowest temperature possible, where there is no molecular movement

and the energy is at a minimum. This condition is the zero point for the absolute temperature

scales.

SUMMARY

Every substance has a specific heat and different substances contain different

amounts of thermal energy.

Energy is in the form of molecular movement of vibration of molecules.

SPECIFIC HEAT is defined as the ratio of heat required to raise the temperature of

a certain weight of substance 1 degree Fahrenheit (measured under constant pressure).

ABSOLUTE ZERO is the lowest temperature possible, where there is no molecular

movement and the energy is at a minimum.

KEO 1.7. DESCRIBE Four commonly used temperature scales, compare their ranges,

applications, and where these scales are used

a. Fahrenheit ( 0F )

b. Rankine ( 0R )

c. Celsius ( 0C )

d. Kelvin ( 0K )

KEO1.7.a The Fahrenheit ( 0F ) temperature scale is the most common temperature scale in

the United States. On the Fahrenheit scale, the freezing point of water is 32 0F.

The boiling point of water is 212 0F at standard atmospheric pressure, and there

are 180 degrees between the fixed points.



KEO1.7.b The Rankine ( 0R ) temperature scale is the absolute equivalent of the Fahrenheit

temperature scale. The Rankine scale has its zero point at the absolute zero, the

scale divisions are the same as the Fahrenheit scale, and the scales are offset by

459.670.

To convert between Fahrenheit and Rankine temperatures add or subtract 459.67

degrees as follows:

0F =

0R + 459.67

0R =

0F - 459.67

The difference between 671.67 abd 491.67 on the Rankine scale is 180 degrees,

the same difference between 212 and 32 on the Fahrenheit scale.

KEO1.7.c The Celsius ( 0C ) temperature scale is another temperature scale that is

sometimes used in the United States, but is primarily used in other countries. The

Celsius scale, formerly known as the Centigrade scale, is universally used for

scientific measurements.

On the Celsius scale, the freezing point of water is 00C, the boiling point of water

is 1000C, at standard atmospheric pressure, and there are 100 degrees between the

fixed points.

EO1.7.d The Kelvin (K) temperature scale is the absolute equivalent of the Celsius scale.

The Kelvin scale has its zero point at absolute zero, the scale divisions are the

same as the Celsius scale, and the scales are offset by 273.15 degrees.

When using the Kelvin scale, the word degree and the degree symbol ( 0

) are not

used. To convert between Celsius and Kelvin temperatures add or subtract 273.15

degrees as follows:

K = 0C + 273.15

0C = K - 273.15



The following depicts the four different temperature scales used in different applications, but

measurements can be converted from one scale to another:

Figure 2-1 page 27

SUMMARY

Fahrenheit temperature scale is the most common temperature scale used in the

United States.

Rankin temperature scale is the absolute equivalent of the Fahrenheit temperature

scale

Celsius temperature scale is also used in the United States, but is primarily used

universally for scientific measurements

Kelvin temperature scale in the absolute equivalent of the Celsius scale



KEO 1.8. CONVERT Temperature readings between Fahrenheit, Rankine, Celsius, and

Kelvin temperature scales

The conversions for temperature scales are depicted below:

Figure 2-2 page 28

KEO 1.9. EXPLAIN the need for Reference Temperatures as applicable to industrial

processes and why boiling and freezing temperatures are inadequate to define a

temperature scale.

For industrial processes that may involve much lower or higher temperatures, the freezing and

boiling temperatures of water are inadequate to define a temperature scale. For example,

processes like cryogenic gases or molten metal’s, require many more known fixed points to

define a temperature scale.



The International Temperature Scale of 1990 (ITS-90), adopted by the International Committee

of Weights and Measures in 1989, uses 17 points to define the Kelvin temperature scale and it

uses the Celsius temperature as depicted below:

Figure 2-3 page 29

The international Temperature Scale of 1990 establishes standards for different temperatures

based on physical properties of pure materials.

The IST-90 also defines the Kelvin as 1

/ 237.16

of the thermodynamic temperature of the triple

point of water. The triple point is the condition where all three phases of a substance: Gas,

Liquid, and Solid can coexist in equilibrium. The triple point of water occurs at 32.018 0F and

273.16 K @ 0.08865 psi (611.657 Pascal’s) pressure.



The Triple Point of a substance is depicted below:

Figure 2-4 page 29

Two important early ideas about heat were the suggestion that heat is conserved and the

distinction between the quantity and quality of heat. Quality of heat is now called temperature

and the study of temperature s called thermometry. The study of the quantity of heat is called

calorimetry.

SUMMARY

For industrial processes that may involve much lower or higher temperatures, the

freezing and boiling temperatures of water are inadequate to define a temperature

scale.

The International Temperature Scale of 1990 (ITS-90), adopted by the International

Committee of Weights and Measures in 1989, uses 17 points to define the Kelvin

temperature scale and it uses the Celsius temperature scale.

The international Temperature Scale of 1990 establishes standards for different

temperatures based on physical properties of pure materials.

The triple point is the condition where all three phases of a substance: Gas, Liquid,

and Solid can coexist in equilibrium.



KEO 1.10. DESCRIBE Heat Transfer as it applies to Thermal Equilibrium

Heat Transfer is the movement of thermal energy from one place to another. When objects are

at the same temperature, they are in thermal equilibrium.

Thermal Equilibrium is the state where objects are at the same temperature and there is no heat

transfer between them. When two substances are at different temperatures, there is heat transfer

from the one with the higher temperature to the substance with the lower temperature until both

are in thermal equilibrium. Heat transfer occurs by: Conduction, Convection, and Radiation as

depicted below:

Figure 2-5 page 30

In the example above, heat transfer occurs through conduction, convection, and radiation in the

operation of a boiler.

SUMMARY



SUMMARY

Heat Transfer is the movement of thermal energy from one place to another

Thermal Equilibrium is the state where objects are at the same temperature and

there is no heat transfer between them.

Heat transfer occurs by: Conduction, Convection, and Radiation.

KEO 1.11. DESCRIBE Heat Conduction

Heat Conduction is heat transfer that occurs when molecules in a material are heated and the

heat is passed from molecule to molecule though the material. For example, conduction occurs

when one end of a metal rod is heated in a flame or when metals are welded. The molecules are

heated and move faster.

The faster moving molecules transfer energy though collisions from molecule to molecule across

the metal until they reach the opposite end of the work-piece. Heat is then transferred through

conduction; there is no flow of material.

Conduction also occurs between two different metals that are in direct contact. This process of

heat transfer is the same, but the rate of heat transfer differs depending on the substances.

Gases and liquids are generally poor heat conductors. Iron is a much greater heat conductor than

water. For example, heat is transferred from a boiler’s heating surface to the boiler water to

produce steam. Heat is then conducted from the hot combustion gases through the metal wall to

the water to produce steam.

The hot side of the metal wall next to the fire is nearly the temperature of the fire generating the

heat. The cool side of the metal next to the water is nearly the temperature of the water.



Another example of conduction is a heat sink as depicted below:

Figure 2-6 page 31

A Heat Sink is a heat conductor used to remove heat from sensitive electronic parts. As

depicted above, a transistor conducts current during its normal operation. A certain amount of

power is lost in the form of heat. That heat must be removed from the transistor to prevent heat-

related failure.

The heat is conducted away through a metal bracket or chassis to radiator fins. From the fins,

heat is transferred away by convection or radiation as will be discussed later in module.

The use of heat sinks is very important to both electrical and electronic equipment. For example

computers use heat sinks and fans to keep the processor and other critical components from

becoming damaged by excessive heat conditions. Electrical Transformers also use heat sinks to

dissipate heat.

KEO 1.12. DESCRIBE Heat Convection

Heat Convection is heat transfer by the movement of gas or liquid from one place to another

caused by a pressure difference. Natural Convection is the unaided movement of a gas or liquid

caused by a pressure difference due to a difference in density within the gas or liquid. Heat is

transferred by currents that circulate between warm and cool regions in a fluid.



For example: A flame heats the boiler’s heating surface. The hot boiler surface heats the water.

The water is heated so quickly that conduction within the water cannot transfer the heat away

fast enough.

The hot water is less dense and begins to rise to the surface and is replaced by cooler, denser

water with moves to the bottom of the boiler near the heat source.

Another type of heat convection is Forced Convection. Forced convection is the movement of a

gas or liquid due to a pressure difference caused by the mechanical action of a fan or pump. An

example of this is in our automobiles heating and cooling system. Liquid is heated by the

engine’s internal combustion and this heat is and fans and pumps allow that heat to be dissipated

or used to keep us warm in cold climates and to keep the engine from overheating.

In Heating Air Conditioning and Ventilation (HVAC) systems for example, warm air in a forced

air heating system travels through the ducts because of the pressure difference created by the

blower fan. Once the air is in a room and mixed, natural sources of heating nd cooling cause air

in parts of the room to cool down or warm up.

This HVAC example is depicted below as the system used forced convection to transfer heat

throughout a building space:

Figure 2-7 page 31



KEO 1.13. DESCRIBE Heat Radiation

Heat Radiation is heat transfer by electromagnetic waves emitted by a higher temperature

object and absorbed by a lower-temperature object.

All objects emit radiant energy. The amount of emitted energy depends of the temperature and

nature of the surface of the object. Radiant energy waves move through air or space without

producing heat.,

Heat is only produced with the radiant energy waves contact an object that absorbs the energy

waves. The energy is transferred to the surface molecules of the object, which are warmed by the

energy the energy from the electromagnet waves. When heat is transferred by radiation, there is

no flow of material.

For example: When a metal is heated to a glowing red, a person that is standing a distance away

from the metal can feel the radiant energy. When a boiler furnace door is opened, the heat can

immediately be felt even though the air temperature between the fire and the person does not

change very much. In both examples, electromagnetic waves emitted by the hot object travel

through the air and warm the person.

One more example is solar heating where radiation from the sun is used to heat water. The

heated water is them moved by convection to where it is needed as depicted below:

Figure 2-8 page 32



KEO 1.14. DESCRIBE Heat Capacity

Heat Capacity of a material is the amount of energy needed to change the temperature of the

material by a certain amount. Heat energy is commonly measured in units of BTU and the

Calorie.

The following depicts Heat Energy Definitions for both units:

Figure 2-9 page 32



There are specific heats of substances. Specific heat has no unit of measurement since it is a

ratio. The following table depicts specific heats of common substances and how they vary

considerably:

Figure 2-10 page 33



The amount of energy to change the temperature of a substance is expressed by the following

formula:

It takes 162 BTU to increase the temperature of 1 lb of water from 500F to 212

0F.

In other words, it takes 162 BTU/lb to increase the temperature of any amount of water from

500F to 212

0F.



SUMMARY

Heat Conduction is heat transfer that occurs when molecules in a material are

heated and the heat is passed from molecule to molecule though the material.

Heat Convection is heat transfer by the movement of gas or liquid from one place

to another caused by a pressure difference.

Heat Radiation is heat transfer by electromagnetic waves emitted by a higher

temperature object and absorbed by a lower-temperature object.

Heat Capacity of a material is the amount of energy needed to change the

temperature of the material by a certain amount.

KEO 1.15. DESCRIBE Temperature Response Time

Temperature Response Time is the time it takes any temperature-measuring instrument to

respond to changes of temperature. A response time is the time required for an instrument to

reach 63.2 % of its final value.

The following depicts how the size of a temperature sensor determines its response time (the

example exhibits the size of the wires used in a thermocouple to transmit the temperature):

Figure 2-11 page 34

Notice the smaller the wire size, the quicker it takes to transmit the temperature by the

Thermometer Response Time chart above.



KEO 1.16. EXPLAIN The Principle of Differential Thermal Expansion

The Principle of Differential Thermal Expansion is the basis of operation for some

thermometers. When one material has a greater coefficient of thermal expansion than another

material, the difference is expansion can be used as a measure of temperature by a direct reading

or by connection to a mechanical linkage.

This same differential expansion can produce a force that actuates devices in direct relation to

the temperature like an alarm or a switching device for temperature control. Common thermal

expansion instruments are liquid-in-glass, bimetallic, and pressure-spring thermometers.

KEO 1.17. DESCRIBE How Thermal Expansion Thermometers work

An example of a Thermal Expansion Thermometer is a Liquid-In-Glass Thermometer. They

consist of a sealed, narrow-bore glass tube with a bulb at the bottom filled with a liquid.

Depicted below are examples of the simplest liquid-in-glass common thermometers:

Figure 2-13 page 35

The volumetric expansion of liquids is typically many times greater than glass. Since the volume

of the liquid changes more than the change in glass, the liquid moves up or down in the tube with

changes in temperature. Liquid in class thermometers can be typically used to measure

temperatures from a -112 0F to 760

0F.



As depicted above, the liquid used in thermometers is usually alcohol or some other organic

liquid with a red dye added to improve visibility. Mercury has been commonly used in the past,

but the use of mercury is being discouraged because of the risk associated with a mercury spill as

mercury is a hazardous material with strict handling federal, state, and local environmental

statutes and regulations.

The most common type of liquid-in-glass thermometer is the industrial thermometer. Both the

tube and glass are enclosed in a metal case with the lower portion of the glass tube extending out

the bottom of the case into a metal bulb chamber.

The chamber contains a liquid with excellent heat transfer characteristics. These thermometers

provide an indication as to the temperature by a raising liquid in a tube or by the liquid raising

attached to linkage that provides an indication on a circular scale as depicted below:

Figure 2-14 page 36

Industrial thermometers are fitted with an external pipe thread that enables them to be screwed

into a pipe or a thermowell for isolation from direct contact to the process being measured and

for and ease of replacement.



SUMMARY

Temperature Response Time is the time it takes any temperature-measuring instrument

to respond to changes of temperature.

The Principle of Differential Thermal Expansion is the basis of operation for many

thermometers devices.

Differential Expansion can produce a force that actuates devices in direct relation to the

temperature like an alarm or a switching device for temperature control.

Common thermal expansion instruments are liquid-in-glass, bimetallic, and pressure-

spring thermometers.

Thermal Expansion Thermometers use liquid-in-glass, bimetallic, and pressure spring

devices that expand or move when temperature is applied.

Mercury has been commonly used in the past for thermal expansion devices, but the use

of mercury is now limited due to the risk associated with a mercury spill, as mercury is a

hazardous material with strict handling federal, state, and local environmental statutes

and regulations.

KEO 1.18. EXPLAIN How Bimetallic Thermometers work

A Bimetallic Thermometer is thermal expansion thermometer that uses a strip consisting of two

alloys with different temperature coefficients of thermal expansion that are fused together and

formed into a single strip, and a pointer or indicating mechanism calibrated for temperature

reading.

Because of the different temperature coefficients of thermal expansion, this means that when

heat is applied the two alloys react and causes a movement because one alloy will be more

responsive to the temperature change than the other, this strip actually bends in a reaction to the

heat being applied. If one alloy were heated, it would bend quickly in response to the heat

applied; however when the alloy is bound to another alloy, it creates a more even and easier

movement for this heat transfer to be measured. This controlled movement is then transmitted

via linkage to provide a calibrated accurate temperature indication that can be used to indicate

and control the temperature being measured.



Bimetalic Elements are usually constructed and wound into a spiral, helix or coil o allow the

element to be placed into a smaller space than a straight element requires as depicted below:

Figure 2-15 page 37

The figure above depicts how bimetallic elements function when heat is applied through thermal

expansion that can be used to indicate, and actuate switches to activate temperature control

circuits.



The following pictures show different bimetallic devices and principles of operation:

SUMMARY

A Bimetallic Thermometer is thermal expansion thermometer that uses a strip

consisting of two alloys with different temperature coefficients of thermal expansion that

are fused together and formed into a single strip, and a pointer or indicating mechanism

calibrated for temperature reading.

Bimetallic Elements are usually constructed and wound into a spiral, helix or coil o

allow the element to be placed into a smaller space than a straight element would require.

Not only is the movement of bimetallic devices used to provide a temperature indication

for local and remote applications, the movement can also be used to open and close

switches for alarm and control purposes.



KEO 1.19. EXPLAIN Pressure-Spring Thermometers work

Pressure - Spring Thermometers are thermal expansion devices consisting of a filled, hollow

spring attached to a capillary tube and bulb where the fluid in the bulb expands or contracts with

the temperature and this movement is detected by linkage.

The spring can be in the C shape of the original Bourdon tube but is often in the form of a spiral

or helix as depicted below:

Figure 2-16 page 39

Pressure – Spring thermometers can be filled with Gas, Liquid, or Vapor – Pressure.

Gas – Filled pressure – spring thermometers measure the increase in pressure of a confined gas

due to a temperature increase. Nitrogen is the gas most often used for such systems and except

for size of the bulb is identical to the liquid – filled types.



Liquid – Filled is filled with a liquid under pressure as depicted below:

Figure 2-17 page 40

Liquid – Filled pressure – spring thermometers uses the thermal expansion of a liquid to

pressurize a Bourdon Tube calibrated in temperature units as indicated above.



Vapor – Pressure pressure – spring thermometers is a pressure uses a change in pressure due to

temperature changes of an organic solution as depicted below:

Figure 2-18 page 41

The Vapor – Pressure pressure – spring thermometer uses the vapor pressure on the liquid in the

bulb to pressurize a Bourdon tube as indicated above.

KEO 1.20. DESCRIBE Temperature Bulb Location considerations Vapor – Pressure Bulbs

The difference in height between the bulb and the pressure spring can introduce error, especially

with Vapor – Pressure pressure – spring thermometers. Since the system is not filled under

pressure, as liquid and gas filled systems, any column of fluid can create a pressure that causes

an erroneous reading. These types need to be installed per manufactures specifications. The



picture below depicts to installation of a Pressure – Spring Bulb when bulb is mounted above the

pressure spring:

Figure 2-20 page 43

When a pressure – spring bulb is mounted above the pressure spring as indicated above, the

instrument must be calibrated to account for the hydrostatic pressure of the liquid in the line

above the pressure spring location.

Vapor – Pressure thermometers provide an accurate measurement and can generate a greater

amount of power to make it easier to operate switch mechanisms and are frequently used for

driving temperature switches.

KEO 1.21. DESCRIBE The Response Time considerations for Pressure-Spring

Thermometers

Response Time is an important consideration in the time it takes to respond to temperature

changes. There are different response times associated with the type of pressure – spring selected

and its location.

Bulbs must be installed so it senses only the temperature of the process or material into which it

is in contact with. It should be shielded from reflected or radiant heat.

At no point should the bulb be in contact with cold metal as this will lower the temperature

reading.

Of the various types of pressure – spring thermometers, the Gas – Filled have the fastest

response time, followed by the Vapor – Pressure and Liquid Filled pressure – spring

thermometers.



Important Note: The response of all systems is faster if the substance whose temperature is to

be measured is a liquid rather than a gas.

PRESSURE SPRING (THERMOMETER) SUMMARY

Pressure - Spring Thermometers are thermal expansion devices consisting of a filled,

hollow spring attached to a capillary tube and bulb where the fluid in the bulb expands or

contracts with the temperature and this movement is detected by linkage.

Spring thermometers can be filled with gas, liquid, or vapor.

Gas – Filled pressure – spring thermometers measure the increase in pressure of a

confined gas due to a temperature increase.

Liquid – Filled pressure – spring thermometers use the thermal expansion of a liquid

to pressurize a device.

Vapor – Pressure – spring thermometer uses the vapor pressure on the liquid in the

bulb to pressurize a device

The difference in height between the bulb and the pressure spring can introduce error

with Vapor – Pressure pressure – spring thermometers and should be installed according

to manufacture specifications to compensate for this difference.

KEO 1.22. DESCRIBE what an Electrical Thermometer is

An ELECTRICAL THERMOMETER is a device having electrical characteristics that change

when heated or cooled. Certain metals when heated or cooled actually change their electrical

characteristics. When they are used as part of an electrical circuit, a change in temperature can

close a switch to start a motor, or cause a solenoid valve to open or close, or the electrical signal

may be converted into a digital signal to be used by a microprocessor.

A common electrical thermometer is a thermostat used in homes to control the temperature.

When air temperature is too low, the heating system is turned on until the air temperature reaches

a preset value. Common electrical thermometers include: Thermocouples, Resistance

Temperature Detectors, Thermisters, and Semiconductor Thermometers.

KEO 1.23. DESCRIBE what a Thermocouple is and how it is used

A THERMOCOUPLE is an electrical thermometer consisting of two dissimilar metals joined

together at one end and a voltmeter connected to the other end to measure voltage. The voltage

generated is in measured in mV (Millivolts 1mV = .001 Volt).



The following picture depicts a thermocouple and how it is configured to measure a change in

temperature:

Figure 2-21 page 43

A thermocouple junction is the point where the two dissimilar wires are joined. The figure

depicts Copper and Constantan as the two dissimilar metals that are joined.

The hot junction is also called the measuring junction and is the joined end of the thermocouple

that is exposed to the process where the temperature measurement is desired.

The cold junction is also called the reference junction and is the end of the thermocouple leads

that is kept at a constant temperature in order to provide a reference point.

When the temperature changes at the hot junction, a measurable voltage (mV) is generated

across the cold junction as shown above at the voltmeter.

KEO 1.24. DESCRIBE The Seebeck Effect as it pertains to a Thermocouple:

The Seebeck Effect is a thermoelectric effect where continuous current is generated in a circuit

where the junctions of two dissimilar conductive materials are kept at different temperatures.



The Seebeck Effect is depicted in the picture below:

Figure 2-22 page 44

When the circuit is opened (as shown above) at the cold junction, an electrical potential (the

Seeback Voltage) exists at that cold junction. The voltage produced by exposing the measuring

junction (hot junction) to heat is dependent on the composition of the two wires and the

temperature difference between the hot and cold junction.

Important Note: The Seebeck Effect goes away when a thermocouple opens either at the hot

junction or in the lead wires. When this happens the instrument detecting the temperature fails to

a zero reading. There are temperature instruments designed to react to this loss of signal by

providing an alarm or causing a fail open or closed event to the process until the problem can be

corrected.



SUMMARY

A THERMOCOUPLE is an electrical thermometer consisting of two dissimilar metals

joined together at one end and a voltmeter connected to the other end to measure voltage.

A Thermocouple Junction is the point where the two dissimilar wires are joined.

The Hot Junction is also called the Measuring Junction and is the joined end of the

thermocouple that is exposed to the process where the temperature measurement is

desired.

The Cold Junction is also called the Reference Junction and is the end of the

thermocouple leads that is kept at a constant temperature in order to provide a reference

point.

The Seebeck Effect is a thermoelectric effect where continuous current is generated in a

circuit where the junctions of two dissimilar conductive materials are kept at different

temperatures.

Seebeck Voltage is the voltage generated across the leads of a thermocouple

If a Thermocouple Opens, the temperature reading will fail to a reading equivalent to

the reference junction and will lose its Seebeck Effect and the temperature measuring

instrument will fail to a loss of its signal.

KEO 1.25. STATE The Law of Intermediate Temperatures

The Law of Intermediate Temperatures states that in a thermocouple circuit, if a voltage is

developed between two temperatures T1 and T2, and another voltage is developed between T2 and

T3, the thermocouple circuit generates a voltage that is the sum of those two voltages when

operating between temperatures T1 and T3.

To summarize this law, the law states:

The temperature at the end of the wires determines the electrical potential

regardless of the intermediate temperatures.



The picture below depicts the Law of Intermediate Temperature:

Figure 2-25 page 46

This law says it is possible to use a reference junction with any fixed temperature T2 that is lower

than T3. This is the basis of cold junction temperature compensation in thermocouples. A

temperature sensitive resistor, or thermistor, is used to measure the reference temperature and an

adjustment is made to the measured voltage to determine the temperature at the measured

junction.



KEO 1.26. STATE The Law of Intermediate Metals

The Law of Intermediate Metals states that the use of a third metal in a thermocouple circuit

does not affect the voltage, as long as the temperature of the three metals at the point of junction

is the same.

The following picture illustrates this Law of Intermediate Metals:

Figure 2-26 page 47

To summarize this law, the law states:

Other metals may be used in a thermocouple circuit as long as the junctions are at the same

temperature.

Therefore, metals different from the thermocouple materials can be used as extension wires in

the circuit. This is common practice in industry.



Modern devices can measure a direct digital readout of a temperature that converts mV to

degrees with built in compensation to accommodate the reference junction. This device will

provides a direct digital readout of the temperature as depicted below:

Figure 2-30 page 50

SUMMARY

Law of Intermediate Temperatures states the temperature at the end of the wires

determines the electrical potential regardless of the intermediate temperatures.

Law of Intermediate Metals states other metals may be used in a thermocouple circuit

as long as the junctions are at the same temperature.

There are a lot of principles and laws associated with thermocouples.

Connecting a volt meter to a thermocouple will provide a (mV) reading.

Modern thermocouple systems include a direct readout along with automatic reference

junction compensation for an accurate temperature reading of the thermocouple junction.



KEO 1.27. LIST The Standard Color Code, Wire Type, Polarity, Maximum Temperature

Range, and uses for the following types of Thermocouples in the United States &

Canada:

a. “J” e. “N”

b. “K” f. “R”

c. “T” g. “S”

d. “E” h. “B”

The following table of International Thermocouple Color Codes lists the color code, wire type,

polarity, maximum temperature range and uses for thermocouples:

Figure 2-31 page 51



SUMMARY

There are several types of thermocouples with specifications that need to be factored into

the correct selection and use

Many measurement errors are caused by unintended thermocouple junctions.

Care must be taken to ensure that the proper extension wires are used.

Additional information on thermocouple characteristics to include temperature ranges and uses

are illustrated below:

Figure 2-32 (top half) page 52



Figure 2-32 (bottom half) page 52

KEO 1.28. DESCRIBE A brief description of the following type of Thermocouple

Measurement Circuits:

a. Difference Thermocouples

b. Thermopiles

c. Averaging Thermocouples

d. Pyrometers



KEO1.28.a Difference Thermocouples are made of two thermocouples wired in series with

reversed polarity to measure a temperature difference between two objects. A

difference thermocouple is illustrated below:

Figure 2-33 page 54

The lower temperature thermocouple is wired so that the polarity is reversed from the high-

temperature thermocouple. Therefore, the voltage output of the two thermocouples is equivalent

to the temperature difference of the two measurements. A difference TC can usually measure a

difference of about 50o or more in temperature.

KEO1.28.b A Thermopile consists of several thermocouples wired in series in order to

amplify the temperature signal. A thermopile is illustrated below:



Figure 2-34 page 55

With the thermocouples connected in series, they provide a higher voltage output. In a

thermopile, the individual voltages of each thermocouple are added together. A thermopile can

be used to measure extremely small temperature differences or it can be used to increase the

voltage of a circuit to be able to trip a contact in a temperature control circuit.

Thermopiles have been designed that can measure temperature differences as small as a few

millionths of a degree. The output of thermopiles is sufficient to be used by a transmitter,

recorder, or controller. This type of temperature sensing is commonly used in glass furnaces,

kilns, and steel mills.

KEO1.28.c Averaging Thermocouples consist of a set of parallel-connected thermocouples

that are used to measure an average temperature of an object or specific area. An



example would be in a large tank, a set of thermocouples inserted in a protective

tube or thermowell in the top of the vessel. The difference thermocouples are

positioned at different depths in the tube and the circuit averages the voltage

readings as depicted below:

Figure 2-35 page 56

In the example above the resistance of the different thermocouple circuits must be similar.

Because the wires are different lengths, this example depicts that each circuit has equivalent

resistors called swamping resistors to ensure the resistances are similar.



KEO1.28.d A thermocouple Pyrometer consist of a plain electrical meter with a

measurement range of 20 to 50 mV, a thermocouple, and a balancing variable

potentiometer resistor to balance loop resistance as illustrated below:

Figure 2-36 page 56

The Thermocouple Pyrometer is entirely self-contained, requiring no external power, and is

ideal for a local thermocouple measurement installation. The disadvantage is that it does not

have a high degree of accuracy and is not acceptable for critical temperature measurement

applications.



SUMMARY

Difference Thermocouples are made of two thermocouples wired in series with

reversed polarity to measure a temperature difference between two objects.

A Thermopile consists of several thermocouples wired in series in order to amplify

the temperature signal.

Averaging Thermocouples consist of a set of parallel-connected thermocouples that

are used to measure an average temperature of an object or specific area.

A thermocouple Pyrometer consist of a plain electrical meter with a measurement

range of 20 to 50 mV, a thermocouple, and a balancing variable potentiometer

resistor to balance loop resistance

Thermocouples are constructed of two dissimilar wires joined at one end and encased

in a metal sheath.

The other end of each wire is connected to a meter or measuring circuit.

Heating the measuring junction of the thermocouple produces a voltage that is greater

than the voltage across the reference junction.

The difference between the two voltages is proportional to the difference in

temperature and can be measured on a voltmeter in mV.

KEO 1.29. DESCRIBE What a Resistance Temperature Detector is and how it is used

Resistance Temperature Detectors (RTDs) are wire wound and thin film devices that measure

temperature because of the physical principle of the positive temperature coefficient of electrical

resistance of metals. The hotter they become, the larger or higher the value of their electrical

resistance.

RTDs are also called Resistance Thermometers, or Resistive Thermal Devices. They are

usually encapsulated in probes for temperature sensing and measurement with an external

indicator, controller or transmitter, or enclosed inside other devices where they measure

temperature as a part of the device's function, such as a temperature controller or precision

thermostat.

RTD General Description:

An RTD is a thermometer consisting of a high precision resistor with resistance that varies with

temperature. Unlike a thermocouple, an RTD does not generate its own voltage. An external

source of voltage or current must be incorporated into the circuit to transmit its temperature



signal. The voltage drop across and RTD provides a much larger output than the Seebeck voltage

of a thermocouple, allowing an RTD to be more precise over a small temperature range.

There are many RTD categories; Carbon Resistors, Film Thermometers, and Wire-Wound

types are the most widely used.

Carbon Resistors are widely available and are very inexpensive. They have very

reproducible results at low temperatures. They are the most reliable form at extremely

low temperatures. They generally do not suffer from hysteresis or strain gauge effects.

Carbon resistors have been employed by researchers for years because of the many

advantages associated with them.

Film Thermometers have a layer of platinum on a substrate; the layer may be extremely

thin, perhaps one micrometer. Advantages of this type are relatively low cost and fast

response. Such devices have improved performance although the different expansion

rates of the substrate and platinum give “strain gauge” effects and stability problems.

Film RTD

http://en.wikipedia.org/wiki/File:Thin_Film_PRT.png



Wire-Wound Thermometers can have greater accuracy, especially for wide temperature

ranges. The coil diameter provides a compromise between mechanical stability and

allowing expansion of the wire to minimize strain and consequential drift.

Wire-Wound RTD

Coil Elements have largely replaced wire wound elements in the industry. This design

allows the wire coil to expand more freely over temperature while still provided the

necessary support for the coil. This design is similar to that of a SPRT (Sequential

Probability Ratio Test), the primary standard which ITS-90 is based on, while still

providing the durability necessary for an industrial process.

Coil Element RTD

The current international standard which specifies tolerance and the temperature to electrical

resistance relationship for platinum resistance thermometers is IEC 751:1983. By far the most

common devices used in industry have a nominal resistance of 100 ohms at 0 °C, and are called

Pt-100 sensors ('Pt' is the symbol for platinum). The sensitivity of a standard 100 ohm sensor is a

nominal 0.385 ohm/°C. RTDs with a sensitivity of 0.375 and 0.392 ohm/°C as well as a variety

of others are also available.

http://en.wikipedia.org/wiki/File:Wire_Wound_PRT.png

http://en.wikipedia.org/wiki/File:Coil_Element_PRT.png

http://en.wikipedia.org/wiki/File:Wire_Wound_PRT.png

http://en.wikipedia.org/wiki/File:Coil_Element_PRT.png



RTD Wiring Configurations

The simplest resistance thermometer configuration uses two wires as shown below. It is only

used when high accuracy is not required as the resistance of the connecting wires is always

included with that of the sensor leading to errors in the signal. Using this configuration you will

be able to use 100 meters of cable. This applies equally to balanced bridge and fixed bridge

system.

Two Wire

In order to minimize the effects of the lead resistances a three wire configuration as shown

below can be used. Using this method the two leads to the sensor are on adjoining arms, there is

a lead resistance in each arm of the bridge and therefore the lead resistance is cancelled out. High

quality connection cables should be used for this type of configuration because an assumption is

made that the two lead resistances are the same. This configuration allows for up to 600 meters

of cable.

Three Wire

The four wire resistance thermometer configuration below even further increases the accuracy

and reliability of the resistance being measured. In the diagram below, a standard two terminal

RTD is used with another pair of wires to form an additional loop that cancels out the lead

http://en.wikipedia.org/wiki/File:Twowire.gif

http://en.wikipedia.org/wiki/File:Threewire.gif

http://en.wikipedia.org/wiki/File:Twowire.gif

http://en.wikipedia.org/wiki/File:Threewire.gif



resistance. The Wheatstone Bridge method uses a little more copper wire and is not a perfect

solution.

Four Wire

Below is a better alternative configuration of a Four-Wire Kelvin connection that should be

used in all RTDs. It provides full cancellation of spurious effects and cable resistance of up to 15

Ω can be handled. Actually in four wire measurement the resistance error due to lead wire

resistance is zero.

Four Wire Kelvin

http://en.wikipedia.org/wiki/File:Fourwire.gif

http://en.wikipedia.org/wiki/File:4wirebetter.gif

http://en.wikipedia.org/wiki/File:Fourwire.gif

http://en.wikipedia.org/wiki/File:4wirebetter.gif



KEO 1.30. DESCRIBE How the Wheatstone Bridge Circuit us used to measure the

resistance change of an RTD

A Bridge Circuit is a resistance bridge circuit used to provide a precise measurement of an

unknown resistor. A Wheatstone Bridge is often used as the bridge circuit as shown below:

Figure 2-50 page 68

The fixed resistors R1 and R2 are matched to each other to have the same resistance. In the

Balanced Bridge, the variable resistor R3 is adjusted to match the resistance of the RTD in order

to balance the bridge to have equal current flow in the bridge legs with zero potential across the

voltmeter. Then R3 is proportional to the temperature of the RTD.



The Unbalanced Bridge with fixed resistors provides a voltage across the bridge proportional to

the temperature of the RTD.

RTD SUMMARY

RTDs are called Resistance Temperature Detectors, Resistive Thermal Devices,

and Resistance Thermometers Devices.

RTDs are usually wire-wound or film resistance devices that measure temperature by

providing an increase of resistance with an increase of temperature.

RTDs are high precision resistors that are usually encapsulated in a probe assembly

and require an external voltage or current source.

Carbon RTDs are inexpensive and provide a reproducible results at low

temperatures.

Film RTDs are low cost and have a fast response time to temperature changes.

Wire-Wound RTDs have better accuracy for a wider temperature range.

Coil-Wound RTDs are most frequently used over wire-wound as this design allows

the wire coil to expand freely with temperature changes.

SPRT (Sequential Probability Ratio Test) is the standard which the ITS-90

(International Temperature Standard of 1990) is based on.

2-Wire RTDs are used when high accuracy is not required.

3-Wire RTDs are used to minimize the effects of lead wire resistance and allows for

up to 600 meters of lead wire to be utilized.

4-Wire RTDs further increase accuracy and reliability of temperature measurement.

4-Wire Kelvin RTDs are the best alternative as they provide full cancellation of

spurious effects of cable / lead wire resistance.

Wheat Stone Bridge circuitry is used to provide a precise measurement of an RTD

device.



KEO 1.31. DESCRIBE A basic overview of a Thermistor and its application

Thermistors are tiny a temperature-measuring devices. They are sensitive resistors consisting of

solid-state semiconductor material and are hermetically sealed in glass. They are available in

several shapes as illustrated below:

Figure 2-53 page 71

The electrical resistance for most thermistors, decreases with an increase of temperature and

therefore have a negative temperature coefficient (NTC). However there are some applications

where a positive temperature coefficient exists, (PTC) thermistors are used. Thermistors have a

much higher resistance than RTDs ranging from 100 ohms to 100 M ohms. Therefore, lead wire

resistance is not a concern. NTC thermistors are well suited for many applications that require a

large change in resistance when a small change of temperature occurs.



KEO 1.32. DESCRIBE How a Thermistor can be used as a temperature switch

A Thermistor can be used as a temperature switch to sound an alarm if the temperature

increases above setpoint. With a NTC Thermsistor, as temperature increases, the resistance of

the thermistors decreases. As the resistance of the thermistors decreases, the current flow

increases and there is a larger voltage drop across the alarm/circuit. The alarm sounds as long as

the temperature is high. The below picture illustrates how thermistors can be used as a

temperature switch:

Figure 2-55 page 72



With a PCT Thermistor with resistance increasing with temperature makes it suitable for

current-limiting applications as illustrated below:

Figure 2-56 page 73

For currents lower than the limiting current, the power generated in the unit is in-sufficient to

heat the thermistor to its switch temperature. However, as current increases to the critical level,

the resistance of the PTC thermistors increases at a rapid rate so that any further increase in

power dissipation results in a current reduction.

SUMMARY

Thermistors are tiny temperature measuring devices in many different shapes and sizes

having both a negative temperature coefficient (NTC) where temperature increases the

resistance decreases, and a positive temperature coefficient (PTC) where when

temperature increases the resistance increases; NTC is most commonly used.

Thermistors have a much higher resistance rating over RTDs, therefore lead wire

resistance is not a concern.

NTC Thermistors are well suited for many applications requiring a large change in

resistance when a small change of temperature occurs.

NTC Thermistors are used as temperature switches to respond to temperature increases,

as the resistance decreases allowing a greater current or voltage drop to activate an alarm

or control function.

PTC Thermistors having the resistance increase with increased temperature make them

suitable for current limiting applications.



KEO 1.33. DESCRIBE The principle of operation of a Semiconductor Thermometer

A Semiconductor Thermometer is a semiconductor device having change in electrical

properties with a change in temperature and are either RTDs or Thermistors. They are typically

produced in the form of integrated circuits as an individual circuit within the IC.

This temperature sensing circuit can be incorporated into many devices at low cost and allows

easy measurement of critical electrical circuits.

Semiconductor Thermometers are typically used as a coarse measurement in thermal shutdown

applications and have a range from -10o F to 400

o F.

These devices have an important function in preventing over temperature failure of integrated

circuits and components those components that are being controlled by these circuits.

KEO 1.34. COMPARE advantages and disadvantages associated with Thermocouples,

Resistance Temperature Detectors, Thermistors, and Integrated Circuit Sensors.

The following table compares the advantages and disadvantages for four common temperature

transducers:



SUMMARY

Semiconductor Thermometers are typically used as a coarse measurement in thermal

shutdown applications and range from -10o F to 400

o F.

Thermocouples are self-powered, yet are the least sensitive.

RTDs are most stable, yet they are expensive and slow in response time and require an

external voltage or current source.

Thermistors have the highest output and fast in response, yet they are non-linear and

have a limited temperature range.

IC Sensors are the most linear with the highest output and are the least expensive, yet

they have a low temperature range and are slow in response time.

KEO 1.35. DESCRIBE The principle of operation of an Infrared Radiation Thermometer

An Infrared Radiation Thermometer is a thermometer that measures the infrared radiation

emitted by an object to determine the object’s temperature.

Infrared radiation is that part of the electromagnetic spectrum with longer wavelengths than

visible light as illustrated below:

Figure 2-57 page 74



Bodies at low temperature emit little infrared radiation at long wavelengths. As the temperature

increases, the amount of emitted infrared radiation from the surface increases dramatically.

Infrared Radiation Thermometers generally have a quick response time. They can typically

make several measurements per second in areas where it is difficult to use a contact

thermometer.

The heart of the IR Thermometer is the Detector, which provides an electrical output

proportional to the amount of infrared radiation focused on it. This output is used in additional

circuitry to provide a control signal and other features.

An IR Thermometer focuses the IR waves onto the IR Detector by means of a lens and

aperture. For best accuracy, the body to be measured must fill the field of view and the angle on

incidence must be as close to 90 degrees as practical. The following picture illustrates this:

Figure 2-59 page 76



If the angle of incidence is not close to 90 degrees, erroneous reading can occur because the spot

size gets larger and the amount of incident radiation changes.

The following picture exhibits a typical Infrared Radiation Thermometer used to measure

temperature in a hard to reach space:

Picture page 85



The following picture illustrates the use of an Infrared Radiation Thermometer used in the

calibration of a temperature sensing device:

KEO 1.36. DESCRIBE Calibration Considerations for Temperature Measuring Instruments

using the following:

a. Dry Well and Mircobath Calibrators

b. Blackbody Calibrators

c. Electronic Calibrators

All temperature-measuring instruments have to be calibrated with known temperature sources

like regulated ice or sand baths, thermal ovens, refrigerated coolers, sub zero freezers, or with

instrument calibration devices that simulate the same temperature signal temperature measuring

instruments provide at their output.

A comparison is made between the actual temperature readings of the calibration source, to what

the device signal is indicating. The sources used are calibrated to set calibration standards and

verified periodically to ensure accuracy to these standards. Infrared Radiation Thermometers are

calibrated against a blackbody calibrator to verify its accuracy. Temperature-Measuring

Transmitters also need to be calibrated to ensure the signal from them is properly sent on to a

Temperature Indicating device (TI) or a Temperature Indicating Controller (TIC).



KEO1.36.a. A Dry Well Calibrator is a temperature controlled well or box where a

thermometer can be inserted and the output compared to the know dry well temperature as

illustrated below ( a Hart Scientific Dry Well Calibrator:

Figure 2-70 page 84

Dry wells are constructed of high-stability metal blocks with holes drilled in them to accept a

reference and a thermometer under test.

A Mircobath is a small tank containing a stirred liquid used to calibrate thermometers. The use

of a thermal bath eliminates problems resulting from poor fit in a dry well block, so microbaths

are especially suited for calibrating odd-shaped probes.

Both Dry Well and Mircrobaths have a temperature controller that maintains the calibrator at a

constant temperature. The actual temperature of the well or bath is measured with a reference

thermometer. The external reference thermometer is usually a Platinum RTD for mazimum

precision and accuracy.

To perform calibrations, set the well or bath calibrator to the desired temperature at the

low, middle, and high setting to verify the as-found settings of the temperature measuring

device being calibrated. Verification of the finding needs to be within the desired

tolerances. If the device is a thermometer and is not within tolerances, it needs to be

discarded or used with an offset reading. If the device is an electrical thermometer and

found to be out of calibration specifications, it should be replaced or the conversion

coefficients may be able to be adjusted in the readout device for continued use.



SUMMARY

An Infrared Radiation Thermometer is a thermometer that measures the infrared

radiation emitted by an object to determine the object’s temperature.

When using an Infrared Thermometer, for best accuracy, the body to be measured

must fill the field of view and the angle on incidence must be as close to 90 degrees as

practical.

A Dry Well Calibrator is a temperature controlled well or box where a thermometer can

be inserted and the output compared to the know dry well temperature

A Mircobath is a small tank containing a stirred liquid used to calibrate thermometers.

KEO 1.36.b A Blackbody Calibrator is a device used to calibrate infrared thermometers.

Blackbody Calibrators have a surface that is either unheated or heated. The

temperature is measured with a certified internal RTD sensor. They are commonly

used to calibrate IR Thermometers as indicated below:

Figure 2-71 page 85



KEO 1.36.c Electronic Calibrators are instruments that are connected to the input of an

electronic temperature measuring device to generate an electrical signal that

replicates the signal from an electrical thermometer as illustrated below:

SUMMARY

Blackbody Calibrators are used to calibrate Infrared Thermometers.

Electronic Calibrators are used to measure actual temperature signals or to replicate

them for calibration of instruments that receive a temperature signal.

KEO 1.37. STATE three basic functions of temperature detectors

Three functions of Temperature indicators are:

1. To provide an INDICATION of the degree of temperature detected.

2. To provide an ALARM when the degree of temperature has been achieved (either

a high temperature, a low temperature, or the preferred temperature – but usually

is for HIGH Temperature)

3. To provide a temperature signals for a circuit to CONTROL / MAINTAIN a set

temperature for the process being measured



Temperatures being monitored may normally be displayed in central locations like a control

room with visible indications and alarms associated with preset limits that must not be exceeded.

The temperatures being measured have control functions associated with them so that equipment

can be started or stopped to support a given temperature condition so a protective action can

occur to prevent equipment damage or injury to personnel.

KEO 1.38. DESCRIBE the two alternate methods of determining temperature when the

normal temperature detection sensing devices are inoperable

1. If an installed spare temperature sensing device is not available, a contact

pyrometer (portable thermocouple) may be used to obtain a temperature reading.

2. If the malfunction is in the circuitry and the detector itself is still functional, it

may be possible to obtain temperature readings by connecting an external bridge

circuit to the detector. Resistance readings may then be taken and a corresponding

temperature obtained from the detector calibration curves.

KEO 1.39. STATE two environmental concerns which can affect the accuracy and reliability

of temperature detection instrumentation.

1. Ambient Temperature variations will affect the accuracy and reliability

of temperature detection. Variations in ambient temperature can directly

affect the resistance of components in a bridge circuit and the resistance of

the reference junction for thermocouples. These variations can also affect

the calibration of electric/electronic equipment. These temperature

variations can be reduced by design of the circuitry and by maintaining the

temperature detection instrumentation in the proper environment via

HVAC.

2. Presence of Humidity will also affect most electrical equipment,

especially electronic equipment. High humidity causes moisture to collect

on equipment and can cause short circuits, grounds, and corrosion to

damage components. These effects are controlled by maintaining the

equipment in the proper environment via HVAC.



SUMMARY

Temperature detectors are used for Indication, Alarm Functions, and Control

Functions

If a temperature detector became inoperative, a Spare Detector may be used if installed

or a Contact Pyrometer can be used

Two environmental concerns are Ambient Temperature and Humidity

KEO 1.40. Given a simplified schematic diagram of a basic bridge circuit, STATE the

purpose of the following components:

a. R1 and R2

b. Rx

c. Adjustable Resistor

d. Sensitive Ammeter

Basic Bridge Circuit

Resistors R1 and R2 are the ratio arms of the bridge. They ratio the two variable resistances for

current flow through the ammeter. R3 is a variable resistor known as the standard arm that is

adjusted to match the unknown resistor. The sensing ammeter visually displays the current that

is flowing through the bridge circuit. Analysis of the circuit shows that when R3 is adjusted so

that the ammeter reads zero current, the resistance of both arms of the bridge circuit is the same.



The following Equation shows the relationship of the resistance between the two arms of the

bridge:

R1 R2

------- = -------

R3 Rx

Since the values of R1, R2, and R3 are known values, the only unkown is Rx. The value of Rx

can be calulated for the bridge during an ammeter zero current condition. Knowing this

resistance value provides a baseline point for calibration of the instrument attached to the bridge

circuit. The unknown resistance, Rx, is given by the following Equation:

R2 R3

Rx = ---------

R1

KEO 1.41. DESCRIBE the bridge circuit conditions that create a balanced bridge

Basic Bridge Circuit

The Bridge Circuit above operates by placing Rx in the circuit and then adjusting R3 so that all

current flows though the arms of the bridge circuit. When this condition exists, there is no

current flow through the ammeter, and the bridge is said to be balanced.

When the bridge is balanced, the currents through each of the arms are exactly proportional.

They are equal if R1 = R2. When this is the case, and the bridge is balanced, then the resistance of

Rx is the same as R3 or Rx = R3.



KEO 1.42. Given a block diagram of a basic temperature instrument detection and control

system, STATE the purpose of the following blocks:

a. RTD

b. Bridge Circuit

c. DC-AC Converter

d. Amplifier

e. Balanced Motor/Mechanical Linkage

Temperature Detection Circuit

The above block diagram consists of a temperature detector (RTD) that measures the

temperature. The detector is felt as resistance to the bridge network.

The Bridge Network converts this resistance to a DC voltage signal.

.

The DC – AC Converter sees the electronic signal developed in the bridge circuit across the

bridge potentiometer and converts the DC voltage to an AC voltage.

The AC voltage is then Amplified by the Amplifier to a higher (usable) voltage that is used to

drive a bi-directional motor.

The bi-directional (Balanced Motor) motor positions the slider on the slide-wire to the Balance

Circuit Resistance (Drive Linkage) to provide an indication.

If the RTD becomes open in either the unbalanced and balanced bridge circuits, the resistance

will be infinite and the meter will indicate a very high temperature. If it becomes shorted,

the resistance will be zero and the meter will indicate a very low temperature.



KEO 1.43. DESCRIBE the temperature instrument indication(s) for the following RTD

circuit faults:

a. Short Circuit

b. Open Circuit

A Short Circuit in a temperature instrument will indicate a very high temperature.

An Open Circuit in a temperature instrument will indicate a very low temperature.

KEO 1.44. EXPLAIN the three methods of bridge circuit compensation for changes in

ambient temperature:

1. Measuring Circuit Resistor Selection - because of changes in ambient

temperature, the resistance thermometer circuitry must be compensated. The

resistors that are used in the measuring circuitry are selected so that their

resistance will remain constant over the range of temperature expected.

2. Electronic Circuitry Design - Temperature compensation is also

accomplished through the design of the electronic circuitry to compensate for

ambient changes in the equipment cabinet.

3. Use of 3-Wire or 4-Wire RTD Circuits - It is also possible for the resistance

of the detector leads to change due to a change in ambient temperature. To

compensate for this change, three and four wire RTD circuits are used. In this

way, the same amount of lead wire is used in both branches of the bridge

circuit, and the change in resistance will be felt on both branches, negating the

effects of the change in temperature.



SUMMARY

The basic bridge circuit consists of:

Two known resistors (R1 and R3 that are used for rationing the adjustable and

known resistances.

One known variable resistor (R3) that is used to match the unknown variable

resistor.

One unknown (Rx) that is used to match the unknown variable resistor.

A sensing ammeter that indicates the current flow through the bridge circuit.

The bridge circuit is considered balanced when the sensing ammeter through the bridge

circuit.

A basic temperature instrument is comprised of:

An RTD for measuring temperature.

A Bridge network for converting resistance to voltage.

A DC to AC voltage converter to supply an amplifiable AC signal to the

amplifier.

An AC signal amplifier to amplify the AC signal to a usable level.

A balancing motor/mechanical linkage assembly to balance the circuits’

resistance.

An open circuit in a temperature instrument is indicated by a very high temperature

reading.

A short circuit in a temperature instrument is indicated by a very low temperature

reading.

Temperature instrument ambient temperature compensation is accomplished by:

Measuring circuit resistor selection.

Electronic circuitry design.

Use of three or four wire RTD circuits.



STEP TWO

Temperature Measurement Course

Skill/Performance Objectives

Skill Introduction:

Below are the skill objectives. How these objectives are performed depend on equipment and

laboratory resources available. With each skill objective it is assumed that a set of standard test

equipment and tools be provided.

For example, to be able to perform temperature calibration tasks, the following tools and

equipment will be required:

1. A temperature source such as a hot or cold bath, oven or subzero container, etc.

2. A calibration standard to measure the applied temperature

3. Equipment capable of measuring temperature such as a gauge, transducer, transmitter,

switch, etc.

4. A measuring device capable of measuring / indicating the output signal such as meter or

smart calibrator

5. An appropriate power supply to power the equipment being calibrated

Skill Terminal Objective (STO):

STO 1.0 Given a Temperature Measurement Task Checklist, under the direction of an

instructor, COMPLETE A SERIES OF TASKS using calibration equipment,

temperature indicating devices, and temperature transmitting devices to

demonstrate mastery of both knowledge and skill objectives associated with the

measurement of temperature.



Skill Enabling Objectives (SEO)

SEO 1.1. Calibrate a Rosemount Temperature Transmitter



SEO 1.2. Calibrate a Moore Temperature Transmitter



SEO 1.3. Perform a temperature calibration using a Thermo Electric TC Source



SEO 1.4. Perform a temperature calibration using a FLUKE 744 Smart Calibrator

FLUKE 744 SMART CALIBRATIOR NOTE:

A standard temperature transmitter calibration is performed by removing the input wires from

the transmitter and replacing them with the Fluke 744 electronic calibrator. The calibrator

generates an electric signal that replicates the signal from an electronic device like a

thermocouple or an RTD. This is accomplished by connecting the test leads from the calibrator

to the transmitter as depicted above (the black cable leaving the top side of the calibrator is the

temperature simulated signal being sent to the transmitter via the optional HART interface cable

connected to the RS-232 serial port).

The output from the thermocouple jacks (black HART cable - top side of calibrator or the two

pin yellow cable with plug to plug into lower right side of meter ) simulates a temperature input

to the transmitter. The red and black leads from the calibrator provide loop power to the

transmitter and measure the 4-20 mA current resulting from temperature changes into the

transmitter.



SEO 1.5. Perform a temperature calibration using the Smart Hart 375 Calibrator



SEO 1.6. Perform a calibration of a Rosemount temperature transmitter using an RTD

temperature detection device as its input



SEO 1.7. Perform the measurement of temperature using an Infrared Temperature Detector

device



SEO 1.8. Perform the calibration of a Temperature Switch



SEO 1.9. Perform a direct read measurement of temperature signals received from the

following three temperature detection devices:

a. Thermo Couple (TC)

b. Resistance Temperature Detector (RTD)

c. Thermister Temperature Detector (TTD)

Thermocouple Readout

RTD Readout



SEO 1.10. Perform a Furnace Temperature Comparison test using temperature detection

devices to compare responses and accuracies



SEO 1.11. Perform at three manual Temperature Transmitter calibrations with calibration

devices of your own choice



SEO 1.12. Perform a Temperature Response Time lab activity with different thermo-well

devices to plot response times

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