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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.
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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 Universitys 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
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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
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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
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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
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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
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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
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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 bodys thermal energy increases;
this together with pressure
determines the bodys temperature and physical state. For
example, assume that heat is added to
water in a container at atmospheric pressure, increasing the
waters 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).
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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.
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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.
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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
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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
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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 metals, require many
more known fixed points to
define a temperature scale.
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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 Pascals) pressure.
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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.
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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
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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 boilers 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.
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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.
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For example: A flame heats the boilers 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
engines 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
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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
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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
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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
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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
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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.
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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).
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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.
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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.
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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.
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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.
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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.
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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.
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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
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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
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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
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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:
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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
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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.
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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.
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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
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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
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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.
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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
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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
can be handled. Actually in four wire measurement the resistance
error due to lead wire
resistance is zero.
Four Wire Kelvin
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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.
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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.
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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.
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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
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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.
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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:
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SUMMARY
Semiconductor Thermometers are typically used as a coarse
measurement in thermal shutdown applications and range from -10
o 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 objects temperature.
Infrared radiation is that part of the electromagnetic spectrum
with longer wavelengths than
visible light as illustrated below:
Figure 2-57 page 74
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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
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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
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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).
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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.
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SUMMARY
An Infrared Radiation Thermometer is a thermometer that measures
the infrared radiation emitted by an object to determine the
objects 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
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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
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Skill Enabling Objectives (SEO)
SEO 1.1. Calibrate a Rosemount Temperature Transmitter
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SEO 1.2. Calibrate a Moore Temperature Transmitter
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SEO 1.3. Perform a temperature calibration using a Thermo
Electric TC Source
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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.
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SEO 1.5. Perform a temperature calibration using the Smart Hart
375 Calibrator
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SEO 1.6. Perform a calibration of a Rosemount temperature
transmitter using an RTD
temperature detection device as its input
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SEO 1.7. Perform the measurement of temperature using an
Infrared Temperature Detector
device
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SEO 1.8.