This document provides an overview of different temperature measurement devices and concepts. It discusses liquid-in-glass thermometers, bimetallic thermometers, pressure/filled system thermometers including classifications based on liquid, vapor, gas and mercury filling. It also covers electrical temperature measurement using resistance temperature detectors (RTDs), thermistors, and thermocouples. Sources of error and advantages/disadvantages are described for each type of temperature measuring device.

1. Mechanical Measurement & Metrology
2141901
Chapter - 05
Temperature Measurement
Presented by
Department of Mechanical Engineering

2. Chapter - 05 (Temperature Measurement)
 Temperature measuring device
 Liquid in glass Thermometers,
 Bimetallic strip.
 Pressure Thermometers,
 Resistance Temperature Detectors
 Thermistor,
 Thermocouples,
 Pyrometer, (Optical Pyrometer & Total radiation Pyrometer.)
 Calibration of temperature measuring devices,
 Numerical Examples on Flow Temperature Measurement.
4 hours 12% Weightage (Approx. 9 Marks)

3. Temperature Measurement

4. Temperature Scale

7. Classification of Thermometers

8. Expansion Thermometers
“Liquid-in-glass thermometers” It is the difference in expansion of
liquid and the containing glass.
“Bimetallic thermometers” The indication is due to the difference in
expansion of two solids.

9. 1.Liquid - in - Glass Thermometer
Construction
1.Bulb: The reservoir for containing most of the
thermometric liquid (mercury).
2.Stem: The glass tube having a capillary bore
along which the liquid moves with
changes in temperature.
3. Scale: A narrow-temperature-range scale for
reading a reference temperature .

10. The volume of mercury changes
slightly with temperature; the small
change in volume drives the narrow
mercury column a relatively long way
up the tube.
The space above the mercury may be
filled with nitrogen or it may be at less
than atmospheric pressure, a
partial vacuum.
Liquid Range (°C)
Mercury -35 to 510
Alcohol -80 to 70
Pentene -200 to 30
Toluene -80 to 100
Accuracy ±120 of span

11. Advantages
1) Simplicity in use & low cost.
2) Portable device.
3) Checking physical damage is easy.
4) Power source not require.
Disadvantages
1) Can not used for automatic recording.
2) Time lag in measurement.
3) Range is limited to about 600 °C .
4) Fragile construction

12. 2.Bimetallic Thermometer
In an industry, there is always a need to measure and
monitor temperature of a particular spot, field or
locality.
The industrial names given to such temperature
sensors are Temperature Indicators (TI) or
Temperature Gauges (TG).
All these temperature gauges belong to the class of
instruments that are known as bimetallic sensors.

13. Bimetallic Thermometer
Two basic principles of operation is to be followed in
the case of a bimetallic sensor.
1) A metal tends to undergo a volumetric dimensional
change (expansion/contraction), according to the
change in temperature.
2) Different metals have different co-efficient of
temperatures. The rate of volumetric change
depends on this co-efficient of temperature.

14. Bimetallic Strip
Invar (An alloy of Nickel and Iron) is the most commonly
used low expansion materials.
Nickel-Iron alloys with Chromium and Manganese added
are often used for thermal expansion materials.

15. Construction

16. The device consists of a bimetallic strip of two different
metals .
They are bonded together to form a spiral or a twisted helix.
Both these metals are joined together at one end by either
welding or riveting.
It is bonded so strong that there will not be any relative
motion between the two.
The image of a bimetallic strip is shown below.

18. Construction
A change in temperature causes the free end of the
strip to expand or contract due to the different
coefficients of expansion of the two metals.
This movement is linear to the change in
temperature and the deflection of the free end can be
read out by attaching a pointer to it.
This reading will indicate the value of temperature.
Bimetallic strips are available in different forms like
helix type, cantilever, spiral, and also flat type.

19. Advantages
1) Power source not required
2) Robust, easy to use and cheap.
3) Can be used to 500 °C.
Disadvantages
1) Not very accurate.
2) Limited to applications where manual reading is
acceptable.
3) Not suitable for very low temperatures because
the expansion of metals tend to be too similar,
so the device becomes a rather insensitive
thermometer

20. Radius of curvature of bimetal strip

22. FILLED
SYSTEM THERMOMETER
22

23. INTRODUCTION
Filled system thermometer are temperature measuring
device which works on pressure or volume change of a g
or changes in vapour pressure of a liquid.
A filled system thermometer consists of four parts:
1. Bulb,
2. Capillary tube,
3. Pressure- or volume-sensitive element &
4. Indicating device.
23

24. CONSTRUCTION
24

25. CONSTRUCTION
• The bulb contains a fluid that is sensitive to temperature
changes.
• The capillary tube connects the bulb to the element that is
sensitive to pressure or volume changes.
• The pressure-sensitive or volume-sensitive element may be
Bourdon tube, a helix, a diaphragm or bellows.
• The motion of the temperature or volume-sensitive element
couples mechanically to the indicating, recording, or
controlling device.
25

26. OPERATING PRINCIPLE
• Almost all liquids, gases or vapours expands when heated &
contracts when cooled. This phenomenon is utilized to
measure temperature by filled system thermometer.
• Bulb contains liquid, gases or vapour whose pressure or
volume changes in response to a change in temperature.
• This variation in pressure is transmitted through capillary tu
to the bourdon tube.
• The movement of bourdon tube is used to drive a pointer by
linkages for indicating temperature.
26

27. CLASSIFICATION
• As per the classification made by
of the USA, filled system
thermometers are divided into four classes as shown below
I
Liquid filled
( other than Mercury)
Volumetric Expansion
II Vapour filled Pressure Generation
III Gas filled Pressure Generation
IV Mercury filled Volumetric Expansion
27

28. Class - I - LIQUID FILLED
• These types of thermometers are completely filled with a liquid other than
Mercury & operates on the principle of volumetric expansion of liquid with a
increase in temperature.
• The filling fluid are usually inert hydrocarbon liquids such as Xylene, Toluene
Alcohol etc. which has coefficient of expansion six times of Mercury so that
system has high sensitivity.
• Range: -87 to 371oC
• Accuracy : + 0.5 of full range
• Advantages: (i) Wide temperature span (ii) Smaller bulb size
(iii) Lower cost
• Disadvantages: (i) Compensation of errors is difficult
(ii) It has a shortest capillary.
28

29. Class - II - VAPOUR FILLED
• In these thermometers the bulbs are partially
filled with volatile liquid while capillary &
bourdon tubes are filled with vapor.
• It operates on the principle that the pressure
in a vessel containing a liquid and its vapor
increases with increase in temperature.
• The commonly used volatile liquids are
Argon, Methyl Bromide, Methyl Chloride,
Butane, Diethyl Ether,Toluene, Ethyl Alcohol.
29

30. Class - II - VAPOUR FILLED
• Range: -184 oC below ambient temperature &
Above ambient temperature to 343 oC
• Accuracy: 1% of differential range
• Speed of response : 4 - 5 seconds except when passing
through ambient
• Advantages: (i) Ambient temperature effect is negligible.
(ii) Good response against temperature.
(iii) Long capillary length available.
• Disadvantages: (i) Nonlinear scale
(ii) Difficult to measure ambient
temperature.
30

31. Class - III - GAS FILLED
• These thermometers operates on the principle of ideal gas law which state
that volume of a gas increases with increase in temperature if pressure is
maintained constant or vice versa i.e. pv = RT
• These thermometers are filled with gases such as N2, H2, He which makes
system more sensitive to temperature changes.
• As N2 is chemically inert & inexpensive and also has good coefficient of
thermal expansion, it is used mostly.
• Range: - 268 oC to 760 oC
• Accuracy: +0.5% upto 320oC & +1% above this range
• Speed of response : 4 - 7 seconds
• Advantages: (i) Greatest working range of temperature.
(ii) No head or elevation effect.
• Disadvantages: (i) Larger bulb size
(ii) Generates less deflection force for controlling
device.
31

32. Class - IV - MERCURY FILLED
• Mercury is a liquid & in this respect a mercury filled thermometer is simila
to liquid filled thermometer.
• These two are separated due to the unique characteristic of Mercury & it
importance as a temperature measuring medium.
• Range: -40 to 649oC
• Advantages over liquid filled thermometer:
(i) Rapid response
(ii) High accuracy
(iii) Large power for operating elements.
(iv) At high temperature, pressure is also high
which reduces head or elevation effect error.
(v) As mercury is incompressible, ambient
temperature effect error is also very less.
(vi) Least difficult to compensate.
32

33. SOURCES OF ERRORS
• The common sources of errors in the filled system thermometers
are :
1) Ambient temperature effect : The
capillary tube & bourdon tube are exposed to surroundings &
subjected to a variable temperature widely different from that of th
bulb, which results as an error in temperature measurement.
2) Head or elevation effect : If the long
capillary tube is used & thermometer bulb is placed at a different
height with respect to the bourdon tube, then due to elevation
difference between bulb & bourdon tube deflecting end, pressure
head is created which results as an error in temperature
measurement.
33

34. 3) Barometric effect : The tip
travel of a bourdon tube is due to pressure difference between
atmosphere & filled system pressure. So if any variation in atmosph
pressure from designed pressure, the bourdon tube tip travels & ca
an error in temperature measurement.
4) Immersion effect : If the
bulb is not properly immersed or head of bulb is not properly
insulated, then heat from the bulb may be lost due to conduction fro
bulb & hence temperature at the bulb may be reduced, so that, it
indicates lower temperature than the actual.
5) Radiation effect :
This error occurs due to temperature difference between the bulb &
other solid bodies around it.
34

35. ADVANTAGES
• Advantages of filled system thermometer :
1. Simple & low cost.
2. Quite rugged construction & less chances of damage in handling.
3. Self operated, no power requires & generate sufficient power for
controller mechanism.
4. Good response, accuracy & sensitivity
5. Vapor actuated thermometer are most widely used as they are less
costly & simple. It has good speed of response.
6. It can be used for remote indication.
35

36. DISADVANTAGES
• Disadvantages of filled system thermometer :
1. Lower accuracy, sensitivity & temperature span compared to
electrical temperature instruments.
2. Compensation is required in case of variation in ambient
temperature.
3. The transient response is primary dependant on the bulb size &
thermal properties of the filled fluid.
4. To get good accuracy, requires large size of bulb results in poor
response.
5. In case of damage,entire system has to be replaced.
6. As filled system thermometers don’t produce electrical signals, it is
difficult to implement them in process controls that rely on
electrical and computerized control.
36

37. Electrical Temperature Measuring Instruments
Change in electrical properties of sensor for estimating temperature
1. Resistance Temperature Detectors (RTD)
2. Thermistors
3. Thermocouples and Thermopiles
Resistance Temperature Detectors (RTD)
Use change in resistance of suitable metals to indicate temperature.
Commonly used metals are platinum, nickel, copper which show a
positive change in resistance with increase in temperature.
The variation in resistance R at temperature can be written as
RT = R0 [ 1 + α T + β T2]
RT = Resistance at temperature T °C
R0 = Resistance at 0° C
or RT = R0 [ 1 + α T]
α and β are constant whose value depends on the RTD materials.

38. Resistance Temperature Detectors (RTD)
Material Temperature coefficient of resistance Melting Point Temperature range
(α) Ω/Ω/°C °C °C
Platinum 0.00392 1773 -240 to +650
Nickel 0.00630 1455 -190 to +310
Copper 0.00425 1083 -40 to +250

39. Resistance Temperature Detectors (RTD)
RTD elements
1) Resistance element or bulb
2) Suitable electrical leads
3) An indicating-recording or resistance measuring instrument
Small laboratory type - wound on a crossed mica former and enclosed in
a Pyrex tube.
Larger industrial type - Insulated ceramic former
The resistance coil and the ceramic former are protected the metal wire.
The tube may be evacuated or filled with an inert gas to protect the
metal wire.
Copper leads are generally preferred for connecting the thermometers to
the Wheatstone bridge.

40. Resistance Temperature Detectors (RTD)
Advantages
1) The sensor can be easily installed and replaced.
2) Good measurement accuracy (± 0.25% of scale range)
3) Fast response and good reproducibility.
4) Resistance to corrosion, chemicals, and oxidation.
5) Wide temperature range. (-180 °C to 760 °C )
6) Remote temperature sensing is possible.
7) No reference junction required.
8) Stable and accurate performance over a long time.
9) No necessity for any temperature compensation.
Disadvantages
1) More expansive than other instruments.
2) Large bulb size compared to thermocouple.
3) Required a separate bridge circuit system.
4) External power source required.
5) The self heating caused by current flow affects the accuracy of the
instrument.

41. Thermistors
Thermistor are thermally sensitive semiconductor materials having a
negative temperature coefficient of resistance.
The resistance of a Thermistor decrease as the temperature increase and
vice versa.
RT = Resistance at absolute temperature T (K)
R0 = Resistance at absolute temperature T0(K)
β = Constant depending upon material of the
Thermistor
Temperature range -108°C to 300°C.

42. Thermistors
Thermistors are made from mixtures of oxides of manganese, nickel,
copper, iron, cobalt, titanium, and uranium.
For cryogenic application doped germanium and carbon impregnated
glass are used.
Production of Thermistors - Powder metallurgy part m/f.
For temperature measurement its attached to the body surface and
connected one arm with Wheatstone bridge.
Balance position no change in galvanometer, temperature change the
resistance change and unbalanced the bridge circuit.

43. Thermistors
Advantages
1) Small size and low cost.
2) Fast temperature response.
3) High sensitivity.
4) Suitable for precise temperature measurement and control.
5) Need simple electric circuitry.
6) No need reference junction.
Disadvantages
1) Response is non linear.
2) Not suitable for measurement of high temperature.
3) Required external power source and bridge circuit.
4) Temperature span is limited.

44. Comparison of RTD and Thermistors
# Characteristic RTD Thermistors
1 Change in resistance Positive, Resistance increase Negative, Resistance decreases
with temperature with increase in temperature with increase in temperature
2 Temperature resistance Approximately linear
relationship RT = R0 [ 1 + α T]
3 Operating range -160°C to 600°C -100°C to 300°C
4 Composition Platinum, Copper, Nickel Metallic oxides like Manganese,
Nickel, Copper, Iron, Titanium,
Uranium
5 Sensitivity Lower Higher
6 Cost High Low

45. Thermocouples
Laws of Thermoelectric Thermometry
1) Seebeck effect
2) Peltier effect
3) Thomson effect

46. Seebeck effect :- When the junctions of two different metals are
maintained at different temperature, the EMF is produced in the
circuit. This is known as Seebeck effect.
The conductor 1 is maintained at
T+∆T temperature (Hot Junction)
The conductor 2 is maintained at
temperature ‘T’. (Cold Junction)
Since the junctions are maintained at
different temperature, the EMF‘U’
flows across the circuit.

47. Peltier effect :- Whenever current passes through the circuit of two
dissimilar conductors, depending on the current direction, either heat is
absorbed or released at the junction of the two conductors. This is known
as Peltier effect.
absorbed
released

48. Thomson effect :- Heat is absorbed or produced when current flows in
material with a certain temperature gradient. The heat is proportional to
both the electric current and the temperature gradient. This is known as
Thomson effect.

49. Experiment - THERMOELECTRIC GENERATOR
Thermoelectric power generator is a device that
converts the heat energy into electrical energy based on the
principles of Seebeck effect.
Apparatus
Hot water Ice water
Beakers
Hot Plate
Ice
Fan
Digital Thermometer
This experiment converts thermal energy to electrical energy by
Seebeck effect. Immerse the metal plates in two different temperature
baths. This Unit will generate10mV/degree temperature
difference. Show this with a multi-meter or use it to run a small fan.

50. Thermocouples

51. Law of Thermocouples
Law of homogeneous material
A thermo-electric current cannot be produced or sustained in a circuit of
a single homogeneous metal by the application of heat alone.
Application:-
Suggests two different metal required for any thermocouple.
Law of intermediate temperatures
A thermo-electric EMF is produced when the junction of two dissimilar
homogeneous metal kept at different temperature, then net EMF is not
affected by temperature gradients along the conductors.
Application:-
Suitable correction incase of reference junction temperature. ( Thermocouple
Calibration)
Law intermediate metals
Insertion of an intermediate (third) metal into a thermocouple basic
circuit will not affected the net EMF, provided the two junction introduced by
the third metal used are at identical temperature.
Application:-
To use extension (lead) wires different from the metals.
To introduce the EMF measuring and indicating device into circuit.
To allow the use of joining materials such as hard or soft solder in fabrication

52. Thermocouples
A Thermocouple consist of two dissimilar metals or alloys which develop
EMF.
When the reference and measuring junctions are at different temperature.
The reference junction or cold junction is usually maintained at some
constant temperature, such as 0°C.
Thermocouples are temperature sensors that are made from two different
metals. A voltage is generated when the metals are brought together to
form a junction and there are temperature differences between them.
Thermocouple circuits are governed by fundamental physical laws that
affect their ability to take measurements.

53. Thermocouples
Thermo means heat and couple means to make the junction.
Thermocouple works on thermoelectric effect.
It consist of
(1) Sensing element with thermal wall (Sheathing), measuring Junction
(2) Lead wire
(3) Reference junction
(4) Current measuring and indicating device, millivoltmeter or potentiometer
type instrument.

54. Thermocouples
The two dissimilar wires welded or soldered to from two junction.
The wires are insulated from each other and covered by protective
sheathing.
Thermal EMF developed upon the difference between measuring junction
and reference junction temperature.
The magnitude of the electric voltage developed by thermocouple due to
temperature different between junction.
Measure the small voltage change using millivoltmeter or the
potentiometer type instrument.

55. Components of Thermocouples
type JIS + -
B -
70 % Platinum 94 % Platinum
30 % Rhodium 6 % Rhodium
R PR
S -
K CA
E CRC
87 % Platinum
13 % Rhodium
90 % Platinum
10 % Rhodium
Chromel
[90 % Ni · 10 % Cr]
Chromel
[90 % Ni · 10 % Cr]
100 % Platinum
100 % Platinum
Alumel
[95 % Ni· 2 % Mn · 2 % Al]
Constantan
[55 % Cu · 45 % Ni]
J IC 99.5 % Iron
T CC 1 0 0% Copper
Constantan
[55 % Cu · 45 % Ni]
Constantan
[55 % Cu · 45 % Ni]
N -
Nicrosil
[84 % Ni ·14.2 % Cr ·1.45 % Si]
Nisil
[95 % Ni · 4.4 % Si · 0.15 % Mg]

56. Types of Thermocouples

58. Thermocouple
Advantages
1) Thermocouple have a better response.
2) They have a higher range of temperature measurements.
3) They sensing elements of thermocouple is easily installed.
4) Cheaper than RTD.
5) Very convenient for measuring the temperature at one particular point in
a piece of apparatus.
Disadvantages
1) Lower accuracy and as such they cannot be used for precision work.
2) Limited life of thermocouple.
3) The circuitry for thermocouple is very complex.
4) Expensive wire.
5) Need known reference.

59. Comparison of Thermocouple and Thermistors
# Characteristic
1 Speed of response
2 Measurement accuracy
3 Use
4 Temperature range
Thermocouple Thermistors
High Low
Good Less
Not much reliable and Very reliable and convenient
complex
-250°C to 2500°C -160°C to 600°C
Comparison of Thermocouple and RTD
# Characteristic Thermocouple RTD
1 Response Faster Slower
2 Temperature range Higher Lower1
3 Sensitivity Low High
4 Cost and Life Low and shorter High and Longer
5 Reference junction Required Not required
6 Accuracy Lower Higher
7 Effect of ambient temp.Effected on accuracy Not effected on accuracy

61. Given Data:- V = 45.15 m V; To = 1100°C; Tj = 840°C; Tc = 25°C

62. PYROMETERS

63. PYROMETERS
 ‘Pyro’ is the Greek word which means Fire and ‘meter’ means
measure.
 The measurement of high temperature (Above 1400° C) without
physical contact between hot body and the measuring
instrument is called Pyrometry and the measuring instruments
are called pyrometers.
 It depends upon the relationship between the temperature of a
hot body and the electromagnetic radiation emitted by the body.
 The radiant energy emitted from a body increase with
temperature, is used in measuring temperature particularly in
the higher ranges.
TYPES PYROMETERS
1) Total radiation Pyrometer.
2) Optical Pyrometer.

64. Total radiation Pyrometer
 A device which measure the total intensity of radiation emitted from
a body is called radiation pyrometer.
1) Total Radiation Pyrometer – In this method, the total heat emitted
from the hot source is measured at all wavelengths.
2) Selective Radiation Pyrometer – In this method, the heat radiated
from the hot source is measured at a given wavelength

65. Total radiation Pyrometer
 The radiation pyrometer has an optical system, including a lens, a mirror and an
adjustable eye piece.
 It collects the radiation from an object (hot body) whose temperature is
measure.
 A mirror is used to focus this radiation on a thermocouple.
 This energy which is concentrated on the thermocouple raise its temperature
and in turn generates an EMF.
 This EMF is then measured either by the galvanometer or potentiometer
method (Milli-voltmeter).
 Thus rise of the temperature is a function of the amount of radiation emitted
from the object.

66. Total radiation Pyrometer
Advantages
1) Temperature range 700° C to 2000° C.
2) Accuracy ± 2 % of scale.
3) High speed of response.
4) Can be measure the temperature of an object may either stationary or
moving.
5) Direct contact is not necessary with the object whose temperature is to be
measured.
6) Measurement is independent of the distance between the target and
measuring instrument.
Disadvantages
1) The scale is non-linear.
2) Cooling is required to protect the instrument when overheating.
3) Presence of dust and dirt on the mirror or lens causes instrument to read too
low.
Application
1) They are used for temperatures above the practical operating range of
thermocouples.
2) They can be used in the environments which contaminate or limit the life of
thermocouple.
3) Used for moving targets.

67. Optical Pyrometer
 An optical pyrometer works
on the principle that
matters glow above 480° C
and the colour of visible
radiation proportional to
the temperature of the
glowing matter.
 The amount of light
radiated from the glowing
matter (Solid or Liquid) is
measured and employed to
determine the temperature.

68. Optical Pyrometer
 An optical pyrometer is sighted at
the hot body and focused.
 In the beginning filament will
appear dark as compared to the
background which is bright (being
hot).
 By varying the resistance (R) in the
filament circuit more and more
current is fed into it, till filament
becomes equally bright as the
background and hence disappears.
 The current flowing in the filament
at this stage is measured with the
help of an ammeter which is
calibrated directly in terms of
temperature.
 If the filament current is further
increased, the filament appears
brighter as compared to the
background which then looks dark.
 An optical pyrometer can measure
temperature ranging from 700° C to
4000° C.

69. Optical Pyrometer
Advantages
1) Temperature range 700° C to 4000° C.
2) Accuracy ± 5 % of scale.
3) High speed of response.
4) Can be measure the temperature of an object may either stationary or
moving.
5) Direct contact is not necessary with the object whose temperature is to
be measured.
6) Measurement is independent of the distance between the target and
measuring instrument.
Disadvantages
1) The lower measuring is limited to 700°C.
2) The device is not useful for obtaining continuous values of temperatures
at small intervals.
Application
1) Used to measure temperatures of liquid metals or highly heated
materials.
2) Can be used to measure furnace temperatures.

70. Calibration of
Temperature measuring devices
Self Study

71. Selection of Temperature Measurement Instrument

72. Assignment No:- 5
1) List and explain with sketch types of expansion thermometer stating application, advantages
and limitations.
2) Explain the construction and working of a resistance thermometer and thermocouple with a
neat sketch.
3) Distinguish between “RTD” , “ THERMISTOR” , “THERMOCOUPLE”
4) Explain briefly the construction and working of a Total radiation and Optical Pyrometer,
stating its application, advantages and disadvantages.
5) A bimetal strip is constructed of strips of nickel chrome iron alloy and invar bonded together
at 25°C. The strips are 50 mm long and each material has a thickness of 1 mm. Calculate the
radius of curvature produced when the strip is subjected to a temperature of 200°C Assume
the following data: α1= 1.7 * 10 -6 / °C E1 = 1.5 * 10 -6 kgf/cm2
α2= 12.5 * 10 -6 / °C E2 = 2.2 * 10 -6 kgf/cm2
6) A Chromel- Alumel thermocouple is assumed to have nearly linear operating range up to
1100° C with EMF (reference 0° C) 45.14 mV at this temperature. The thermocouple is
exposed to a temperature of 840° C. The potentiometer is used as cold junction and its
temperature is estimated to be 25° C. Calculate the EMF indicated on the potentiometer.
Submit Assignment before Friday; 17/02/2017; 4:30 PM

73. Department of Mechanical Engineering
Shroff S.R. Rotary Institute of Chemical Technology