Unit 5 temperature measurement

INDUSTRIAL MEASUREMENTS
UNIT 05: TEMPERATURE
MEASUREMENT
1

TEMPERATURE - Definition
• Temperature is an objective measurement of how hot or cold an
object is. It can be measured with a thermometer or a calorimeter.
• It is a means of determining the internal energy contained within
a given system.
• Hotness and coldness are the result of molecular activity. The
faster the movement of a substance, the more heat it contains.
• The units of temperature are °C, °K, °F and °R.
• In this chapter we have to study Temperature scales and
transducers
2

• There are four major temperature scales that are used around the world –
Fahrenheit and Celsius are frequently used in everyday, around the house
measurements, while the absolute zero-based Kelvin and Rankine scales
are more commonly used in industry and the sciences.
TEMPERATURE - Scales
Temperature Scales
Fahrenheit (°F) Reaumur (°R’)Rankine (°R)Kelvin (°K)Celsius (°C)
3

FAHRENHEIT SCALE °F
• Fahrenheit scale is the temperature scale in
which 212 degrees is the boiling point of
water and 32 degrees is the freezing point of
water.
• The scale was invented in 1714 by the
German physicist G.D. Fahrenheit (1686-
1736).
• The difference in height between the two
points would then be marked off in 180
divisions with each division representing 1
°F.
• Conversions:
32 °F = 0 °C
212 °F = 100 °C
1 °F =(5/9) °C
T(°F) = (9/5)T(°C) + 32
4

CELSIUS SCALE °C
• About 20 years after Fahrenheit proposed its
temperature scale for thermometer, Swedish
professor Anders Celsius defined a better scale for
measuring temperature. He proposed using
the boiling point of water as 100° C and the
freezing point of water as 0° C.
• Water was chosen as the reference material
because it was always available in most
laboratories around the world.
• Celsius temperature scale is also called centigrade
temperature scale because of the 100-
degree interval between the defined points.
• The Celsius temperature for a state colder than
freezing water is a negative number. The Celsius
scale is used, both in everyday life and in science
and industry, almost everywhere in the world.
• Conversion:
T(°C) = (5/9)[T(°F) - 32]
5

KELVIN SCALE K
• Kelvin temperature scale is the base unit
of thermodynamic temperature measurement
in the International System (SI) of
measurement.
• The Kelvin scale was determined based on
the Celsius scale, but with a starting point
at absolute zero. Temperatures in the Kelvin
scale are 273 degrees less than in the Celsius
scale. The kelvin is defined as the fraction
1⁄273.
• Kelvin was named in honor of Lord Kelvin
who had a great deal to do with the
development of temperature measurement
and thermodynamics.
• Conversion:
Kelvin – Celsius
K = °C + 273.15
°C = K – 273.15
6

• Rankine is a temperature scale named
after the physicist William John
Macquorn Rankine (1820-1872), who
proposed it in 1859.
• The Rankine scale is similar to the
Kelvin scale in that zero is Absolute
Zero; however, a degree Rankine is
defined as equal to one degree
Fahrenheit as opposed to one degree
Celsius (as used by the Kelvin scale).
• A temperature of -459.67 F is equal to 0
R.
• Conversion:
R = F + 459.67
RANKINE SCALE °R
7

REAUMUR SCALE °R’
• The Réaumur scale, also known
as the is a temperature scale for
which the freezing and boiling
points of water are defined as 0
and 80 degrees respectively.
• This scale is often used in alcohol
industries.
8

TEMPERATURE SCALES -
Summary
9

Absolute Zero Temperature
Absolute zero is the lowest temperature possible. At a
temperature of absolute zero there is no motion and no
heat. Absolute zero occurs at a temperature of 0 degrees
Kelvin, or -273.15 degrees Celsius, or at -460 degrees
Fahrenheit.
10

CLASSIFICATION OF TEMPERATURE
MEASURING TRANSDUCERS
11

Vapour Pressure
Thermometer:
a) Filled System Thermometer
12

1. The vapour filled system uses a volatile liquid/ vapour combination to generate
a temperature dependent fluid expansion. The mechanism of operation is as
shown in fig.
2. Vapour pressure systems are quite accurate and reliable. They also do not
require any compensation for temperature effects.
3. This form of measurement is based on the vapour-pressure curves of the fluid
and measurement occurs at the transition between the liquid and vapour
phases.
4. This transition occurs in the bulb and will move slightly with temperature, but
it is the pressure that is affected and causes the measurement.
5. If the temperature is raised, more liquid will vapourize and pressure will
increase.
6. A decrease in temperature will result in condensation of some of the vapour,
and the pressure will decrease.
7. According to change in pressure, the spiral bourdon tube will expand and
contract respectively.
8. The pointer mechanism will move over the scale calibrated in terms of
temperature and give the indication of temperature.
Vapour Pressure Thermometer: Working
13

b) Expansion Thermometer
Bimetallic Thermometer:
The bimetallic thermometer uses the bimetallic strip which converts the
temperature into the mechanical displacement. The working of the bimetallic strip
depends on the thermal expansion property of the metal. The thermal expansion is
the tendency of metal in which the volume of metal changes with the variation in
temperature.
14

Working Principle of Bimetallic Thermometer:
The working principle of bimetallic thermometer depends on
the two fundamental properties of the metal.
1. The metal has the property of thermal expansion, i.e., the
metal expand and contract concerning the temperature.
2. The temperature coefficient of all the metal is not same.
The expansion or contraction of metals is different at the
same temperature.
15

1. The bimetallic strip is constructed by bonding together the two thin
strips of different metals. The metals are joined together at one end
with the help of the welding.
2. The bonding is kept in such a way that there is no relative motion
between the two metals.
3. The physical dimension of the metals varies with the variation in
temperature.
4. Since the bimetallic strip of the thermometer is constructed with
different metals. Thereby, the length of metals changes at different
rates.
5. When the temperature increases, the strip bends towards the metal
which has a low-temperature coefficient. And when the
temperature decreases, the strip bends towards the metal which has
a high-temperature coefficient.
Constructions of Bimetallic Thermometer
16

6. The figure below shows the bimetallic strip in the form of the
straight cantilever beam. The strip fixed at one end and deflects at
the other end.
17

7. The range of deflection of bimetallic strip depends on the type of metals
used for construction. The deflection of the metal is directly proportional to
the length of the strip and the variation of temperature and is inversely
proportional to the thickness of the strips.
18

Types of Bimetallic Strip
1) Spiral Strip bimetallic thermometer
19

1) Spiral Strip bimetallic thermometer:
1. In bimetallic strip thermometer, the spiral-shaped strip is
used.
2. This type of thermometer is used for measuring the
ambient temperature.
3. Because of the thermal expansion property of metal the
deformation occurs in the spring with the variation of
temperature.
4. The pointer and dials attached to the spring, which
indicates the variation of temperature.
20

2) Helical bimetallic thermometer
21

2) Helical bimetallic thermometer:
1. The helix type bimetallic strip is mostly used for
industrial applications.
2. In this thermometer, the helix shape strip is used for
measuring the temperature.
3. The free end of the strip is connected to the pointer.
4. The deflection of the strip shows the variation of
temperature.
22

Advantages bimetallic thermometer:
• Low cost
• Tough and cannot be broken easily
• Easy to install and maintain
• Good accuracy
• Wide temperature range
Disdvantages bimetallic thermometer:
• Possibility of calibration change due to rough handling
• Limitations to local mounting
Applications bimetallic thermometer:
The bimetallic thermometer is used in household devices likes oven, air conditioner,
and in industrial apparatus like refineries, hot wires, heater, tempering tanks etc. for
23

ELECTRICAL METHODS
There are three types of electrical temperature instruments generally
used in industries:
1. Thermistor
2. Resistance Thermometer (RTD)
3. Thermocouple
24

ELECTRICAL METHODS
A) Thermistor
1. Definition: The thermistor is a kind of resistor whose resistivity depends
on surrounding temperature. It is a temperature sensitive device. The
word thermistor is derived from the word, thermally sensitive resistor.
The thermistor is made of the semiconductor material that means their
resistance lies between the conductor and the insulator.
2. The variation in the thermistor resistance shows that either conduction or
power dissipation occurs in the thermistor. The circuit diagram of
thermistor uses the rectangular block which has a diagonal line on it.
25

3) Types of Thermistor:
The thermistor is classified into types. They are the negative
temperature coefficient and the positive temperature coefficient
thermistor.
1. Negative Temperature Coefficient Thermistor – In this type
of thermistor the temperature increases with the decrease of the
resistance. The resistance of the negative temperature
coefficient thermistor is very large due to which it detects the
small variation in temperature.
2. Positive Temperature Coefficient Thermistor – The
resistance of the thermistor increases with the increases in
temperature.
26

4) Construction of Thermistor:
1. The thermistor is made with the sintered mixture of metallic oxides
like manganese, cobalt, nickel, cobalt, copper, iron, uranium, etc.
2. It is available in the form of the bead, rod and disc. The different
types of the thermistor are shown in the figure below.
3. The bead form of the thermistor is smallest in shape, and it is
enclosed inside the solid glass rod to form probes.
4. The disc shape is made by pressing material under high pressure
with diameter range from 2.5 mm to 25mm.
5. Following figures show various types of construction of
thermistors:
27

DIFFERENT FORMS OF THERMISTOR
28

5) Resistance Temperature Characteristic of Thermistor
1. The relation between the absolute temperature and the resistance of
the thermistor is mathematically expressed by the equation shown
below:
Where RT1 – Resistance of the thermistor at absolute temperature T1 in
Kelvin.
RT2 – Resistance of the thermistor at absolute temperature T2 in
Kelvin.
Β – a temperature depending on the material of thermistor.
29

5) Resistance Temperature Characteristic of Thermistor
2. The resistance temperature coefficient of the thermistor is shown in the
figure below.
3. The graph below shows that the thermistor has a negative temperature
coefficient, i.e., the temperature is inversely proportional to the
resistance.
30

6) Advantages of Thermistor:
• The thermistor has fast response over narrow temperature range.
• It is small in size.
• Contact and lead resistance problem not occurred due to large
resistance.
• It has good sensitivity in NTC region.
• Cost is low.
7) Disadvantages of Thermistor:
• The thermistor need of shielding power lines.
• The excitation current should be low to avoid self heating.
• It is not suitable for large temperature range.
• The resistance temperature characteristics are non linear.
31

ELECTRICAL METHODS
B) Resistance Temperature Detector (RTD)
1. Definition – The resistance thermometer or resistance temperature
detector (RTD) uses the resistance of electrical conductor for
2. The resistance of the conductor varies with the time. This property
of the conductor is used for measuring the temperature.
3. The main function of the RTD is to give a positive
change in resistance with temperature.
4. The metal has a high-temperature coefficient that means their
temperature increases with the increase in temperature. The carbon
and germanium have low-temperature coefficient which shows that
their resistance is inversely proportional to temperature.
32

1. The resistance thermometer uses a sensitive element made of extremely pure metals
like platinum, copper or nickel. The resistance of the metal is directly proportional to
the temperature. Mostly, platinum is used in resistance thermometer. The platinum has
high stability, and it can withstand high temperature.
2. Gold and silver are not used for RTD because they have low resistivity. Tungsten has
high resistivity, but it is extremely brittles. The copper is used for making the RTD
element. The copper has low resistivity and also it is less expensive. The only
disadvantage of the copper is that it has low linearity. The maximum temperature of
the copper is about 120ºC.
3. The RTD material is made of platinum, nickel or alloys of nickel. The nickel wires are
used for a limited temperature range, but they are quite nonlinear.
4. The following are the requirements of the conductor used in the RTDs.
5. The resistivity of the material is high so that the minimum volume of conductor is
used for construction.
6. The change in resistance of the material concerning temperature should be as high as
possible.
7. The resistance of the material depends on the temperature.
8. The resistance versus temperature curve is shown in the figure below. The curves are
nearly linear, and for small temperature range, it is very evident.
Material used in Resistive Thermometer:
33

Material used in Resistive Thermometer:
34

Construction of Resistive Thermometer
1. The resistance thermometer is placed inside the protective
tube for providing the protection against damage.
2. The resistive element is formed by placing the platinum
wire on the ceramic bobbin.
3. This resistance element is placed inside the tube which is
made up of stainless steel or copper steel.
4. The lead wire is used for connecting the resistance
element with the external lead.
5. The lead wire is covered by the insulated tube which
protects it from short circuit.
6. The ceramic material is used as an insulator for high-
temperature material and for low-temperature fibre or
glass is used. 35

Construction of Resistive Thermometer
36

Operation of Resistive Thermometer
The tip of the resistance thermometer is placed near the measurand
heat source. The heat is uniformly distributed across the resistive
element. The changes in the resistance vary the temperature of the
element. The final resistance is measured. The below mention
equations measure the variation in temperature:
where Rθ – approximation resistance at θºC
Rθ0 – approximation resistance at θ0ºC
Δθ – θ – θ0 change in temperature ºC and the αθ0 – resistance
temperature coefficient at θ0ºC
37

Advantages of RTD:
1. The accuracy of RTD is good.
2. Temperature compensation is not necessary.
3. The RTD available in wide range.
4. The stability maintained over long period of time.
5. It can be used to measure differential temperature.
6. It is suitable for remote indication.
7. It can be easily installed and replaced.
Disadvantages of RTD:
1. The cost of RTD is high.
2. It has large bulb size.
3. Low sensitivity occurs in RTD.
4. Affected by shock and vibration.
5. Possibility of self heating.
6. Bridge circuit is needed with power supply. 38

ELECTRICAL METHODS
C) Thermocouple
• A thermocouple is comprised of at least two metals joined
together to form two junctions.
• One is connected to the body whose temperature is to be
measured; this is the hot or measuring junction.
• The other junction is connected to a body of known temperature;
this is the cold or reference junction.
• Therefore the thermocouple measures unknown temperature of
the body with reference to the known temperature of the other
body.
39

Thermocouple: Working Principle
The working principle of thermocouple is based on three effects, discovered by
Seebeck, Peltier and Thomson. They are as follows:
1) Seebeck effect: The Seebeck effect states that when two different or unlike
metals are joined together at two junctions, an electromotive force (emf) is
generated at the two junctions. The amount of emf generated is different for
different combinations of the metals.
2) Peltier effect: As per the Peltier effect, when two dissimilar metals are joined
together to form two junctions, emf is generated within the circuit due to the
different temperatures of the two junctions of the circuit.
3) Thomson effect: As per the Thomson effect, when two unlike metals are joined
together forming two junctions, the potential exists within the circuit due to
temperature gradient along the entire length of the conductors within the circuit.
In most of the cases the emf suggested by the Thomson effect is very small and it
can be neglected by making proper selection of the metals. The Peltier effect plays
a prominent role in the working principle of the thermocouple. 41

Thermocouple: Schematic Diagram
42

Thermocouple: Working
1. The general circuit for the working of thermocouple is shown in the figure 1
above.
2. It comprises of two dissimilar metals, A and B. These are joined together to
form two junctions, p and q, which are maintained at the temperatures T1 and
T2respectively.
3. Remember that the thermocouple cannot be formed if there are not two
junctions.
4. Since the two junctions are maintained at different temperatures the Peltier emf
is generated within the circuit and it is the function of the temperatures of two
junctions.
5. If the temperature of both the junctions is same, equal and opposite emf will be
generated at both junctions and the net current flowing through the junction is
zero.
6. If the junctions are maintained at different temperatures, the emf’s will not
become zero and there will be a net current flowing through the circuit.
7. The total emf flowing through this circuit depends on the metals used within
the circuit as well as the temperature of the two junctions.
8. The total emf or the current flowing through the circuit can be measured easily
by the suitable device.
44

9. The device for measuring the current or emf is connected within the circuit of
the thermocouple.
10.It measures the amount of emf flowing through the circuit due to the two
junctions of the two dissimilar metals maintained at different temperatures.
11.In figure 2 the two junctions of the thermocouple and the device used for
measurement of emf (potentiometer) are shown.
12.Now, the temperature of the reference junctions is already known, while the
temperature of measuring junction is unknown.
13.The output obtained from the thermocouple circuit is calibrated directly against
the unknown temperature.
14.Thus the voltage or current output obtained from thermocouple circuit gives the
value of unknown temperature directly.
Thermocouple: Working
45

Advantages of Thermocouple:
1. The thermocouple is less expensive than RTD.
2. It has wide temperature ranges.
3. It has good reproducibility.
4. The temperature range is 270 to 2700 degree Celsius.
5. It has rugged construction.
6. It does not required bridge circuit.
7. It has good accuracy.
8. It has high speed of response.
Disadvantages of Thermocouple:
1. The stray voltage pick up is possible.
2. As output voltage is very small, it needs amplification.
3. The cold junction and lead compensation is essential.
4. It shows non linearity.
Applications:
1. To monitor temperatures of liquids and gases in storage and flowing pipes and ducts.
2. In industrial furnaces
3. For temperature measurement in cryogenic range.
48

PYROMETERS
• All the temperature measuring methods studied earlier require physical
contact of thermometer with the body whose temperature is to be
measured.
• But at high temperatures above 1400 °C, the thermometer may melt due to
direct physical contact.
• To solve these problems, a non-contact method of temperature
measurement is used. This method is also suitable for moving bodies.
• Pyrometry is a technique for measuring temperature without physical
contact.
• A pyrometer is a device that is used for the temperature measurement of an
object.
• The device actually tracks and measures the amount of heat that is radiated
from an object.
• The thermal heat radiates from the object to the optical system present
inside the pyrometer.
• The optical system makes the thermal radiation into a better focus and
passes it to the detector.
• The output of the detector will be related to the input thermal radiation. 49

PYROMETERS- TYPES
There are two types of Pyrometers:
1)Radiation Pyrometers
2)Optical Pyrometers
50

2) OPTICAL PYROMETERS
Definition:
1. The optical pyrometer is a non-contact type temperature measuring device.
2. It works on the principle of matching the brightness of an object to the brightness
of the filament which is placed inside the pyrometer.
3. The optical pyrometer is used for measuring the temperature of the furnaces,
molten metals, and other overheated material or liquids.
4. It is not possible to measures the temperature of the highly heated body with the
help of the contact type instrument.
5. Hence the non-contact pyrometer is used for measuring their temperature.
Construction of Optical Pyrometer:
1. The construction of the optical pyrometer is quite simple.
2. The pyrometer is cylindrical inside which the lens is placed on one end and the
eyepiece on the other end.
3. The lamp is kept between the eyepiece and the lens. The filter is placed in front
of the eyepiece.
4. The filter helps in getting the monochromatic light. The lamp has the filament
which is connected to the battery, ammeter and the rheostat. 54

Working of Optical Pyrometer:
1. The optical pyrometer is shown in the figure below.
2. It consists the lens which focuses the radiated energy from the
heated object and targets it on the electric filament lamp. The
intensity of the filament depends on the current passes through it.
Hence the adjustable current is passed through the lamp.
3. The magnitude of the current is adjusted until the brightness of the
filament is similar to the brightness of the object.
4. When the brightness of the filament and the brightness of the object
are same, then the outline of the filament is completely disappeared.
5. The filament looks bright when their temperature is more than the
temperature of the source.
6. The filament looks dark if their temperature is less than that
required for equal brightness
56

Advantages:
1. The optical pyrometer is useful for high temperatures.
2. It is useful for monitoring the temperature of moving object and
distant objects.
3. It has good accuracy.
4. No need for contact with target of measurement.
5. It is light in weight.
57

Disadvantages:
1. The accuracy may be affected by dust, smoke and thermal
background radiation.
2. The optical pyrometer is not useful for measuring the temperature
of clean burning gases that do not radiate visible energy.
3. It is more expensive.
4. It causes human errors.
58

Unit 5 temperature measurement

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