Measurement and Control
(ME37502)
Unit II
 Pressure Measurement
 Temperature Measurement
 Flow Measurement
Temperature Measurements
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Contents
 Introduction
 Methods of measuring temperature
 Thermocouples
 Thermocouple Materials
 Thermistors
 RTD
 Pyrometers
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Introduction
• The first thermometer was developed by Galileo in the 17th century.
• In 1724, D.G. Fahrenheit, a German physicist, contributed significantly
to the development of thermometery.
• The Swedish physicist Anders Celsius, in 1742, developed the mercury-
in-glass thermometer.
• Baron Kelvin, popularly known as Lord Kelvin, a British physicist,
introduced a new concept, known as the Kelvin scale.
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Introduction
 The scale used to measure temperature can be divided into:
1.
Relative scale [Fahrenheit (oF) and Celsius (oC)]
2. Absolute scale [Rankine (oR) and Kelvin (K)]
 The various temperature scales are related as follows:
F = 1.8C + 32
or
C = (F - 32)/1.8
R = F + 460
K = C + 273
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Methods of measuring temperature
Contact type sensors are classified as follows:

Thermocouples

Resistance temperature detectors (RTDs)

Thermistors

Liquid in glass thermometers

Pressure thermometers
Non contact type sensors are categorized as follows:

Radiation Pyrometer

Optical Pyrometer

Infrared Pyrometer
In case of non contact type sensors, the radiant power of the Infrared
or optical radiation received by the object or system is measured
Thermocouples
• A thermocouple is an electrical device consisting of two dissimilar
electrical conductors for forming electrical junctions at differing
temperatures.
• A thermocouple produces a temperature dependent voltage as a result of
the thermoelectric effect, and this voltage can be interpreted to measure
temperature.
The Seebeck effect is a phenomenon in which a temperature difference between two
dissimilar electrical conductors or semiconductors produces a voltage
difference between the two substances
Laws of thermocouples
 Law of homogeneous Circuit
 Law of Intermediate Metals
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Law of homogeneous Circuit
• This law states that a thermoelectric current cannot be sustained in a circuit
of a single homogenous material, regardless of the variation in its cross
section and by the application of heat alone.
Thermocouple used as a instrument
Law of Intermediate Metals
• If an intermediate metal is inserted into a thermocouple circuit at any point,
the net emf will not be affected provided the two junctions introduced by
the third metal are at identical temperatures.
Identical temperatures
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Thermocouple Materials
Base metal type
T : Copper (40%)-Constantan (60%): -200 to 350
J : Iron (50%)-Constantan (50%): -150 to 750
E : Chromel (Ni-Cr)-Constantan (57%Cu, 43%Ni): -200 to 1000
K : Chromel (90%Ni-10%Cr)- Alumel (94%Ni-2%Al-3%Mn-1%Si): -200
to 1300
Rare metal type
S : Platinum(90%)- rhodium-platinum (10%): 0 to 1500
R : Platinum-rhodium (87%Pt-13%Rh)-Platinum: 0 to 1500
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Thermopile
• An extension of thermocouples is known as a thermopile.
• A thermopile comprises a number of thermocouples connected in series,
wherein the hot junctions are arranged side by side or in a star formation.
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Resistance Temperature Detectors (RTD)
• RTD is a temperature measuring device composed of a resistance thermometer
element, internal connected wires, a protective shell with or with out means for
mounting a connection head.
• An RTD is a temperature sensor that work on the principle that the resistance of
electrically conductive materials is proportional to the temperature to which it
exposed.
R T
where : R is resistance of the electrically
conductive materials
T is the temperature in which
it exposed
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Resistance Temperature Detectors (RTD)
RTD have the following advantages as compared to other types of temperature sensors:
1.
The resistance verses temperature linearity characteristics of RTD are higher.
2.
They possess greater accuracy (as high as 0.1 oC ). Standard platinum resistance
thermometers have ultra high accuracy of around 0.0001 oC .
3.
It has excellent stability over time.
4.
Temperature sensitive resistance elements can be replaced easily.

The popularity of platinum is due to the following factors:
1.
Chemical inertness
2.
Almost linear relation ship between temperature and resistance
3.
Large temperature coefficient of resistance, resulting in readily measurable values
of resistance changes due to variations in temperature
4.
Greater stability because the temperature resistance remains constant over a long
period of time
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Thermistors
•
A thermistor is a type of resistor whose resistance is dependent on
temperature, thermister means combination of thermal and resistor.
•
The relationship between temperature and resistance given by the following
equation:
R  RR e
•
1 1 

 T TR 
 
The temperature coefficient of resistance is given by: dR dT
R


T2
•
The temperature coefficient of platinum at 25 oC is +0.0036/K and, for
thermisters, it is generally around -0.045/K, which is more than 10 times
sensitive when compared to platinum.
•
A variety of ceramic semiconductor materials qualify as thermistor
materials.
•
Among them , germanium containing precise proportions of arsenic,
gallium or antimony is most preferred. The temperature measurement range
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of thermistors is -250 to 650 oC
• The resistance temperature coefficient of the thermistor is shown in the figure
below.
Resistance Temperature Characteristics
• The graph shows that the thermistor has a negative temperature coefficient,
i.e., the temperature is inversely proportional to the resistance.
• The resistance of the thermistor changes from 105 to 10-2 at the temperature
between -100 oC to 400 oC.
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Liquid-in-glass Thermometers
•
In the LIG thermometer the thermally sensitive element is a liquid
contained in a graduated glass envelope.
•
The principle used to measure temperature is that of the thermal
expansion of liquid.
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Pressure Thermometers
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Pyrometers
(Non-contact type Temperature Measuring instrument)
 Total Radiation Pyrometer
 Optical Pyrometer
 Infrared Pyrometer
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Pyrometers
•
If E is the radiation impinging on a body (W/m2), a is the fraction of
radiation power absorbed by the body, and e is the total emissivity, then
e
•
E
E

Radiation emitted from a body
Radiation falling upon the body
The Stefan-Boltzmann law states that the total energy radiated by a black
body is a function of its absolute temperature only and is proportional to
the fourth power of that temperature.
E  Ta 4
•
or
E   Ta4
If e is the emissivity of the body at a given temperature,
E  e Ta4
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Total Radiation Pyrometers
•
The total radiation pyrometer receives all the radiation from a particular
are of hot body.
•
The term total radiation includes both the visible and invisible radiations.
•
It consists of radiation receiving element and a measuring device.
•
The diaphragm unit along with a mirror is used to focus the radiation on a
thermocouple.
•
The distance between the mirror and the thermocouple is adjusted for
proper focus.
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Total Radiation Pyrometers
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Optical Pyrometers
•
Optical pyrometers work on the disappearing filament principle.
•
In order to measure temperature, the brightness generated by the radiation
of the unknown source or hot body whose temperature is to be
determined is compared with that of reference lamp.
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Optical Pyrometers
Brightness
Filament
1. Filament is dark: that is, cooler than the temperature source.
2. Filament is bright: that is, hotter than the temperature source.
3. Filament disappears: Thus, equal brightness between filament and
temperature source.
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Optical Pyrometers
Advantages:
1. Simple assembling of the device enables easy use of it.
2. Provides a very high accuracy with +/-5 degree Celsius.
Applications:
1. Used to measure temperature of liquid metals or highly heated materials.
2. Can be used to measure furnace temperature.
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Infrared Pyrometers
•
IR pyrometer is a non contact type sensor that can detect infrared
radiation from a heated body.
•
It essentially measures the amount of radiation emitted by an object.
•
Infrared pyrometers are ideally suited for high temperature measurement.
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The End
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