We discuss the working principle and construction of different temperature sensors like radiation pyrometer ,filled system thermometer and bimetallic thermometer.their advantages disadvantages and industrial application etc.

1. 1
Abstract
We discuss the working principle and construction of different temperature sensors like
radiation pyrometer ,filled system thermometer and bimetallic thermometer.their advantages
disadvantages and industrial application etc.
Introduction
Temperature sensors are used to measure temperature in circuits which control a wide variety
of equipment. Various processes require temperature monitoring for effective control. Such
processes include manufacturing processes, transportation, security, maintenance, and other
types of processes during which monitoring the thermal characteristics of devices is necessary
or advisable. Temperature sensors are widely used in many fields, such as household electrical
appliances and medical appliances.
Body
Radiation pyrometers
Pyrometer relies on a quantitative measurement of the radiation which is emitted from an object. The
main advantage of pyrometers is that they work without physical contact with the hot object. The two
types of pyrometers use are the optical pyrometer and the radiation pyrometer.
Radiation pyrometers use a radiation detector which, when pointed at an object detects the amount of
infrared radiation impinging on the detector. The temperature of the detector is measured (usually with
a thermopile or other electronic device) and the radiation emitted from the source is inferred.
An optical pyrometer works by comparing the visible radiation that is emitted from a radiation source to
the visible radiation emitted from a filament wire. The current supplied to the filament wire is adjusted
until the wire "disappears", inferring that it is at the same temperature as the object whose temperature
is being measured. The temperature of the filament wire is a known function of the supplied current and
therefore the temperature of the object is inferred.The radiation pyrometer primarily based upon the
Stefan-Boltzmann equation of energy transfer by radiation from a black body
Temperature sensors
(Radiation pyrometers,Filled system thermometer and
bimetallic thermometer)

2. 2
Where J is the total amount of energy radiated per unit area and unit time from a black body at an
absolute temperature T, and is an empirical constant the value of which depends only on the units of
measurement.
The energy received by a total radiation pyrometer may be measured in a variety of ways:
calorimetrically e.g. certain pyrheliometers; thermoelectrically e.g. the thermopile; electrically e. g. the
bolometer; mechanically e.g. the angular deflection of a bimetallic spiral spring or the elongation of a
metallic strip; and radiometrically, e.g. the pressure of radiation exerted on delicate vanes mounted in
vacuum etc.
The thermoelectric and the mechanical (bimetallic spring) methods are the only total radiation methods
which have been quite generally applied strictly for the purpose of temperature measurement.
The quantity of energy a body receives by radiation from another body depends on certain conditions
relative to each of the two bodies and area of surface, distance apart, emissive and absorbing power
and temperature.
Energy receivers may be divided into three classes, as follows
(1) A black receiver is one which absorbs all the energy falling upon it and reflects none whatever
be the wave length of the incident radiation. Its absorption coefficient is accordingly unity.
(2) A gray receiver is one having an absorption coefficient which is independent of the wave
length of the incident radiation the value of the coefficient being less than unity.
(3) A selective receiver is one having an absorption coefficient which is afunction of the wave length
of the incident radiation.
The errors which may occur in total radiation pyrometry may be classified as follows:
(1) Limitations or approximations of the fundamental formulas
(2) Imperfections of the radiating source or uncertainties in its radiometric properties
(3) Effects of the intervening medium i. e. air more or less charged with water vapor and gases such
as CO and C02
(4) Construction of the pyrometric receiver
(5) Errors of the measuring or recording instruments.
In the ideal radiation pyrometer the energy J received from the radiating source at an absolute
temperature T, by the receiver at an absolute To ,is proportional to the factor ( T4-To
4).
J=const * ( T4-To
4)
as follows directly from the Stefan-Boltzmann radiation law Various factors enter, however, into the
actual construction of the radiation pyrometer which slightly alter this ideal relation. For example,
consider the thermoelectric type of radiation pyrometer, in which the energy of the radiator is indirectly
measured by the emf developed in a thermoelectric circuit.Here the emf developed is not exactly
proportional to the temperature of the receiver and the temperature of the receiver is not exactly

3. 3
proportional to the energy received. mechanical defects in construction may cause deviations from the
ideal condition. Stray reflection, selective reflection, and convection currents in the pyrometer must
necessarily vary in magnitude, depending upon the temperature of the radiating source. The
temperature of the hot junction of the thermocouple T increases with T4
and the relation
approximately linear.The loss of energy expressed as a fraction of the energy incident at the receiver is
entirely different for different values of Tc , not because of changes in radiation from the receiver, but
mainly because of the different rates of energy loss by conduction and by convection currents, i. e.,
departure from Newton's law of cooling.For these reasons the radiation pyrometer does not follow
exactly the Stefan-Boltzmann radiation law
TYPES OF RADIATION PYROMETER
1. MIRROR AND THERMOCOUPLE PYROMETER
2. MIRROR AND SPIRAL SPRING PYROMETER (FERY SPIRAL PYROMETER)
3. LENS AND THERMOCOUPLE PYROMETER (FERY LENS THERMOELECTRIC PYROMETER)
4. CONE THERMOELECTRIC PYROMETER
In the ordinary use of a thermoelectric radiation pyrometer a galvanometer is employed for the
measurement of emf, but for the highest accuracy a potentiometer use to the measurement of small
electromotive forces is desirable. Potentiometers are now available for the measurement of emf 's as
small as 0.000 1 millivolts In the use of a potentiometer the resistance or length of the lead wires from
the pyrometer the resistance of the thermocouple and the variation with temperature in the resistance
of the pyrometer circuit produce no effect whatever upon the emf reading.
When a radiation pyrometer is exposed to the radiation from a source at a constant temperature the
pyrometer does not immediately indicate the temperature of the source but exhibits a certain time lag
during which the receiving system is heating up and the receiver emits or loses by conduction radiation
and convection as much heat as it receives and a condition of equilibrium is maintained between the
source and the receiver.
If the radiation pyrometer is to be used with a galvanometer it is desirable that both the resistance of
the thermocouple and its variation in resistance with the temperature of the source be small.
EFFECT OF DIRT AND OXIDATION UPON THE CONDENSING DEVICE
Pyrometers subjected to severe use in steel mills and other industries soon become coated with dust
and dirt The importance of keeping the mirror free from dirt is therefore evident. When necessary the
mirror may be taken from the telescope and carefully washed with water.
EFFECT OF DISTANCE AND SIZE OF SOURCE AND INCREASING THE FOCUSING DISTANCE
Reading increases on account of
1. Variable aperture
2. Shading of concave mirror by thermocouple box

4. 4
Reading decreases on account of
1. Atmospheric absorption
2. Convection currents from source to couple box receiver
3. Stray reflection in receiver and telescope tube
4. Reradiation to couple from side walls of pyrometer
5. Image of source becoming smaller
APPLICATIONS
1 DETERMINATION OF TOTAL EMISSIVITY OF NONBLACK MATERIALS
2 THE DETERMINATION OF TEMPERATURES

5. 5
Filled-system thermometers
Filled-system thermometers are thermometers that are filled with any of the matter used and use
the phenomenon of thermal expansion of matter to measure change in temperature.
The filled thermal device consist of a primary element that takes the form ofReservoir or bulb, a
flexible capillary tube, and a hollow bourdon tube that actuates a signal-transmitting device. In
this system, the filling fluid, either liquid or gas, expands as a temperature increase. This cause
the bourdon tube to uncoil and indicates the temperature on a calibrated dial. Thermometer of this
type are commonly used in industry in the temperature range from - 60° to 550°C. With long
capillaries of up to 60m such thermometer may be used for remote temperature measurments .
Principle of operation
The operation of filled-system thermometer is based on one of three principles:
the thermal expansion of liquid,the temperature depends on the pressure of a
gas, or the temperature depends of the saturated vapor pressure of the liquid.
The deformation of the bourdon tube which depend on the pressure of a gas or
on the volume of a liquid filling the system would indicate the temperature on
the calibrated dial .
Classification
1. Mercury-filled
2. Liquid-filled
3. Gas-filled
4. Vapor-filled
But they generally come in two main classification : the mercury type and the organic-liquid
type. Since mercury is considered an environmental hazard, soThere are regulations governing
the shipment of that type of devices that contain it. Now a day, there are filled system
thermometers which employ gas instead of liquids
Liquid-filled
A liquid system completely fills with liquid. This type of system operates on the
principle that expands with an increase in temperature. When the liquid expands
It cause the pressure to increase, which cause the bourdon tube to uncoil and
move the needle on scale Typically, inert hydrocarbons such as xylene see more
use because of their low coefficient of expansion. In some cases, you can
even use water. Another common liquid is mercury.

6. 6
Vapor-filled
A vapor system contains a volatile liquid and vapor and operates on the principle
That pressure in a vessel containing only a liquid and its vapor increase with
temperature and is independent on volume. With a vapor system, you measure
temperature at the interface between the liquid and the vapor. For a vapor system
to operate properly, the interface must remain in the bulb.
Four subclasses of liquid system exist. Class IIA operates with the measured
temperature above the temperature of the rest of the system. The class IIB
system operates with the measured temperature below the temperature of the
rest of the system. The class IIC vapor system measures temperatures above
and below the temperature of the system. Because of cross-ambient effect,
vapor system thermometers often see use either exclusively below ambient
or exclusively above ambient.The class IID vapor system can over come the
cross- ambient limitation by using the second nonvolatile liquid.
Gas-filled
Gas-filled system see use in industrial applications. And in some cases, in
laboratory measurments. The operation of gas-filled system is based on the Ideal
gas law, and their measurments is thus an approximation at normally
encountered temperatures and pressures. In a typical gas-filled system, the
gas (usually nitrogen) is not perfect, so their may be a slight change in volume

7. 7
Mercury-filled
It consists of a bulb containing mercury attached to a glass tube of narrow
diameter ; the volume of mercury in the tube is much less than the volume in
the bulb. The volume of the mercury changes slightly with temperature; the
small change in the volume drives the narrow mercury column a relatively long
way up the tube.
General industrial applications
1. Petroleum industries
2. Storage facilities need to know the temperature of the material in tanks.
3. Various stages of refinement.
Advantages
1. They do not require any electric power .
2. They do not pose any explosion hazard
3. They are stable even after repeated cycling.

8. 8
Disadvantages
1. They do not generate data that are easily recorded Or can be transmitted.
2. They do not make spot or points measurements
BIMETALLIC THERMOMETER
A bimetallic strip is used to convert a temperature change into mechanical displacement. The strip
consists of two strips of different metals which expand at different rates as they are heated,
usually steel and copper, or in some cases steel and brass. The strips are joined together throughout
their length by riveting, brazing or welding. The different expansions force the flat strip to bend one way if
heated, and in the opposite direction if cooled below its initial temperature. The metal with the
higher coefficient of thermal expansion is on the outer side of the curve when the strip is heated and on
the inner side when cooled
As a temperature measuring device the bimetallic element similar in design to that of the actuator can
be used to determine the ambient temperature if the degree of bending can be measured. The
advantage of such a system is that the amount of bending can be mechanically amplified to produce a
large easily measurable displacement.
The basic principle of a bimetallic thermometer is shown in Figure Here, two metal strips of differing
thermal expansion are bonded together. When the temperature of the assembly is changed in the
absence
of external forces the bimetallic strip will take the shape of an arc. The total displacement of the strip
out of the plane of the metal strips is much greater than the individual expansions of the metallic
elements. To maximize the bending of the actuator, metals or alloys with greatly differing coefficients of
thermal expansion are normally selected. The metal having the largest thermal expansitivity is known as
the active element, while the metal having the smaller coefficient of expansion is known as the passive
element. For maximum actuation, the passive element is often an iron–nickel alloy, Invar, having an
almost zero thermal expansivity (actually between 0.1 and 1×10
–6
K
–1
, depending upon the
composition).

9. 9
The active element is then chosen to have maximum thermal expansivity given the constraints of
operating environment and costs.In addition to maximizing the actuation of the bimetallic element,
other constraints such as electrical and thermal conductivity can be made. In such cases, a third metallic
layer is introduced, consisting of either copper or nickel sandwiched between the active and passive
elements so as to increase both the electrical and thermal conductivity of the actuator. This is especially
important where the actuator is part of an electrical circuit and needs to pass current in addition to
being a temperature sensor.
Different common forms of bimetallic sensors are listed
1. Helix type.
2. Spiral type.
3. Cantilever type.
4. Flat type
Linear Bimaterial Strip
The analysis of the stress distribution and the deflection of an ideal bimetallic strip was first deduced by
Timoshenko the general equation for the curvature radius of a bimetallic strip uniformly heated from T0
to Tin the absence of external forces is given by
the width of the strip is taken as equal to unity

10. 10
The principle of operation of the bi metalic thermometer is an application of the expansion of a solid
material caused by a change in temperature. The expansion coefficient relates the change in length
of a solid material to a change in temperature T2 - TI as follows:
where L1 is the length of metal at temperture T1 and L2 is the length at final temperature T2 .
Temperature-measuring instrument requires two metals with substantially different thermal expansion
coefficients so that the difference in elongation is large for relatively small changes in temperature. In
general, such a temperature probe consists of two parallel members of dissimilar materials a and b
joined together at one end so that a change in temperature along the probe length produces a
difference in elongation at the free ends of the two materials. When the temperature along the probe
length is nonuniform the total difference in elongation at the free ends is a summation of local
differences as generated by local temperatures along the probe. Thus, the total difference in elongation
becomes a measurement of the average temperature along the length of the probe.The difference in
elongation of the probe members a and b is related to temperature by the following equation for
L1,a = L1,b = L1 at temperature T1
where
a,b probe materials
L1 length of materials a and b at temperature T1
difference in elongation of materials a and b at temperature T2
T1 reference temperature
T2 final temperature
mean thermal expansion coefficient of material a for temperature range T1 to T2
mean thermal expansion coefficient of material b for temperature range T1 to T2
Advantages
1. They are simple, robust and inexpensive.
2. Their accuracy is between +or- 2% to 5% of the scale.
3. They can with stand 50% over range in temperaures.
4. They can be used where evr a mecury –in-glass thermometer is used.

11. 11
Limitations
1. They are not recommended for temperature above 400’C.
2. When regularly used, the bimetallic may permanently deform, which inturn will introduce errors
Industrial Applications
A direct indicating dial thermometer (such as a patio thermometer or a meat thermometer) uses a
bimetallic strip wrapped into a coil. One end of the coil is fixed to the housing of the device and the other
drives an indicating needle.they are low cast and easy to install.
Heating installations, heating technology,combustion and industrial plants,engine, machine and ship-
building turbines, ovens, ventilation and air-ducts,fluegas measurement (chimney sweeping),
refrigeration, breweries, galvanizing, photo developing fluids
Results and discussions
The start of the topic is an account of the principles which form the basis for the operation of total
radiation pyrometers, and types of this instrument, together with the results of an experimental study
of their calibration and behavior under various conditions of use, and as modified by changing the
several factors which may influence the readings of such pyrometers. A considerable portion of the text
is devoted to the examination of the sources of error and their elimination or correction. Finally, there is
considered the application of the radiation pyrometer to the determination of the total emissivity of
nonblack substances and to the measurement of temperatures.
Many physical properties change with temperature, such as the volume of a liquid, the length of a metal
rod, the electrical resistance of a wire, the pressure of a gas kept at constant volume, and the volume of a
gas kept at constant pressure. Filled-system thermometers use the phenomenon of thermal expansion of
matter to measure temperature change.

12. 12
All metals change in dimension, that is expand or contract when there is a change in
temperature. The rate at which this expansion or contraction takes place depend on the
temperature co-efficient of expansion of the metal and this temperature coefficient of
expansion is different for different metals.Hence the difference in thermal expansion rates is
used to produce deflections which is proportional to temperature changes.
Conclusions
Radiation pyrometer are use to maesure high temperatuer where physical contect is not
possible and difficult to use in dusty condition.emissivity depend on
temperature,wavelength,shape,angle and the texture of the surface and we can not find the
temperature of the objects with unknown emissivity.Pyrometer are expensive due to their
complex structure.
Filled system thermometer and bimetallic thermometer are measure the temperature due to
direct contect with the system they have certain ranges with in which they can measure the
temperature and their accuracy is between +or- 2% to 5% of the scale . they are inexpensive and
simple in construction and use.
List of symbols
empirical constant
J total amount of energy radiated per unit area and unit time
To absolute temperature
T Temperature
mean thermal expansion coefficient of material a for temperature range T1 to T2
mean thermal expansion coefficient of material b for temperature range T1 to T2
a,b probe materials
L1 length of materials a and b at temperature T1
difference in elongation of materials a and b at temperature T2
expansion coefficient

13. 13
Acknowledgements
This work was done with the help of my grope members and we acknowledge Dr. Faizan Ahmad
for their assignment.
References
1. Hidnert, Peter; and Souper, Wilmer: Thermal Expansion of Solids. Optics,
Metrology, and Radiation. Vol. 3 of Precision Measurement and Calibration.
Handbook 77, vol. 3, National Bureau of Standards, Feb. 1, 1961, pp, 349/1-
380/29.
2. H.B. Callen, Thermodynamics and an Introduction to Thermostatistics, 2nd ed.,
New York, John Wiley & Sons, 1985
3. J.E.K. Schawe, Principles for the interpretation of modulated temperature DSC
measurement. Part1. Glass transition, Thermochim. Acta,261, 183-194, 1995.
4. Measurement of Thermal Radiation of Solids Edited by J. C. Richmond, NASA
Special Publication SP-31 , U.S. Government Printing Office, (1963)
5. Noncontact Temperature Measurement, Edited by M.C. Lee, NASA Conference
Publication 2503, U.S. Government Printing Office (1987)
6. http://www.temperatures.com/sensors.html
7. http://www.globalspec.com/reference/10946/179909/chapter-7-temperature-
measurement-bimetallic-thermometers
8. Dougherty, E.P. ,Temperature Control Principles for Process Engineers, Hanser
Publishers, 1993 ISBN 0344615980
9. Liptak,B.G. ,Instrument Engineer's Handbook, 4th
Edition,Volume 2. Taylor and
Francis Group,2005 ISBN 0849310814