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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)

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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-To4).

J=const * ( T4-To4)

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

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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

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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

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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.

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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

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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.

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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).

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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

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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.

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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.

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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

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Acknowledgements

This work was done with the help of my grope members and we acknowledge Dr. Faizan Ahmad

for their assignment.

References

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2. H.B. Callen, Thermodynamics and an Introduction to Thermostatistics, 2nd ed.,

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