7. temperature measurement

8/1/2019 7. Temperature Measurement

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DBIT Pvt. circulationNotes by Prof.S.P.Sabnis

1

Temperature measurement

To most people, temperature is an intuitive concept which tells whether abody is hot or cold. In second law of thermodynamics temperature isrelated to heat, as it is observed in nature that heat flows only from high

temperature to low temperature. In kinetic theory of gases and statistical thermodynamics it is shown that

temperature is related to average kinetic energy of the molecules of anideal gas. A further extensions of statistical thermodynamics show therelationship between temperature and energy level in liquids and solids.

Temperature is a fundamental quantity, similar to mass, length and time.However unlike other quantities direct comparison with standards is notpossible in case of temperature,

The law that is used in temperature measurement is known as the Zerothlaw of thermodynamics. This states, if two bodies are in thermalequilibrium with a third body, then they are all in thermal equilibrium witheach other. In other words the three bodies have the same temperature.

It is therefore required to have a third body (a Thermometer) which can beconfirmed to have a thermal equilibrium with other two bodies then the twobodies can be said to have same temperature.

Since direct comparison of temperature with a standard is not possible it isdifficult to establish a scale of temperature as a standard for comparison

with the same. A theoretical scale was proposed by Lord Kelvin on thebasis of second law of thermodynamics and concept of an ideal reversibleCarnot cycle. The reversible heat engine working on Carnot cycle takesan amount of heat Q2 from a reservoir of infinite capacity at temperatureT2 and supplies an amount of heat Q1 to another reservoir at temperatureT1 according to relation

Q2 = T2Q1 T1

Kelvins thermodynamic scale is absolute in the sense that it isindependent of any material properties. But it is not realisable because ofits dependence on an ideal cycle. Fortunately, it can be shown thattemperature scales based on constant volume or constant pressure gasthermometer using an ideal gas is identical to the thermodynamic scale.


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International Temperature Scale 1990 (ITS-90)

It should be noted that Ideal gas is a theoretical concept and in practice the

Ideal gas thermometers have shortcomings. According to ideal gas therory

1. Gas molecules are point masses, occupying no space

2. Collisions between them are elastic

3. No intermolecular force acts between them.

Unfortunately no real gas satisfies the above description. For all practicalpurposes, temperature measurement by gas thermometers is cumbersome.

Therefore an International practical temperature scale (IPTS) is defined at

which thermal transducers can be calibrated. (in 1927 for the first time)


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The latest standard ITS-90 defines a number of fixed referencetemperature points as shown in the table. Between these fixed points,elaborate interpolation equation are specified by ITS-90for use withvarious interpolation standards.

From 0.65 K to 5.0 K standard is based on measurement of vapour

pressure of helium and use of equation relating vapour pressure withtemperature

3.0 K to 24.5561 K (Triple point of neon) a constant volume helium gasthermometer is used.

Over a broad range of 13.8033 K (triple point of hydrogen) to the normalfreezing point of silver (1234.95 K) the standard is defined by means of aplatinum resistance as a function of temperature. Elaborate process ofcalibration at several fixed point temperatures for platinum resistancethermometer to cover various sub ranges is provided by the standard.

Finally above the melting point of silver, temperatures are determined bymeasurement of the thermal radiation emitted by a black body cavity invacuum and the plank radiation law.

The radiant energy is typically measured by optical pyrometry. Theunknown temperature is then calibrated by comparing the emission of asource at the unknown temperature to that from a source at the referencetemperature.


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Temperature scales

Two temperature scales which are in common use are Celsius andFahrenheit scales. These are based on freezing point and boiling point ofwater at standard atmospheric pressure. The Celsius scale has 100 unitsbetween these points and Fahrenheit scale has 180 points. They are

related by equation

The absolute temperature scales based on the thermodynamic idealCarnot cycle has been correlated with Celsius and Fahrenheit scales asfollows

The zero points on both the absolute scales represent the same physicalstate and the ratio of two values is the same regardless of which absolutescale is used i.e.

Freezing point and boiling point of water at a pressure of std oneatmosphere (101.3 kN/m2) are taken as 0o and 100o on Celsius scale and32o and 212o on the Fahrenheit scale.

The relation ship between Rankine and Kelvin scale is as follows.


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Measurement of Temperature

Temperature cannot be measured directly but must be measured byobserving the effect that temperature variation causes on themeasuring device. The temperature measurement methods are

broadly classified as1. Non electrical methods

2. Electrical methods

3. Radiation methods

Non electrical methods are based on one of the following principles

1. Change in Physical state - Temp. at which a pure substance changetheir physical state are used for calibration of temperature scales butthese devices give one particular value of unique temperature and

hence are not suitable for measurement of temperatures over a range.2. Change in Chemical properties For using the substances which

change chemical properties, we have to look for a reversible processin order to have a repeatable/reproducible scale with respect tochange in temp. However such processes are very few in number

3. Change in physical properties Most non electrical methods arebased on change in physical properties e.g. thermal expansion. Sincethese devices have no electrical connections they can be used inexplosion risk areas such as petrol storage tanks.

e.g. Bimetallic thermometers, Liquid in glass thermometers, Pressurethermometer mercury in steel, constant volume inert gasthermometer, vapour pressure thermometer.

Electrical Methods are mainly of two types.

1. Thermo resistive variable resistance transducers e.g. mettalicresistance, semiconductor (thermistor)

2. Thermo electric type emf generating type. E.g. thermocouples, solidstate temp. sensors, Quartz thermometer

Radiation methods (Pyrometry)

1. Total radiation pyrometer2. Selective radiation pyrometer


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Bimetallic Thermometer

This type of thermometer employs the principle of solid expansion andconsist of a bimetal strip usually in the form of a cantilever beam. Thiscomprises strips of two metals, having different coefficients of thermalexpansion, welded or riveted together so that relative motion between

them is prevented. An increase in temperature causes the deflection ofthe free end of strip.

When a bimetal strip is formed by firmly bonding two metal strips A & Bhaving different thermal coefficient of expansion a & b. If this bimetalstrip is a straight line at temperature T1, then at elevated temperature T2the strip will form a uniform circular arc of radius of curvature r such that

Where t is the thickness of the strip

Different forms of bimetal such as

cantilever, U-Tube, spiral etc. are

used


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The deflection with temperature is nearly linear, depending mainly on coeff. of

linear expansion. Invar is generally used as the low expansion metal. This is an

iron / nickel (36%) alloy. Its coefficient of expansion is around 1/20th of ordinary

metals (nearly zero). Brass is used as high expansion metal for measurement

of low temperatures, whereas nickel alloys are used when high temperatures

are to be measured.

A plain bimetallic strip has less sensitivity, however the sensitivity can be

improved by using a longer strip in a helical form as shown above. One end of

helix is anchored to the casing and the other end which is free is connected to

the pointer which sweeps over a circular dial graduated in degrees of

temperature. A long stem can be provided between pointer and the free end of

bimetal element so that, the element enclosed in protective sheath, can be

submerged in the hot area and dial and pointer can be located at safe distance.

Bimetallic thermometers are usually employed in the range of -30 to 550oC,

with inaccuracies of the order of +0.5 to +1% of FSD in highly accurate

thermometers. In addition to temperature indication, they are also used in

application for combined sensing and control purpose. Control action is usually

on-off type (thermostats), where the strip has sufficient force to actuate a control

switch in electric ovens, irons and refrigerators. These can also be used as

compensating element for ambient temperature is pressure thermometers,

aneroid barometers and watches. The bimetallic strip has advantage of being self-generating type instrument

with low cost, practically no maintenance expenses and stable operation over

extended period of time. Main disadvantage is its inability to measure rapidly

changing temperatures due to relatively high thermal inertia.


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Liquid in glass Thermometers

The liquid in glass thermometer is one of the most common temperature

measuring devices. Both liquid and glass expand on heating and their

differential expansion is used to indicate the temperature. The lower limit for

mercury is -37.8oC while -130oC for pentane. The higher temperature range is

340oC (boiling point of mercury is 357oC) This may be extended to 560oC byfilling the space above mercury with CO2 or N2 at high pressure to increase

the boiling point and the range.

The precision of these thermometers depend on the care used in

calibration. The calibration should also be checked regularly to take into

account aging effects. Accuracy of these thermometers does not exceed

0.1oC. However when increased accuracy is required Beckmann range

thermometer can be used. It has big bulb attached to a very fine capillary. The

range of this thermometer is limited to 5-6oC with an accuracy of 0.005oC

Liquid in glass thermometers have qualities like low cost, simplicity in useportability and convenient visual indication without use of any externalpower. However their use is limited to laboratory applications.

It is not preferred in industrial applications because of its fragility and its

adaptability to remote indication. Also it introduces a time lag in dynamicmeasurements due to thermal inertia of the bulb.


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Filled System Thermometers (Filled Thermometers)

Filled system thermometer works on pressure or volume change of a fluid,gas or changes in vapour pressure of a liquid. It consists of four parts

1. Bulb

2. Capillary Tube3. Pressure or volume sensitive element

4. Indicating Device.

The capillary tube connects the bulb containing a fluid that is sensitive to

temperature changes to the element that is sensitive to pressure or volume

changes. The pressure sensitive or volume sensitive element may be a

Bourdon tube, a helix of bourbon tube, a diaphragm or bellows.

The gas filled systems are sometimes called gas thermometers or pressure .

thermometers.


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Operating principle

The volume expansion of a liquid is given by the equation

Where VT and Vo are the volumes of the liquid at temperatures T and 0oC

respectively. and is the coefficient of volume expansion.

The bulb which contains the liquid also expands or contracts with the

variation of temperature. Those these expansion / contraction are much

smaller than those of the liquids, they introduce some degree on non linearity

in the relation.

For the gas filled system, Boyles law is guiding principle. Hence

In this case the absolute (Kelvin) temperature will show linear relationshipwith the pressure. The non linearity introduced by volume change of bulbwith temperature is negligible.

Classification of filled system thermometers is made by SAMA(Scientific Apparatus Makers Association, USA. Into four classes as perthe table below.


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Class I (Liquid filled thermometers Other than mercury)

The liquids generally used in these thermometers are alcohol, pentane and

toluene. The minimum temperature that can be measured by such system

depends on the freezing point of the organic liquid used. This lies usually

between -200oC and -75oC depending on the liquid used.

The maximum temperature depends, not only on the boiling points ofliquids but also on the linearity of their volume expansion at higher temperatures.

For organic liquids usually 300oC is the upper limit.

Class V (Mercury or Mercury-Thallium eutectic amalgam filled)

The mercury in steel thermometer has a near linear characteristic. Sufficient

power is available to operate a recording pen if required. The range of operation

is from -38oC to 650oC

Class II Vapour Filled Thermometer (vapour pressure thermometer)

The system in the vapour pressure thermometer is partly filled with liquid andpartly with vapour of the same liquid, so that there is a liquid vapour interface in

the bulb. The liquid vapour system does not have any error as long as a free

liquid surface exists in the sensing bulb, since such system follows Daltons law

of partial pressure which states that if both liquid and vapour are present, there

is only one saturation pressure corresponding to the given temperature. The

general usefulness of this thermometer is restricted due to limited number of

liquids providing suitable saturation vapour pressure ranges. Commonly used

liquids are ethane, ethyl alcohol, ethyl chloride, methyl chloride, chlorobenzene.

toluene, pentane etc. Scale range is of the order of 100oC and accuracy up to

+1%. The pressure is roughly logarithmic function of temperature

log P = a b/T

Thus the scale is non linear.

Class III Gas Filled Thermometer (Constant volume thermometer)

This uses an inert gas (usually N2), the principle of working is pressure of the

gas increases as temperature is increased while operating at constant volume.

In reality the volumes of bulb, capillary and Bourdon tube increase with pressureand temperature increase.

Operating range -130oC to 540oC mostly linear and accuracy of the order of +1%

at lower range and +2% above 300oC


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In full compensation both Bourdon tube and Capillary are covered. The fullcompensation is necessary if the capillary length is greater than 3 m.

Despite compensation, to obtain sufficiently accurate data from sucharrangements, the volume of the temperature sensing bulb has to besufficiently larger than the Bourdon tube.

Another potential cause of error is the change in pressure head which is

introduced by change in relative levels of the bulb and display. If the bulb is

raised by height h there is an increase in pressure equal to gh. This error is

constant for specific setup and may be removed by means of zero adjustment.

Errors in measurement with filled systems :The filled system thermometers are vulnerable to errors arising out of thedifferential expansion of fluid contained in the bulb and also in the capillarytubeand the Bourdon tube, which are subjected to ambient temperatures. These

errors are minimised through case compensation and full compensationmethods.

In case compensation onlythe error arising out of thedifferential expansion of theFluid in the Bourdon tube isCompensated for.


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Temperature measurement using changein electrical properties

Electrical methods in general are preferred for the measurement oftemperature as they furnish a signal which can be easily detected, amplifiedor used for control purposes

The two main electrical methods are

Thermo resistive i.e. variable resistance transducer

Thermo electric i.e. emf generating transducers

Electric Resistance Thermometers

In resistance thermometers, the change

in resistance of various materials, which

varies in reproducible manner with

temperature, forms the basis of this

technique. The material in actual use

Fall in two classes namely, conductors

(metals) and semiconductors.

In general, the resistance of the highly

conducting materials (metals) increases

With increase in temperature. Whereas

The resistance of semiconductor materialsgenerally (not always) decreases with

increase in temperature. These are called

negative temperature characteristic (NTC)

Thermistors.

The fig shows typical variation of specific resistance of the metals (e.g.

Platinum) and the NTC Thermistor.


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Resistance Temperature Detectors (RTDs) Metallic resistance

thermometers are commonly known as RTD. The relation between electrical

resistance of a metal RT and the correspondingtemperature T is generally

given as

RT = Ro (1 + C1T + C2 T2 + .. + CnT

n)

Where the Cs are constants and Ro is the resistance at temperature T =0oC

Although the above relationship is non linear it can be seen from the fig that

the curve is nearly linear for Copper and Platinum over a long range.

The copper being easily susceptible to chemical reactions in process industry

platinum is preferred for the RTDs. The platinum resistance thermometers are

also referred as PRT, hence the terms RTD and PRT are used interchangeably.

Platinum is very widely used sensor and its operating range is from 4 K to

1064oC. Because it provides extremely reproducible output, it is used in

establishing International Practical Temperature Scale from 13.8K to 961.93o

C.RTDs are very suitable for both laboratory and industrial applications because of

high degree of accuracy as well as long term stability, wide operating range and

linear characteristics. Limitations are low sensitivity, higher cost compared to

other sensors and errors due to contact resistance, shock & accelerations.


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Construction :- Metallic resistance Thermometers (RTDs) are constructed

in many forms , but the temperature sensitive element is usually in the form

of a coil or fine wire supported in stress free manner. A typical construction is

shown below.

The wire of metal is wound on a grooved ceramic former and is

covered with protective cement. The ends of the coils are welded to stiff

copper leads that are taken out, to be connected in a Wheatstone bridgecircuit. Most of the times there is a protective metal sheath above the

cemented portion to provide rigidity and mechanical strength.

Alternatively RTD sensors may be fabricated by depositing thin films of

platinum, nickel or copper on a ceramic substrate. These thin film sensors

have the advantage of extremely low mass and consequently more rapid

thermal response.

Platinum, in spite of its low sensitivity and high cost compared to

nickel and copper, is the most widely used material for RTDs because of

following

1. The temperature resistance characteristics of pure platinum are welldefined and stable over a wide range of temperatures

2. It has high resistance to chemical attack and contamination.

3. It has very good accuracy and reproducibility. +0.01oC upto 500oC and+0.1oC up to 1200oC.

The variation of resistance of sensing element is normally measured using

some form of electrical bridge circuit which may employ either deflection mode

or null mode of operations. Lead length and their resistance are sometimescritical hence should be kept to minimum. In addition some modifications may

be required for lead compensation. It is also essential that thermoelectric emfs

do not affect the system and hence techniques like ac excitation or manually

varying dc polarity are used.


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Cable Compensation arrangements for RTD:- One of the primary sources

of error in the electrical resistance thermometer is the effect of the resistance

of the leads which connect the element to the bridge circuit. Some of the

arrangements that may be used to correct for this effect are shown in the fig.

above. The Siemens three lead arrangement is the simplest type. At balance

condition the central lead carries no current and the effect of resistance of the

other two leads is cancelled out. The Callenders four lead arrangement solves

the problem by inserting two additional lead wires in the adjustable leg of thebridge so that the effect of the lead wires on the resistance thermometer is

cancelled out.

Practical Problems with RTDs: are due to lead error and relatively bulky size

which is sometimes the reason for poor transient response and conduction error.

RTD also has a relatively fragile construction. As current has to be fed to the

RTD for bridge measurement, there is the possibility of self heating (=i2R) which

may alter the temperature of the element. This is especially prominent in air

temperature measurement where self heating effect is more prominent.


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Semiconductor Resistance Sensors (Thermistors)Thermistor (shortform of thermal resistor) is a thermally sensitive variable

resistor made of ceramic like semi conducting material. They are available in a

greater variety of shapes and sizes having cold resistance ranging from a few

ohms to mega ohms. The size range can range from extremely small bead, a

thin disc, thin chip of wafer to a large sized rod as shown in fig. Most thermistorsunlike metals respond negatively to temperature and their coefficient of

resistance is of the order of 10 times higher than of platinum and copper.

If R is the change in resistance corresponding to a temperature change ofT, approximately they are related as

R=TTWhere T is the temperature coefficient of resistivity. Thermisters can bedivided into two categories depending on whether T is positive or negative.Those with positive temperature coeff. are called PTC and those with negative

temperature coeff. are called NTC thermistors. Thermistors are tiny or small in

size and have a small temperature range with a high response and sensitivity,

hence they are quite different from RTDs.

Thermistors are fabricated from the semiconducting materials which

include the oxides of copper, manganese, nickel, cobalt, lithium and titanium.These are blended in a suitable proportion and compressed into desired shape

from powders and recrystalised them to form a dense ceramic body with

required characteristics.


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Positive temperature coefficient thermistors (PTC) : The commercial PTCs

are of two major categories. The first category is of thermally sensitive silicon

resistors (also known as silisters). They exhibit fairly uniform positive temp.

coeff. (about +0.77%/oC) over most of their operating range but may exhibit

negative coeff. at temperatures above 150 oC. These devices are used for

temperature compensation of silicon semiconducting devices in the range of

66 oC to +150 oC

The other category of PTC are made up of doped polycrystalline ceramic

Materials. These devices have a resistance-temperature characteristics that

exhibits a very small negative temp coeff until the device reaches a critical

temperature Tc, referred as Curie temperature. Below Curie point, it shows a

low resistance and a small negative temp coeff.. At curie point the resistance

increases sharply.

Other group of PTC

thermistors are made

with polymers and

carbon.grains. They are

called polyswitch, They

also exhibit similar sharp

rise in resistance with

Increase in temperature.

Thus the PTC type thermistors are mostly used for switching in temperature

controllers rather than proportional temperature measurement.

Negative temperature coefficient thermistors (NTC) : Owing to their larger

resistance change with temperature, NTC devices are usually more suitable

for precision temperature measurement, even though the relevant

characteristic curve is non linear.


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Construction: Oxides of Manganese, nickel or cobalt are milled & mixed in

proper proportion with binders, pressed into desired shapes and then sintered

to form thermistors in the form of rods, discs, flakes or beads.

Lastly, the wire leads are attached, and the combination is coated with

glass or epoxy. This coating provides mechanical strength and electrical

Insulation thereby reducing signal noise. The thermistor element and the

connecting wires along with the glass insulation are put inside a metal sheath

to form a thermistor probe as shown below.

By varying the mixture of oxides, it is possible to produce thermistors having

range of resistances from 30 to 20M (at 25oC). Speed of response is faster

in case of small thermistors. Beads are made as small as 0.3mm to 2mm.


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Thermistor resistances are usually specified at room temperature (i..e. 25oC).

The resistances of commercially available thermistors are usually 2252 ohms,

5000 ohms and 10000ohms. The standard temperature ranges are -55oC to

150oC. Longer range -100oC to 400oC are also available.

No special leads are required to connect thermistors because the deviceoperates at very high resistance compared to leads. Another aspect which has

to be considered is that the source of error is the temperature rise caused by self

heating.

Thermistors have following advantages for temperature measurement,

1. A large temperature coeff. Which makes thermistor extremely sensitivedevice, thus measuring accuracy up to +0.01oC is achievable.

2. Ability to withstand electrical and mechanical stresses

3. Fairly good operating range -100oC to 300oC4. Low cost and easy adaptability to the available resistance bridge circuit.

5. High sensitivity and availability in extremely small size (size of pin head)enable fast speed of thermal response.

6. They are very useful for dynamic measurements.

Disadvantages

1. Highly non linear resistance-temperature characteristics and

2. Problems of self heating which necessitate the use of much lower

current compared to metallic sensors.


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Thermoelectric Temperature MeasurementThe most common method of measuring and controlling temperature uses

thermocouple. A thermocouple consists of two electrical conductors that are

made of dissimilar metals and have at least one electrical connection. This

electrical connection is referred to as a junction.

A thermocouple junction can be created by welding, soldering, or by anyother method that provides good electrical contact between the twoconductors, such as twisting the wires around one another.

The output of a thermocouple circuit is a voltage, and there is a definiterelationship between the voltage and the temperatures of the junctions thatconstitute the circuit.

Thermoelectricity

Consider the basic thermocouple circuit shown above. The junction 1 is attemperature T1 and the junction 2 is at temperature T2. If T1 and T2 are notequal, a finite open circuit potential, (emf1), will be measured. Themagnitude of potential difference will depend on the difference in the

temperatures and the particular metals that form the thermocouplejunctions.

Thus the open circuit emf in the circuit is the basis for temperaturemeasurement. In the above circuit emf will measure the difference betweenT1 and T2 .

Thermoelectric phenomena is a result of simultaneous flow of heat and

electricity in an electrical conductor. In an electrical conductor subject to

temperature gradient, there will be both flow of thermal energy and electricity.

Both these phenomena are closely linked with free electrons in metal. The

behavior of the free electrons in an electrical circuit composed of dissimilar

metals results in useful relationship between temperature & emf. There are

three basic phenomena that can occur in the thermocouple circuit.


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There are three basic phenomena that can occur in thermocouple circuit

(1) The Seebeck effect

(2) The Peltier effect

(3) The Thomson effect.

The Seebeck Effect : refers to the generation of an emf in an open

thermocouple circuit caused by a difference in temperature between junctions in

the circuit. The Seebeck emf can be measured when there is no current flow in

the circuit. Micro-voltmeter of very high input impedance may be included in the

circuit to measure the resulting emf. There is a fixed reproducible relationship

between the emf and the junction temperatures T1 and T2. This relation ship is

expressed by the Seebeck coefficient, AB defined as

For any pair of materials, the relative Seebeck emf can be measured and

tabulated as a function of temperature.

The Peltier Effect: If a current flows through

a thermocouple, heat is absorbed at one

junction and liberated at other. Unlike Jouleheating (i2R) which liberates heat irrespective

of the direction of current flow, the reversal of

current direction in thermocouple circuit will

result into reversal of heat absorption and rejection ends in the circuit.

2

1

T

TABdT

12 TTAB


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The Peltier effect is due to the thermodynamically reversible conservationof energy as current flows across the junction, in contrast to theirreversible dissipation of energy associated with i2R losses.

The Peltier heat is the quantity of heat in addition to the quantity i2R thatmust be removed from the junction to maintain the junction at a constanttemperature. This amount of energy is proportional to the current flowingthrough the junction. The proportionality constant is the Peltier coefficient

AB

and the heat transfer required to maintain a constant temperature is

QAB.i caused by Peltier effect alone. This principle is used in thermoelectric

refrigerator which works without any moving parts.

The Thomson Effect : In addition to theSeebeck effect & Peltier effect, there is athird phenomenon that occurs inthermoelectric circuits. Consider aConductor as shown in the figure which

Is subjected to a longitudinal temperatureGradient and also a potential difference,Such that there is a flow of current andHeat in the conductor. To maintain aConstant temperature in the conductor it isfound that quantity of energy differentthan i2R must be removed from the conductor.This energy is expressed in terms of the Thomson Coefficient as follows

Q = i (T1 -T2. )

It may be experimentally observed that if currentis allowed to flow through a conductor maintained

in a temperature Gradient (ref. fig.), left half sidebecomes cooler than the right half. This is called the

positive Thomson effect. For Cadmium, antimony,silver and zinc exactly reverse happened which is

called the negative Thomson effect.

For a thermocouple circuit, all three effects maybe present and may contributte to the overall emfof the circuit.


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Thermo-emf measurement

The magnitude of the thermo emf depends on the materials and thetemperature difference between the junctions.

The process converts heat energy to electrical energy and theconversion is reversible.

Measurement of thermo emf is linked to temperature measurementunless the current flow within thermocouple is inhibited. Because if thecurrrent is allowed to flow, it will change the temperature of the junction(by Peltier effect)

Thermocouple laws

The behavior of thermocouples can be summarised into two laws.

(1) Law of intermediate temperatures

(2) Law of intermediate metals

Law of intermediate temperatures : If a thermocouple is made of two metals

A & B, then

(1) If the junctions of the thermocouple are maintained at temperatures T1and T2., then it will produce the same emf whatever be the value of thetemperatures at other parts of the thermocouple.

(2) If E12 is the emf generated for junctions at T1 and T2. , E23 is that for

Junctions at T2 and T3. then the emf E13 for junctions at T1 and T3 isE12 + E23

The law allows a thermocouple calibrated for one reference temperature to be

used with another reference temperature.


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Law of intermediate Metals : If a thermocouple junctions are maintained at

T1 and T2., then this law has the following implications.

1) If we have a thermocouple AB made up of metals (1) & (2) and anotherthermocouple also made up of metals (1) & (2) but another metal (3) isincorporated somewhere in between, then the net emf will be same in

both the cases

2) If EAC is the emf generated by the thermocouple AC and EBC is that bythe thermocouple BC then the emf EAB generated by the thermocoupleAB will be EAC+ EBC (= EAB)

In the following circuit the measured emf will be same as the open circuit emf,

which corresponds to temperatures T1 & T2 as long as the temperatures of

junctions AC & BC are same such as in the case of terminal box of the

thermocouple from where copper wires are taken to voltmeter.

Thus copper wires or the copper terminals in the terminal box are like thematerial C shown above but that will not alter the emf generated by

thermocouple AB. Hence copper extension wires can be used to transmit

thermocouple emf as long as all terminals and wires are at same temperature.


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The basic thermocouple circuit

shown above can be used

to measure the difference

between the two temperatures

T1 and T2 . For practicalTemperature measurements,

One of these junctions becomes

a reference junction and is

maintained at some known constant temperature reference point (such as ice

bath shown). The other junction then becomes the measuring junction.

It may be noted that extension wires are needed when the measuring

instrument is to be placed at a considerable distance from the reference

junction. Maximum accuracy is obtained when the leads are of the same

material as the thermocouple element as shown in fig.

However this approach is not economical while using the expensive

Thermocouple materials. Further a small inaccuracy is still possible if the binding

post of the instrument is made of say copper and the two binding posts are at

different temperature. Therefore it is preferable to employ the system shown in

fig below to keep the copper iron and copper constantan junctions in the

thermos flask at 0oC and provide binding posts of copper this ensures

maximum accuracy in the thermocouple operation. The law of intermediate

materials ensures that neither the potentiometer nor the lead wires will change

emf in the circuit., as long as the connecting junctions are at same temperature.


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To be able to measure temperature with this circuit it is necessary to know

the relationship between the output emf and the temperature of the measuring

Junction, for the specific reference temperature.

One method of determining the relationship is to calibrate the thermocouple.

However for reasonable level of uncertainty for temperature measurement,

standard materials and procedures allow thermocouples to be accurate devices,without the necessity of calibration.

As illustrated earlier, a very common reference junction temperature is

provided by ice point, 0oC, which is created by filling a vacuum flask, or Dewar

flask. With finely crushed ice and adding a small quantity of water.

Electronic reference junctions provide a convenient means of themeasurement of measurement of temperature without the necessity to construct

an ice bath. Numerous manufacturers produce commercial temperature

measuring device with built in reference junction compensation. Electronics

generally rely on a thermistor to determine the local ambient temperature.

Thermocouple materials : The choice of materials for thermocouples isgoverned by following factors

(1) ability to withstand the temperature at which they are used(2) Immunity from contamination / oxidation etc. which ensures maintenance of

the precise thermo electric properties with continuous use and

(3) Linearity characteristics

It may be noted that the relectionship between thermo electric emf and the

difference between hot and cold junction temperatures is approximately of the

parabolic form

E = aT = bT2


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Thermocouples are broadly classified as1. Base metal thermocouples &2. Rear metal thermocouples.

Base metal thermocouples use combination of pure metals and alloys of iron,copper and nickel and are used for temperature up to 1450 K. These are mostcommonly used in practice as they are more sensitive cheaper and have nearlylinear characteristics.Their main limitation is the lower operating range because of their low meltingpoint and vulnerability to oxidation.

Rare metal thermocouples use a combination of pure metals and alloys of

platinum for temperatures up to 1600o

C and Tungsten, Rhodium andMolybdenum for temperatures up tp 3000oC

Thermopile : For special purposes where high sensitivity is needed,thermocouple may be attached in series. The out put is then the numericalsum of the voltages expected from each single thermocouple. Thiscombination is known as thermopile.

When connected in parallel, a group of thermocouples will give a readingthat is numerical average of the individual once provided the resistance ofeach individual thermocouple is the same.


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Typical thermocouples with their temperature ranges and other salient

operating characteristics are given in the table below.

NIST provides specifications for materials and construction of standard

thermocouple circuits for measurements. Many material combinations exist

for thermocouple, these are identified by thermocouple type and denoted

by letters E , J,K,S,T. For each standard thermocouple charts of

temperature and emf are available with reference junction at 0oC.


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Type Material Combination Application

Positive (+) Negative (-)

E Chromel Constantan Highestsensitivity(


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The measurement of temperature through the detection of thermal

radiation presents some unique advantages. All the temperature measuring

devices viz. filled system thermometers, thermister or thermocouples require themeasuring sensor to be brought in physical contact with the body whose

temperature is to be measured. This means that the sensor must be capable of

withstanding this temperature. In the case of very hot bodies, the thermometer

may melt at high temperature.

Secondly for bodies that are moving, a non contacting device for

measuring temperature is more convenient. Thirdly if estimation of temperature

over the surface of an object is required then a non contact type device can

readily scan the surface.

For temperatures above 650oC, the heat radiations emitted from the

body are of sufficient intensity to be used for measuring temperature. The sensor

of thermal radiation need not be in contact with the surface to be measured,

which makes this method attractive for a wide variety of applications.

Instruments that employ radiation principles fall into three general classes(a) Total radiation pyrometer

(b) Selective (Partial) radiation pyrometer

(c) Infrared (IR) pyrometer

The first one is sensitive to all radiation that enters the instrument while second

one is sensitive to only a particular wavelength. IR pyrometer employ only the

infrared portion of the spectrum for measurement.

Radiative Temperature Measurements


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Radiation Fundamentals

Radiation refers to electromagnetic waves from the surface of an object.The radiation has characteristic of both waves and particles i.e. the radiationis composed of photons. Photons usually travel in straight line from point ofemission to another surface where they may get absorbed, reflected or

transmitted. Radiation exists over a large range of wavelengths thatincludes x-rays, Ultra violet radiation, visible light and thermal radiation.

The thermal radiation emitted from an object is related to its temperature. Ithas wavelengths ranging from 10-7 to 10-3 m

Radiation emitted by an object is proportional to 4th power of temperature.

Eb=T4

Where Ebis the flux of energy radiating from an ideal surface or the black body

emissive power. For any non black finite surface, the radiant energy received by

body B from body A is given by

EA/B= C [TA4 - TB4 ] {W/m2}

Where c is shape factor of B relative to A

is emissivity of the body B &

Is the Stefan Botzmann constant = 56.7x10-12 [kW/m2.K]

The emissive power is a direct measure of the total radiation emitted by an

object. However it is emitted over a range of wavelengths. At any temperature

the distribution of energy emitted as a function of wavelength is unique. Total

emissive power at given wavelength is given by Planks law.

Eb= . 2 hp c2

5[ exp(hpc/kBT) -1]Where is wavelength,c is speed of light,

hp is planks constant = 6.6256x10-34 Js

KB is Boltzmanns constant =1.3805x10-23 j/K


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Total Radiation Pyrometer The total radiation pyrometer receives a controlled sample of total radiation

of a hot body (say a furnace) and focuses it on a temperature sensitivetransducer. The term total radiation includes both visible (light) and invisible(infrared) radiations.

Wavelength of light in the visible range is 0.3 to 0.72 m, whereas infrared

radiations are associated with relatively large wavelengths of 0.72 to 1000m. They require special optical material for focusing. As ordinary glassabsorbs infrared radiation it is unsuitable.

Above fig shows a schematic diagram of Ferys total radiation pyrometer. It

consists of blackened tube T open at one end to receive the radiations from the

object, temperature of which is to be measured. The other end of tube has a

sighting aperture in which an adjustable eyepiece is usually fitted. The thermal

radiation impinge on the concave mirror. Its position can be adjusted suitably bya rack and pinion arrangement to get proper focusing of the thermal radiation on

the detector disc S. The detector disc is made up off blackened platinum sheet

and is connected to a thermocouple / thermopile junctions or to a resistance

thermometer bridge circuit.

The energy received by pyrometer is given by Stefan Boltzmann law as

follows

Where EA/B is energy received by the pyrometer in W/m2

C is shape factor of B relative to A

is emissivity of the body B & Is the Stefan Botzmann constant = 56.7x10-12 [kW/m2.K].TA

& TBare steady state absolute temperature of the source and detector disc

EA/B=C ( TA4 -TB4)


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These pyrometers are usually calibrated against known temperatures in

the range of 700 to 2000oC, where thermocouples and resistance thermometers

cannot be employed.

Errors in measurement arise from two sources in actual use

1. Any filtering material such as smoke, dust, gases, etc which werenot present during calibration will reduce the energy received andcause error.

2. Error may be caused due to surface having changed emissivity aftercalibration due to oxidation etc.

To reduce such uncertainties, pyrometers are calibrated from time to time in

actual use.

In view of the errors due to filtering and emissivity, the total radiation pyrometer

is not very accurate temperature indicator. However it can be used to good

advantage in fixed locations where emissivity and optical paths are well known

and constant. A typical use is a large furnace in metal industries.

The advantage is that the signal is electrical and therefore can be used for

control applications.

The principle of this instrument is based on Planks law which states that

the energy level in the radiations from hot body are distributed in different

wavelengths. As the temperature increases the emissive power shifts to shorter

wavelengths. The Planks distribution equation is simplified as.

Selective (Partial) Radiation Pyrometer


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The classical form of this optical pyrometer is the disappearing filamentoptical pyrometer. It is most accurate of all radiation pyrometers. Howeverits use is limited to temperatures greater than 700oC since it requiresvisual brightness match by human operator. This instrument is used byInternational Practical Temperature Scale above 1064oC.

From Planks equation it is evident that for a given wavelength, the radiantintensity (Brightness) varies with temperature. In the disappearing filamentinstrument shown in the fig. an image of target is superimposed on theheated filament.

The tungsten lamp, which is very stable, is previously calibrated so thatthe current through the filament is known, The brightness temperature ofthe filament is also known. A red filter that allows only narrow band of

wavelengths around 0.65m is placed between the observer eye and thetungsten lamp and the target image.

The observer controls the lamp current until the filament disappears in thesuperimposed target image.


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The temperature calibration is made in terms of the lamp heating current.

Because of the manual null balancing principle, the optical pyrometer is notuseful for continuous reading or automatic control applications. However it ismore accurate and less subject to large errors compared to Total radiationpyrometer

Typical accuracy for range 850oC-1200oC is +5%and for range 1100oC-1950oC is +10%

7. temperature measurement

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