Lec 5 Temp Measur

7/30/2019 Lec 5 Temp Measur

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TEMPERATURE MEASUREMENT

Thermometers are based on changes in a broad range of physical properties:1. Changes in physical dimensions

a) Liquid-in-glass thermometersb) Bimetallic elements

2. Changes in gas pressure or vapor pressurea) Constant volume gas thermometersb) Pressure thermometers (gas, vapor, and liquid-filled)

3. Changes in electrical propertiesa) Resistance thermometersb) Thermistorsc) Thermocouplesd) Semiconductor-junction sensors

i. Diodesi i. Integrated circui ts

4. Changes in emitted thermal radiationsa) Thermal and photon sensorsb) Total-radiation pyrometersc) Optical and two-color pyrometersd) IR pyrometers

5. Changes in chemical phasea) Fusible indicatorsb) Liquid crystalsc) Temperature-reference (fixed point cells)

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Liquid-in-glass Thermometers

Several desirable properties for the liquid are: The temperature-dimensional relationship should be linear

permitting a linear instrument scale

Large coefficient of expansione.g. alcohol is better than

mercuryprovides easier reading.

Should accommodate a reasonable temperature range without

change of phase

Mercury is limited at the low end by its freezing point (-38.87oC) and

spirits are limited at the high temperature end by their boiling points.

Should be clearly visible when drawn into fine threadAlcohol is

usable only if dye is added.

Should not adhere to capillary wallsin this respect mercury is

better than alcohol.

Expansion type thermometers

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In liquid-in-glass when the immersion

employed is different from that used for

calibration, an estimate of correct reading

may be obtained from the following relation:

T = T1 + kT(T1-T2)

Where T = the correct temp.

T1 = the actual temp reading

k = the differential expansion coefficient

between liquid and glass

T2 = the ambient temp surrounding the

emergent stem

T = degrees of mercury thread emergence tobe corrected


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Use of Bimaterials

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Pressure Thermometers

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Constant volume gas thermometer Gasusually H2 and He contained Pressure thermometers are called

Liquid-filledif filling medium is completely liquid Gas-filledif the medium is completely gaseous

Vapor-filledif a combination of liquid and its vapor. Are less costly than other systems For the liquid-filled system

The sensing device C acts as differential volume indicator

For the gas- or vapor-filled system, The sensing device serves as pressure indicator

In both cases, both pressure and volume change.


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Electrical resistance of most materials varies withtemperature.

A temp measuring device using an element of a metallicmaterial, is commonly referred to as resistancethermometer, or a resistance temperature detector(RTD).

Semiconductor materialshaving negative resistancecoefficient are called thermistors.

Thermoresistive Elements

RTD Thermistors

Resistance change is small andpositive

Relatively large and negativeresistance

Nearly linear temp-resistancerelationship

Non-linear relationship

Operating range: -100o

C to275oC Operating range: -260o

C to1000oC

More time-stable

provides better reproducibilityand lower hysteresis

Lesser time-stable

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Resistance Thermometers (RTDs)

Properties desirable in RTDs: Should have a resistivity permitting fabrication in convenient

sizes without degrading the time response.

Thermal coefficient of resistivity should be high and as constantas possibleproviding an approximate linear output.

Should be corrosion resistant

Should not undergo phase change

Should be available in condition providing reproducible andconsistent results

No material is universally acceptable for resistancethermometer elementPt, Ni, Cu are most commonlyused. W, Ag and Fe are also employed.

The temp-resistance relationship of a RTD must bedetermined experimentally. For most metals Rt = Ro[1+A(T-To) + B(T-To)

2] Rt = resistance at temp T

Ro = resistance at reference temp To A & B = temp coefficients of resistance depending on material.


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More recently, thin films of metal-glassslurry have been used as resistanceelements.

These films are deposited onto a ceramic

substrate and laser trimmed. Film RTDs are less expensive than metalRTDs and have a larger resistance for agiven size. However, somewhat lessstable.

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Thermistors


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Thermally sensitive variable resistor made of a ceramic-like semiconducting material As temp rises, resistance decreases Often composed of oxides of Mn, Ni, and Co Resistivities range: 100450,000 .cm. In cryogenic applications, doped germanium and carbon-impregnated glass are

used. The temp-resistance function for a thermistor is given by

gradeornformulatiostoron thermidependsthatconstanta

Kin,tempreferenceanyatresistancethe

K,in,any tempatresistancethe

11exp

oo

o

o

TR

TR

TTRR

When current flows, ohmic heating is generated by its resistancethe temp of theelement is raised, the amount depending on the rate with which the heat isdissipated.

For a given ambient condition, a temp eqm. will occur at which a definite value willexist.

The devices can be used for temperature measurement and control by employingproper thermistors and electrical circuit characteristics. Thermistors can be quite small (a few mm in dia.), so their response to changes in

ambient temp may potentially be very rapid.

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EMF exists across a junction of two unlikemetals

The potential comes from two differentsources:

Contact of two dissimilar metals at thejunction temperature, and

Temperature gradient along theconductors in the circuit.

Named as Peltier and Thompsoneffects

Two junctions are always required: Onecold or reference Secondhot or measuring

Law of intermediate metals Insertion of an intermediate metal into a

thermocouple circuit will not affect the netemf, provided the two junctions introduced

by the third metal are at identicaltemperatures. Law of intermediate temperatures

If a simple thermocouple circuit developsan emf e1 when its junctions are attemperatures T1 & T2 and an emf e2 whenits junctions are at potentials T2 & T3, it willdevelop an emf e1+e2 when the junctionsare at temperatures T1 & T3.

Thermocouples


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Thermocouple Materials & Installation

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thermopile


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Pyrometry Electromagnetic radiations extend over a wide range of frequencies. Pyrometry is based on sampling energies in certain bandwidths of this

spectrum.

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At any given wavelength, a body radiates energy

of an intensity that depends on the bodystemperature.

By evaluating the emitted energy at knownwavelengths, the temperature of the body canbe found.

Two types of pyrometers:Thermal detectors

Based on temp rise produced when the energy radiated froma body is focused on to a target, heating it.

The target temp may be sensed with a thermopile, athermistor or RTD, or a pyroelectric element.

Photon detectors Use semiconductors of either the photoconductive orphotodiode type.

The sensor responds directly to the intensity of radiated lightby a corresponding change in its resistance or in its junctioncurrent or voltage.


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Pyrometers may be classified by the set ofwavelengths measured.

1) A total radiation pyrometer

Absorbs energy at all wavelengths, or at least

over a broad range of wavelengths (such as

all visible wavelengths).

2) An optical (or brightness) pyrometer

Measures energy at one specific wavelength;

a variant of this approach, the two-color pyrometer,compares the energy at two specific wavelengths.

The most common is Infrared pyrometer

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Pyrometer Theory

When a piece of steel is heated to 550oC it begins toglowthrough 550oC to 1540oC energy in the form ofvisible light is radiated from it.

Even below 550oC down to room temp, it radiatesenergy or heat in the form of IR radiation.

Energy is transmitted as electromagnetic waves orphotons traveling at the speed of light.

Radiation striking the surface of a material is partiallyabsorbed, partially reflected and partiallytransmittedthese photons are measured in terms ofabsorptance (), reflectivity (), and transmissivity (),where

+ +=1 For an ideal reflector, 1

Many gases represent substances of high transmissivity, forwhich 1

A small opening into a large cavity approaches an idealabsorber, or blackbody, for which 1


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A good absorber is also a good radiator.

For emitted radiation the term emissivity () is

used rather than absorptance ().

Emissivity: the ratio of the power per unit area radiatedby a surface to that radiated by a blackbody at thesame temperature. A blackbody therefore has anemissivity of 1 and that a perfect reflector has anemissivity of 0.

Absorptance: the ratio of the radiant or luminous fluxabsorbed by a body to the flux falling on it

However, these two are related by Kirchhoffs lawwhich states that the emissivity of a body is equal to its

absorptance at the same temperature i.e. =

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According to Stefan-Boltzmann law, the net rateof exchange of energy between two ideal

radiators A and Bthat view only each other is

q..= net radiant heat flux to Bfrom A, in W/m2

TA and TB = absolute temps of objects A and B, in K

...= the Stefan-Boltzmann constant, 5.6697x10-8

W/m2K4.

)(44

BA TTq


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In the case of non-ideal object A radiates to aperfectly absorbing object B (as well as to other

relatively cool object), the expression must bemodified:

A = the emissivity of object AFBA = configurational factor to allow for relative

position and geometry of bodies

This expression forms the basis of thermal-detector total-radiation pyrometry. The detecting element Breceives heat flux qfrom the

measured object A. TB

has already been establishedthrough calibration, which is raised by the detectingflux. Hence the unknown temperature TA can bemeasured.

)(44

BABAA TTFq

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In optical pyrometry, the color changes with

increasing temp

Change in color corresponds to change in

wavelength and the wavelength of maximum

radiation decreases with an increase in temp.

A decrease in wavelength shifts the color from

red towards yellowe.g.

At 850oC the color is bright red and

At 1200oC the color appears white.

The corresponding radiant energy maximums

occur at wavelengths of 3.5, 2.6 and 1.9 m,

respectively.


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Total-Radiation Pyrometry

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Generally used above 550oChowever, low level radiation(50350oC) is also possible.

Appropriate material is selected for a necessary range oftemp. e.g.

Pyrex: 0.3 to 2.7 m,

Fused silica: 0.3 to 3.8 m,

Ca-fluoride: 0.3 to 10 m.

Used ideally where sources approach blackbody

conditionsi.e. the source has an emissivity approachingunity.

Radiated energy is a good measure of temp only if the

applicable value of is accounted for.


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Optical Pyrometry

Optical pyrometers measure radiant intensity at only oneor two specific wavelengths, which are isolated by use ofappropriate filters.

The intensity is found either by visual comparison to acalibrated source or by using the output of a calibratedthermal or photon detector.


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Source emissivity is important for opticalpyrometer like total-radiation pyrometer.

At a given wavelength, imperfect emissivityreduces the source intensity.

Two-color pyrometry is an optical techniquethat minimizes the influence of theemissivity. Specifically, the two-colorpyrometer measures source intensity at twowavelengths, 1 and 2.

If the emissivity is independent of

wavelength or if the wavelengths are nearlyequal, then the ratio of measuredintensities depends only on temperature.

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Is an example of visual comparison type The intensity of an electrically heated fi lament is varied to match the

source intensity at a particular wavelength. In use, the pyrometer is sighted at the unknown temp source at a

distance such that the objective lens focuses the source in theplane of lamp filament.

The eyepiece is adjusted to see the filament and source superimposed. In general, the filament will appear either hotter than or colder than

the unknown source. The current indicated by the mil li-ammeter to obtain this condition may

then be used as the temp readout. A red fi lter is generally used to obtain approximately monochromatic

conditions, and an absorption filter is used so that the filamentmay be operated at reduced intensity, thereby prolonging its life.

Disappearingfilament

pyrometer

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Lec 5 Temp Measur