Temperature Temperature measurementsmeasurements

OutlineOutline1. Liquid-in-glass thermometres1. Liquid-in-glass thermometres

2. Bimaterial thermometres2. Bimaterial thermometres

3. Electrical thermometres3. Electrical thermometres

4. IR-thermometres4. IR-thermometres

5. Pyrometres5. Pyrometres

6. Summary6. Summary

7. Other measurement methods7. Other measurement methods

Liquid-in-glass Liquid-in-glass thermometresthermometres

Liquid-in-glass Liquid-in-glass thermometresthermometres

The “traditional” thermometresThe “traditional” thermometresMeasurement scale from -190 Measurement scale from -190 °C to °C to +600 °C+600 °C

Used mainly in calibrationUsed mainly in calibrationMercury: -39 °C … +357 °CMercury: -39 °C … +357 °CSpirit: -14 °C … +78 °CSpirit: -14 °C … +78 °C

Functionning methodFunctionning method

Method is based on the expansion of Method is based on the expansion of a liquid with temperaturea liquid with temperature

The liquid in the bulb is forced up the The liquid in the bulb is forced up the capillary stemcapillary stem

Thermal expansion:Thermal expansion:

)1(0 TVV

StructureStructure

Causes of inaccuratiesCauses of inaccuraties Temperature Temperature

differences in the differences in the liquidliquid

Glass temperature Glass temperature also affectsalso affects

The amount of The amount of immersion (vs. immersion (vs. calibration)calibration)

Bimaterial thermometresBimaterial thermometres

Method based on different thermal Method based on different thermal expansions of different metalsexpansions of different metals– Other metal expands more than other: Other metal expands more than other:

twistingtwisting– Inaccurary Inaccurary ± 1 ° C± 1 ° C– Industry, sauna thermometresIndustry, sauna thermometres

Bimaterial thermometresBimaterial thermometres

Electrical thermometresElectrical thermometres

Electrical thermometresElectrical thermometres

Resistive thermometresResistive thermometres– Resistivity is temperature dependentResistivity is temperature dependent

– Materials: Platinum, nickelMaterials: Platinum, nickel

)1()( 0 TRTR

Characteristic resistancesCharacteristic resistances

Thermistor thermometresThermistor thermometres

Semiconductor materialsSemiconductor materials Based on the temperature Based on the temperature

dependence of resistancedependence of resistance Thermal coefficient non-linear, 10 Thermal coefficient non-linear, 10

times bigger than for metal resistortimes bigger than for metal resistor NTC, (PTC): temperature coefficient’s NTC, (PTC): temperature coefficient’s

signsign

Example of a characteristic Example of a characteristic curvecurve

Limitations of electrical Limitations of electrical thermometresthermometres

Sensor cable’s resistance and its Sensor cable’s resistance and its temperature dependencytemperature dependency

Junction resistancesJunction resistances Thermal voltagesThermal voltages Thermal noise in resistorsThermal noise in resistors Measurement currentMeasurement current Non-linear temperature dependenciesNon-linear temperature dependencies Electrical perturbationsElectrical perturbations Inaccuracy at least Inaccuracy at least ± 0.1 °C± 0.1 °C

Infrared thermometresInfrared thermometres

Thermal radiationThermal radiation

Every atom and molecule exists in Every atom and molecule exists in perpetual motionperpetual motion

A moving charge is associated with an A moving charge is associated with an electric field and thus becomes a electric field and thus becomes a radiatorradiator

This radiation can be used to This radiation can be used to determine object's temperaturedetermine object's temperature

Thermal radiationThermal radiation

Waves can be characterized by their Waves can be characterized by their intensities and wavelengthsintensities and wavelengths– The hotter the object:The hotter the object:

the shorter the wavelengththe shorter the wavelength the more emitted lightthe more emitted light

Wien's law:Wien's law:

cmKT 2896.0max

Planck's lawPlanck's law

1

21)(

2

5

kT

hc

e

hcF

Magnitude of radiation at particular wavelength (λ) and particular temperature (T).h is Planck’s constant and c speed of light.

BlackbodyBlackbody

An ideal emitter of electromagnetic An ideal emitter of electromagnetic radiationradiation– opaqueopaque– non-reflectivenon-reflective– for practical blackbodies for practical blackbodies εε = 0.9 = 0.9

Cavity effectCavity effect– em-radiation measured from a cavity of em-radiation measured from a cavity of

an objectan object

Cavity effectCavity effect

Emissivity of the cavity increases and Emissivity of the cavity increases and approaches unityapproaches unity

According to Stefan-Boltzmann’s law, According to Stefan-Boltzmann’s law, the ideal emitter’s photon flux from the ideal emitter’s photon flux from area a isarea a is

In practice:In practice:

40 Ta

0 r

Cavity effectCavity effect

For a single reflection, effective For a single reflection, effective emissivity isemissivity is

Every reflection increases the Every reflection increases the emyssivity by a factor (1-emyssivity by a factor (1-εε))

bbr )1(0

Cavity effectCavity effect

Practical blackbodiesPractical blackbodies

Copper most common materialCopper most common material The shape of the cavity defines the The shape of the cavity defines the

number of reflectionsnumber of reflections– Emissivity can be increasedEmissivity can be increased

DetectorsDetectors

Quantum detectorsQuantum detectors– interaction of individual photons and interaction of individual photons and

crystalline latticecrystalline lattice– photon striking the surface can result to photon striking the surface can result to

the generation of free electronthe generation of free electron– free electron is pushed from valency to free electron is pushed from valency to

conduction band conduction band

DetectorsDetectors

– hole in a valence band serves as a hole in a valence band serves as a current carriercurrent carrier

– Reduction of resistanceReduction of resistance

Photon’s energyPhoton’s energy

hE

DetectorsDetectors

Thermal detectorsThermal detectors– Response to heat resulting from Response to heat resulting from

absorption of the sensing surfaceabsorption of the sensing surface– The radiation to opposite direction (from The radiation to opposite direction (from

cold detector to measured object) must cold detector to measured object) must be taken into accountbe taken into account

Thermal radiation from Thermal radiation from detectordetector

PyrometresPyrometres

Disappearing filament pyrometerDisappearing filament pyrometer– Radiation from and object in known Radiation from and object in known

temperature is balanced against an temperature is balanced against an unknown targetunknown target

– The image of the known object The image of the known object (=filament) is superimposed on the (=filament) is superimposed on the image of targetimage of target

PyrometresPyrometres

– The measurer adjusts the current of the The measurer adjusts the current of the filament to make it glow and then filament to make it glow and then disappeardisappear

– Disappearing means the filament and Disappearing means the filament and object having the same temperatureobject having the same temperature

Disapperaring filament Disapperaring filament pyrometerpyrometer

PyrometresPyrometres

Two-color pyrometerTwo-color pyrometer– Since emissivities are not usually known, Since emissivities are not usually known,

the measurement with disappearing the measurement with disappearing filament pyrometer becomes impracticalfilament pyrometer becomes impractical

– In two-color pyrometers, radiation is In two-color pyrometers, radiation is detected at two separate wavelengths, detected at two separate wavelengths, for which the emissivity is approximately for which the emissivity is approximately equalequal

Two-colour pyromererTwo-colour pyromerer

PyrometersPyrometers

– The corresponding optical transmission The corresponding optical transmission coefficients are coefficients are γγxx and and γγy y

Displayed temperatureDisplayed temperature

1

5

5

ln11

yx

xy

xyc CT

MeasurementsMeasurements

– Stefan-Boltzmann’s law with manipulation:Stefan-Boltzmann’s law with manipulation:

– Magnitude of thermal radiation flux, sensor Magnitude of thermal radiation flux, sensor surface’s temperature and emissivity must surface’s temperature and emissivity must be known before calculationbe known before calculation

– Other variables can be considered as Other variables can be considered as constants in calibrationconstants in calibration

44

sc A

TT

Error sourcesError sources

Errors in detection of the radiant flux Errors in detection of the radiant flux or reference temperatureor reference temperature

Spurious heat sourcesSpurious heat sources– Heat directly of by reflaction into the Heat directly of by reflaction into the

optical system optical system Reflectance of the object (e.g. 0.1)Reflectance of the object (e.g. 0.1)

But does not require contact to surface But does not require contact to surface measured!measured!

Pyroelectric thermometresPyroelectric thermometres

Generate electric charce in response Generate electric charce in response to heat fluxto heat flux– Crystal materialsCrystal materials– Comparable to piezoelectric effect: the Comparable to piezoelectric effect: the

polarity of crystals is re-orientedpolarity of crystals is re-oriented

SummarySummary

Only some temperature Only some temperature measurement methods presentedmeasurement methods presented

Examples of phenomenons used: Examples of phenomenons used: thermal expansion, resistance’s thermal expansion, resistance’s thermal dependency, radiationthermal dependency, radiation

The type of meter depends onThe type of meter depends on– Measurement object’s propertiesMeasurement object’s properties– TemperatureTemperature

More temperature More temperature measurement possibilitiesmeasurement possibilities

ThermocouplesThermocouples Semiconductor thermometresSemiconductor thermometres Temperature indicatorsTemperature indicators

– Crayons etc.Crayons etc. Manometric (gas pressure) sensors Manometric (gas pressure) sensors

Questions?Questions?