Improving Uncertainties of Non-Contact Thermometry

Measurements

Mark FinchFluke Calibration

Introduction

Global Emergencies

IR Body Scanner Handheld IR Thermometer

Medical Uses

Tympanic or ear thermometer

Handheld Thermometer

Fixed Thermal Imager

VISIBLE

UV InfraredX-raysGammaRays

Radio

0.1A 1A 100A 1µ 100µ 1cm1mm

1m 1km 100km

Wavelength

30201510864321.510.80.60.4

Wavelength µm

Infrared Measurement Region

TVmm WAVE

VISIBLE

Electromagnetic Spectrum

The Basics of IR

Windowsand Optics

Target Environment

IR DetectorElectronic Displayor Other Output

453¡C

SP1 470¡C

EMS ¯.85

IR Sensor

CollectedIR EnergyIR

Electronics

S

T

IR Radiation From the Target

Calibration Sources

Cavity Type Calibration Device Flat-Plate Device

Flat Plate Calibrators

• What should we know about them?– Uniformity– Emissivity– Reflected Ambient Radiation– Heat Exchange– Calibration Temperature

Uniformity

• You need to know how uniform the temperature is over the surface of the flat plate.

• The IR thermometer is measuring an area, not a point, therefore the reading is an average of temperature in that area. There may be some points of the surface that are hotter than others.

• Currently no standardised test method but manufacturers have developed ways to test the uniformity of the surface.

Emissivity

• Emissivity is one of the largest uncertainties in a budget caused by not knowing the emissivity of the calibration surface temperature. Until quite recently in some Industrial areas apparent temperature was taken as being surface temperature.

• Left Side: Bare Metal ( =0.2)e • Right Side: Painted ( =0.95)e

Emissivity

• Ideal Blackbody • “Real Body”

• Perfect absorber• and emitter

• Some energy is Reflected and some is Emitted

• Emissivity ( e ) =1 • Emissivity ( e ) < 1

• I

• e

• e

• I

• R

Reflected Ambient Radiation

• Often called background radiation.• This is flux in the IR spectral region coming from surfaces

facing the surface being measured. • Quite often this is coming from the walls facing the flat-

plate calibrator.

Heat Exchange

• This is the uncertainty caused by the assumption that the surface temperature is the same as the area of interest.

• Example of this would be a fluid calibration bath. The temperature of the fluid may be different to that of the surface of the fluid where it is in contact with the air.

Contact Calibration Radiometric Calibration

Does not c

alibrate emiss

ivity

Calibration Temperature

Reference radiometer

Inclu

des emiss

ivity:

Traceable

IR Thermometer Uncertainties

• Lets now look at the uncertainties associated with the IR Thermometer– Size-of-Source Effect– Detector Temperature– Ambient Temperature– Atmospheric Absorption– Noise– Interpolation Error– Drift

Size Of Source EffectScatter in this thermometer causes cold/inconsistent temperature readings on smaller surfaces

90%

100%

Detector Temperature

IR DetectorElectronics

Windowsand Optics

• An IR thermometer is measuring radiation.

• The detectors output corresponds to the difference in the incoming radiation and the output radiation generated by the detector.

• Low cost IR thermometers do not have cooled detectors and therefore are slightly higher than ambient temperature meaning that when measuring targets below 200 °C radiation generated by the detector is a significant part of this output.

Ambient Temperature

• Ambient temperature effects need to be considered. Do not confuse this with detector temperature or background radiation.

 

Atmospheric Absorption• This is the effect of radiation being attenuated in the

environment between the surface being measured and the IR thermometer.

• This effect is small at short distances but can be accounted for.

Noise

• Noise uncertainty is really the thermometers ability to make a repeatable measurement on the same surface at the same temperature.

Interpolation Error

• Interpolation error or non linearity is how well the thermometer’s temperature calculation algorithm works between the points of calibration. This may be provided in the instruments specification however it should be much smaller than the thermometers calibration uncertainty.

X

X

10 20 30 40 50 60 70 80 90 °C

Drift

• Drift refers to how the thermometers measurement of temperature has changed since it was last calibrated.

Final Uncertainty Contributions

Flat-Plate Surface Related Uncertainties

Emissivity

Reflected Ambient Radiation

Heat Exchange

Uniformity

IR Thermometer Related Uncertainties

Size-of-Source Effect

Detector Temperature

Ambient Temperature

Atmospheric Absorption

Noise

Interpolation Error

Drift

These are then combined in the normal way.

Conclusion

• IR temperature devices are being developed that open up more applications where these non-contact devices can be used including that of medical science.

• To back up the measurement accuracy of these devices much evaluation work is taking place into how these devices are calibrated and the uncertainty contributions associated with the commonly used flat-plate calibrator.

• By ensuring the calibration of IR thermometers is reliable with repeatable and realistic uncertainties IR instrument uncertainties are becoming smaller for repeatable and stable devices.