Improving Uncertainties of Non-Contact Thermometry Measurements Mark Finch Fluke Calibration

Introduction

Global Emergencies IR Body Scanner Handheld IR Thermometer

Medical Uses Tympanic or ear thermometer Handheld Thermometer Fixed Thermal Imager

VISIBLE UV Infrared X-rays Gamma Rays Radio 0.1A1A100A 1µ 100µ1cm 1mm 1m1km100km Wavelength Wavelength µm Infrared Measurement Region TV mm WAVE VISIBLE Electromagnetic Spectrum The Basics of IR

Windows and Optics Target Environment IR Detector Electronic Display or Other Output 453¡C SP1 470¡C EMS ¯.85 IR Sensor Collected IR Energy IR Electronics S T IR Radiation From the Target

Calibration Sources Cavity Type Calibration DeviceFlat-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 (  Right Side: Painted ( 

Emissivity Ideal Blackbody “Real Body” Perfect absorber and emitter Some energy is Reflected and some is Emitted Emissivity (  ) =1Emissivity (  ) < 1 I   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 CalibrationRadiometric Calibration Does not calibrate emissivity Calibration Temperature Reference radiometer Includes emissivity: 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 Effect Scatter in this thermometer causes cold/inconsistent temperature readings on smaller surfaces 90% 100%

Detector Temperature IR Detector Electronics Windows and 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 °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.