10 May-June 2012/Vol.14, No.67 Introduction This paper was written to share our knowledge which we gained during the training on strengthening of Measurement Standards Institutes of Asia Pacific Countries from 16th January to 24th February at National Measurement Institute, Thailand. Temperature is the one of the most frequently measured physical quantity, second only to time. It can be measured by contact thermometers or non contact thermometers. Non contact measurements are taken by using radiation thermometers. This paper describes the brief outline of radiation thermometry and calibration of radiation thermometers. Blackbody Radiation All forms of materials with temperature (T) above absolute zero emit thermal radiation. An ideal blackbody will absorb all incident electromagnetic radiation at all wavelengths and all directions. A blackbody is simply a perfectly black surface, emitting exactly what it absorbs when it is in equilibrium with its surrounding and it is an idealized object. The term emissivity is used to quantify the energy-emitting characteristics of different objects. Emissivity of the material(є) is the relative ability of its surface to emit energy by radiation. For a given temperature, Emissivity of a material(є) = Energy radiated by a particular material Energy radiated by the blackbody Emissivity of blackbody depends on shape, material, surface and oxidation. Bodies with emissivity less than 1 are called gray bodies. However, the bodies with emissivity changes with wavelength are called non-gray bodies. Plank’s radiation law describes the amount of energy emitted by blackbody in radiation of a certain wavelength. Radiance flux ; Where, λ – wavelength, T – absolute temperature, c 1 -0.595x10 -8 W.m -2 , c 2 =0.014388 m.K The intensity of an object's emitted infrared energy is proportional to its temperature. This energy, measured as the target's emissivity that indicates an object's temperature. By understanding how this radiation depends on temperature we can make accurate measurements of temperature over a wide range. SPECIAL Ms Sujeewa Udakara (Assistant Director, Sri Lanka Standards Institution) Mr. Ganbold Altangerel (Researcher, Mongolian Agency for Standardization and Metrology) Basic Principles of Radiation Thermometry and Thermometer Calibrations ) 1 ) (exp( ) ( 2 5 1 − = T c c T L λ λ λ m m m
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Introduction This paper was written to share our knowledge which we gained during the training
on strengthening of Measurement Standards Institutes of Asia Pacific Countries from 16th January to 24th February at National Measurement Institute, Thailand.
Temperature is the one of the most frequently measured physical quantity, second only to time. It can be measured by contact thermometers or non contact thermometers. Non contact measurements are taken by using radiation thermometers. This paper describes the brief outline of radiation thermometry and calibration of radiation thermometers.
Blackbody RadiationAll forms of materials with temperature (T) above absolute zero emit thermal
radiation. An ideal blackbody will absorb all incident electromagnetic radiation at all wavelengths and all directions. A blackbody is simply a perfectly black surface, emitting exactly what it absorbs when it is in equilibrium with its surrounding and it is an idealized object.
The term emissivity is used to quantify the energy-emitting characteristics of different objects. Emissivity of the material(є) is the relative ability of its surface to emit energy by radiation. For a given temperature,
Emissivity of a material(є) = Energy radiated by a particular material Energy radiated by the blackbody
Emissivity of blackbody depends on shape, material, surface and oxidation.Bodies with emissivity less than 1 are called gray bodies. However, the bodies
with emissivity changes with wavelength are called non-gray bodies. Plank’s radiation law describes the amount of energy emitted by blackbody in
radiation of a certain wavelength.
Radiance flux ;
Where, λ – wavelength, T – absolute temperature, c1-0.595x10-8 W.m-2, c
2=0.014388
m.K
The intensity of an object's emitted infrared energy is proportional to its temperature. This energy, measured as the target's emissivity that indicates an object's temperature. By understanding how this radiation depends on temperature we can make accurate measurements of temperature over a wide range.
SPECIAL Ms Sujeewa Udakara (Assistant Director, Sri Lanka Standards Institution)Mr. Ganbold Altangerel (Researcher, Mongolian Agency for Standardization and Metrology)
Basic Principles of Radiation Thermometry and Thermometer Calibrations
)1)(exp()(
25
1
−=
TccTLλλλ
mmm
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Radiation thermometersInfrared radiation is part of the electromagnetic spectrum and the range of the
spectrum is from 0.7 micron to 1000 micron. However, only the 0.7-micron to 14-micron band is used for Infrared temperature measurement, Noncontact Infrared thermometers can focus on nearly any portion of the 0.7-micron to 14-micron band. It consists of advanced optic systems and detectors.
Spectral Radiation Thermometers
Most radiation thermometers are of the type known as spectral band thermometers. They measure the radiance over a relatively narrow band of wavelengths somewhere within the range 0.5 µm to 25µm. The choice of wavelength depends on the temperature range, the environment, and the type of surface under investigation. The spectral band grouping includes most industrial radiation thermometers and all primary and transfer standard thermometers.
In a radiation thermometer, the sensor collects the energy emitted from the object base on its optics and location. The filter selects the range of wavelength to be measured. Then, the detector and signal processing system measures the energy and converts into an electrical signal.
The factors considered for designing of radiation thermometers are temperature range, target size, wavelength range, response time, and distance between radiation thermometer to target .
How do you select suitable distance for non contact temperature measurement?
The optical resolution of a radiation thermometer is defined as the relationship between the distance of the measuring device from the target, and the diameter of the spot (D:S). The greater this value, the better the optical resolution of the measuring device, and the smaller the target can be at a given distance. For accurate measurements, spot size shall be less or equal to half of the diameter of object.
Figure 1: Schematic diagram of a spectral band thermometer
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Sensitivity of detectorsInfrared detectors fall into two main groups: quantum detectors and thermal
detectors. Quantum detectors (photodiodes) interact directly with the impacting photons, resulting in electron pairs and therefore an electrical signal. Thermal detectors are much slower than quantum detectors due to the self-heating required.
A variety of detectors are used to maximize the sensitivity of sensors. Eg: PbS has the greatest sensitivity, while the thermopile has the least sensitivity.
Advantages of Radiation thermometersi) used for high temperature measurement for long periodii) measurements on moving objectiii) measurements with fast respondiv) avoid the disturbance and contamination of measured system
Calibration of Radiation thermometerCalibration of radiation thermometer can be done by using the following methods.
Uncertainty sources of Radiation thermometer calibrations
Sensor
est
Object is bigger than spot size
Good
Object is equal spot size
Object is smaller than spot size
Wrong
Figure 2 : Suitable distance for accurate non contact temperature measurement
i) Fixed point calibration: used to calibrate transfer standards
(temperature range:157oC to 1084oC)
ii) Spectral Resposivity measurement : used to calibrate radiation thermometer which is using for high temperature measurements.
(temperature range : above 1084oC)
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Figure 5 : Measurement setup
The uncertainties of radiation thermometer calibrations fall into three main groups. Uncertainties related to i) the standard used ii) the blackbody source iii) the radiation thermometer under calibrationExample : Calibration of radiation thermometer using standard radiation thermometer
as a standardUncertainty sources: i) Size of source effectii) Distance effectiii) Emissivity effectiv) Wavelength difference between standard thermometer and calibration thermometerv) Temperature source : uniformity, stability, emissivity
Acknowledgement Finally We wish to express our deep gratitude to the Japan International Cooperation
Agency(JICA) and Thailand International Development Cooperation Agency (TICA) for sponsoring for us for TCPT training programme. Also our special thanks go to Dr. Narudom Noulkhow and the staff of National Measurement Institute, Thailand (NIMT) for organizing and providing a valuable technical training for us in the field of radiation thermometry.
Figure 6 : calibration setup
iv) Comparison with Platinum resistance thermometer or thermocouple : used to calibrate Infrared thermometers
(temperature range: -30oC to 500oC)
iii) Comparison with standard radiation thermometer : used to calibrate standards radiation t h e r m o m e t e r s / r a d i a t i o n thermometers
