Basic Principles of Radiation Thermometry and Thermometer Calibrations
m SPECIALUdakara (Assistant Director, Sri Lanka Standards Institution) Ms Sujeewa Mr. Ganbold Altangerel (Researcher, Mongolian Agency for Standardization and Metrology) Basic Principles of Radiation Thermometry and Thermometer Calibrations 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 describes10 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 May-June 2012/Vol.14, No.67 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. c1 Lλ (T ) = Radiance flux ; λ5 (exp(c 2 λT ) − 1) Where, λ – wavelength, T – absolute temperature, c1-0.595x10-8 W.m-2, c2=0.014388 m.K The intensity of an objects emitted infrared energy is proportional to its temperature. This energy, measured as the targets emissivity that indicates an objects temperature. By understanding how this radiation depends on temperature we can make accurate measurements of temperature over a wide range.
Radiation thermometers Infrared radiation is part of the electromagnetic spectrum and the range of thespectrum is from 0.7 micron to 1000 micron. However, only the 0.7-micron to 14-micronband is used for Infrared temperature measurement, Noncontact Infrared thermometerscan focus on nearly any portion of the 0.7-micron to 14-micron band. It consists of advancedoptic systems and detectors.Spectral RadiationThermometers Most radiation thermometers are of the type known as spectral band thermometers.They measure the radiance over a relatively narrow band of wavelengths somewhere withinthe 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 groupingincludes most industrial radiation thermometers and all primary and transfer standardthermometers. 11 May-June 2012/Vol.14, No.67 Figure 1: Schematic diagram of a spectral band thermometer In a radiation thermometer, the sensor collects the energy emitted from the objectbase 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 intoan electrical signal. The factors considered for designing of radiation thermometers are temperaturerange, target size, wavelength range, response time, and distance between radiationthermometer to target .How do you select suitable distance for non contacttemperature measurement? The optical resolution of a radiation thermometer is defined as the relationshipbetween 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, andthe smaller the target can be at a given distance. For accurate measurements, spot sizeshall be less or equal to half of the diameter of object.
est Good Wrong Sensor Object is bigger Object is equal Object is smaller than spot size spot size than spot size Figure 2 : Suitable distance for accurate non contact temperature measurement Sensitivity of detectors Infrared 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.12 Advantages of radiation thermometers i) used for high temperature measurement for long period ii) measurements on moving object iii) measurements with fast respond May-June 2012/Vol.14, No.67 iv) avoid the disturbance and contamination of measured system Calibration of radiation thermometer Calibration of radiation thermometer can be done by using the following methods. Uncertainty sources of radiation thermometer calibrations i) Fixed point calibration: used to ii) Spectral Resposivity measurement calibrate transfer standards : used to calibrate radiation (temperature range:157oC to thermometer which is using for 1084oC) high temperature measurements. (temperature range : above 1084oC) Figure 3 : Blackbody furnaces Figure 4: Spectral resposivity measurement system
iii) Comparison with standard iv) Comparison with Platinum radiation thermometer : used to resistance thermometer or calibrate standards radiation thermocouple : used to calibrate thermometers/radiation Infrared thermometers thermometers (temperature range: -30oC to (temperature range:150oC to 500oC) 1000oC) 13 May-June 2012/Vol.14, No.67 Figure 5 : measurement setup Figure 6 : calibration 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 calibration Example : Calibration of radiation thermometer using standard radiation thermometeras a standard Uncertainty sources: i) Size of source effect ii) Distance effect iii) Emissivity effect iv) Wavelength difference between standard thermometer and calibration thermometer v) Temperature source : uniformity, stability, emissivityAcknowledgement Finally We wish to express our deep gratitude to the Japan International CooperationAgency(JICA) and Thailand International Development Cooperation Agency (TICA) forsponsoring for us for TCPT training programme. Also our special thanks go to Dr. NarudomNoulkhow and the staff of National Measurement Institute, Thailand (NIMT) for organizingand providing a valuable technical training for us in the field of radiation thermometry.