Performance index of far infrared thermometer
1. Determine the temperature measurement range: The temperature measurement range is the most important performance index of the thermometer. Each type of thermometer has its own specific temperature range. Therefore, the user's measured temperature range must be considered accurately and comprehensively, neither too narrow nor too wide. According to the law of black body radiation, in the short wavelength band of the spectrum, the change of radiant energy caused by temperature will exceed the change of radiant energy caused by emissivity error.
2. Determine the target size: Infrared thermometers can be divided into single-color thermometers and two-color thermometers (radiation colorimetric thermometers) according to the principle. For a monochromatic thermometer, when measuring temperature, the area of the target to be measured should fill the field of view of the thermometer. It is recommended that the measured target size exceed 50[%] of the field of view. If the target size is smaller than the field of view, the background radiation energy will enter the visual and acoustic symbols of the thermometer and interfere with the temperature measurement readings, causing errors. Conversely, if the target is larger than the pyrometer's field of view, the pyrometer will not be affected by background outside the measurement area. For a two-color pyrometer, the temperature is determined by the ratio of radiant energy in two independent wavelength bands. Therefore, when the target to be measured is small, does not fill the field of view, and there are smoke, dust, and obstructions on the measurement path, which attenuate the radiation energy, it will not have a significant impact on the measurement results. For small and moving or vibrating targets, the two-color thermometer is the best choice. This is due to the small diameter of the light rays and their flexibility to transport light radiant energy over curved, blocked and folded channels.
3. Determine the distance coefficient (optical resolution): The distance coefficient is determined by the ratio of D:S, that is, the ratio of the distance D between the probe of the thermometer to the target and the diameter of the measured target. If the thermometer must be installed far away from the target due to environmental conditions, and a small target must be measured, a thermometer with high optical resolution should be selected. The higher the optical resolution, i.e. increasing the D:S ratio, the higher the cost of the pyrometer. If the thermometer is far away from the target and the target is small, a thermometer with a high distance coefficient should be selected. For a pyrometer with a fixed focal length, the focal point of the optical system is the minimum position of the spot, and the spot near and far from the focal point will increase. There are two distance factors.
4. Determine the wavelength range: The emissivity and surface characteristics of the target material determine the corresponding wavelength of the spectrum of the pyrometer. For high reflectivity alloy materials, there is low or variable emissivity. In the high temperature area, the best wavelength for measuring metal materials is near infrared, and 0.8-1.0 μm can be selected. Other temperature zones can choose 1.6μm, 2.2μm and 3.9μm. Since some materials are transparent at a certain wavelength, infrared energy will penetrate these materials, and a special wavelength should be selected for this material.
5. Determine the response time: the response time indicates the reaction speed of the infrared thermometer to the measured temperature change, which is defined as the time required to reach 95[%] of the energy of the final reading. It is related to the photodetector, signal processing circuit and display system. related to the time constant. If the moving speed of the target is very fast or when measuring a fast-heating target, a fast-response infrared thermometer should be selected, otherwise the sufficient signal response will not be achieved, and the measurement accuracy will be reduced. However, not all applications require a fast-response infrared thermometer. For static or target thermal processes where thermal inertia exists, the response time of the pyrometer can be relaxed.
6. Signal processing function: In view of the difference between discrete processes (such as parts production) and continuous processes, infrared thermometers are required to have multi-signal processing functions (such as peak hold, valley hold, average value) to choose from, such as temperature measurement conveyor belt When the bottle is on, it is necessary to use peak hold, and the output signal of its temperature is sent to the controller. Otherwise the thermometer reads a lower temperature value between the bottles. If using peak hold, set the thermometer response time to be slightly longer than the time interval between bottles so that at least one bottle is always under measurement.
7. Consideration of environmental conditions: The environmental conditions of the thermometer have a great influence on the measurement results, which should be considered and properly resolved, otherwise it will affect the temperature measurement accuracy or even cause damage. When the ambient temperature is high and there is dust, smoke and steam, the protective cover, water cooling, air cooling system, air purger and other accessories provided by the manufacturer can be selected. These accessories can effectively address environmental influences and protect the thermometer for accurate temperature measurement. When specifying accessories, standardize service should be requested as much as possible to reduce installation costs.
8. Calibration of the infrared radiation thermometer: the infrared thermometer must be calibrated so that it can correctly display the temperature of the measured target. If the temperature measurement of the thermometer used is out of tolerance during use, it needs to be returned to the manufacturer or the repair center for re-calibration.
