9 Points for Attention in Purchasing Infrared Thermometers
1. Understanding the measurement temperature range The temperature measurement range is an important performance index of infrared thermometers. Each type of thermometer has its own specific temperature measurement range. It is recommended that you choose an infrared thermometer with an appropriate range according to your measurement needs. The measured temperature range must be considered accurate and comprehensive, neither too narrow nor too wide. If the temperature measurement range is too wide, the temperature measurement accuracy will be reduced. If the temperature is too high, the price will be expensive, which is economically uneconomical; fulfil requirements. According to the law of black body radiation, the change of radiant energy caused by temperature in the short-wave band of the spectrum will exceed the change of radiant energy caused by emissivity error. Therefore, it is better to use short-wave as much as possible when measuring temperature. Generally speaking, the narrower the temperature measurement range is, the higher the resolution of the output signal of the temperature monitoring is, and the accuracy and reliability are easy to solve.
2. Understanding measurement accuracy and minimum resolution measurement accuracy are two different concepts, easy to confuse. The measurement accuracy is the only indicator to ensure the accuracy of the measurement, and it is also the key indicator to determine the performance of the infrared thermometer. Resolution is the smallest measure at which a specific temperature can be measured.
3. Understanding emissivity According to customer feedback, when using infrared thermometers, measurement deviations often occur, and in 50% of cases, emissivity is the culprit for errors. Since the infrared thermometer is suitable for various occasions, the material and color of the surface of the measured object are different (especially the various pipes in the HVAC system), and its ability to emit infrared energy to the outside is not the same. Measurement errors due to materials are reduced by emissivity adjustment. So whether the instrument has this function is very important.
4. Understand the target size, that is, the spot size, which is the area of the measuring point of the thermometer. The farther you are from the target, the larger the spot 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, background radiation energy will enter the thermometer and interfere with the temperature reading, 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 colorimetric thermometers, 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 is smoke, dust, or obstruction on the measurement path that attenuates the radiation energy, it will not affect the measurement results. Even in the case of 95% energy attenuation, the required temperature measurement accuracy can still be guaranteed. For small and moving or vibrating targets, colorimetric thermometers are the best choice, because the light diameter is small and flexible, and can transmit light radiation energy on curved, blocked and folded channels, and can measure inaccessible , harsh conditions or targets close to electromagnetic fields.
5. Understand that the distance coefficient ratio (D:S) is the optical resolution, which refers to the ratio of the distance D between the infrared thermometer to the target and the diameter S of the measurement spot. If you are far away from a target with a small diameter, you should choose a high-ratio infrared thermometer. The higher the distance coefficient ratio, the higher the cost of the infrared thermometer. In order to obtain accurate temperature readings, the distance between the thermometer and the test target must be within the proper range. 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. 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. Therefore, in order to accurately measure temperature at a distance close to and far from the focus, the size of the measured target should be larger than the spot size at the focus. The zoom thermometer has a minimum focus position, which can be adjusted according to the distance to the target. If D:S is increased, the received energy will decrease. If the receiving aperture is not increased, the distance coefficient D:S will be difficult to increase, which will increase the cost of the instrument.
6. Knowing the wavelength range The emissivity and surface properties of the target material determine the spectral response wavelength of the pyrometer. For high reflectivity alloy materials, there is low or varying 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 the application of this material should be
Select a particular wavelength. For example, the wavelengths of 1 μm, 2.2 μm and 3.9 μm are used to measure the internal temperature of the glass (the glass to be tested must be very thick, otherwise it will pass through); the wavelength of 5 μm is used to measure the surface temperature of the glass; For example, 3.43μm is used for measuring polyethylene plastic film, 4.3μm or 7.9μm is used for polyester, and 8-14μm is used for thickness exceeding 0.4mm. For example, the narrow band 4.64μm is used to measure CO in the flame, and 4.47μm is used to measure NO2 in the flame.
7. Understand the response time Response time is the time required for the infrared thermometer to reach 95% of the energy of the final reading, indicating the reaction speed of the infrared thermometer to the measured temperature change, and the time between it and the photodetector, signal processing circuit and display system Constants are related. The choice of the response time of the infrared thermometer should be adapted to the situation of the measured target, and the determination of the response time is mainly based on the target's movement speed and the target's temperature change speed. 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 requirement can be relaxed.
8. Understanding signal processing functions 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, minimum value hold, average value) to choose from, such as temperature measurement on the conveyor belt When the bottle is used, it is necessary to use peak hold, and the output signal of its temperature is sent to the controller. Otherwise the thermometer reads the 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 under measurement.
9. Understand the 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. Light colorimetric thermometers are the best choice when smoke, dust, or other particles degrade the measured energy signal under noise, electromagnetic fields, vibration, or inaccessible environmental conditions, or other harsh conditions.
