Performance specifications and selection of infrared thermometers:
The performance indicators of infrared thermometer include: temperature measuring range, display resolution, accuracy, working environment temperature range, repeatability, relative humidity, response time, power supply, response spectrum, size, maximum display, weight, emissivity, etc. Attention should be paid when selecting:
1 determine the temperature range: the temperature range is the most important performance index of the thermometer. Each type of thermometer has its own specific temperature measuring range. Therefore, the user's measured temperature range must be accurate and comprehensive, neither too narrow nor too wide. According to the law of blackbody radiation, the change of radiation energy caused by temperature in the short band of spectrum will exceed the change of radiation energy caused by emissivity error.
2 Determine the target size: According to the principle, infrared thermometers can be divided into monochromatic thermometers and bicolor thermometers (radiation colorimetric thermometers). For a monochromatic thermometer, when measuring the temperature, the measured target area should fill the field of view of the thermometer. It is suggested that the size of the measured object should 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 symbol branch of the thermometer and interfere with the temperature measurement reading, resulting in errors. On the contrary, if the target is larger than the field of view of the thermometer, the thermometer will not be affected by the background outside the measurement area. For a two-color thermometer, its temperature is determined by the ratio of radiation energy in two independent wavelength bands. Therefore, when the measured object is small, does not fill the field of view, and there are smoke, dust and obstacles on the measurement path, which have attenuation on the radiation energy, it will not have a significant impact on the measurement results. For small and moving or vibrating targets, a two-color thermometer is the best choice. This is because light is small in diameter and flexible, and can transmit light radiation energy in 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 thermometer probe and the target to the diameter of the measured target. If the thermometer must be installed far away from the target due to environmental conditions, and it is necessary to measure small targets, a thermometer with high optical resolution should be selected. The higher the optical resolution, that is, the higher the D: S ratio, the higher the cost of the thermometer. If the thermometer is far away from the target and the target is small, you should choose a thermometer with high distance coefficient. For a thermometer with a fixed focal length, the spot is the minimum position at the focal point of the optical system, and the spot will increase near and far from the focal point. There are two distance coefficients.
Determine the wavelength range: the emissivity and surface characteristics of the target material determine the spectrum of the thermometer, and the corresponding wavelength has low or variable emissivity for high reflectivity alloy materials. In the high temperature area, the optimal wavelength for measuring metal materials is near infrared, which can be selected from 0.8～1.0μm m.. Other temperature zones can be 1.6μm,2.2μm and 3.9 μ m. Because some materials are transparent at a certain wavelength, infrared energy will penetrate these materials, so we should choose a special wavelength for this material.
5 Determine the response time: The response time indicates the response speed of the infrared thermometer to the measured temperature change, and is defined as the time required to reach 95% of the energy of the last reading, which is related to the time constants of the photodetector, the signal processing circuit and the display system. If the moving speed of the target is fast or when measuring the rapidly heated target, the fast response infrared thermometer should be selected, otherwise the signal response will not be enough and the measurement accuracy will be reduced. However, not all applications require a fast response infrared thermometer. For static or target thermal process with thermal inertia, the response time of thermometer can be relaxed.
6 Signal processing function: In view of the difference between discrete process (such as parts production) and continuous process, the infrared thermometer is required to have multiple signal processing functions (such as peak holding, valley holding and average value) to choose from. For example, when measuring bottles on the conveyor belt, the peak holding is required, and the output signal of its temperature is transmitted to the controller. Otherwise, the thermometer reads the lower temperature between the bottles. If peak holding is used, set the response time of the thermometer 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 solved, otherwise it will affect the temperature measurement accuracy and even cause damage. When the ambient temperature is high and there are dust, smoke and steam, accessories such as protective sleeve, water cooling, air cooling system and air blower provided by the manufacturer can be selected. These accessories can effectively solve the environmental impact and protect the thermometer to achieve accurate temperature measurement. When determining accessories, standardized services should be required as much as possible to reduce installation costs.
Calibration of infrared radiation thermometer: The infrared thermometer must be calibrated to show the temperature of the measured object correctly. If the thermometer used is out of tolerance in use, it should be returned to the manufacturer or maintenance center for re-calibration.
