Performance indicators of the thermometer
Temperature measurement range: Each type of thermometer has its own specific temperature measurement range, neither too narrow nor too wide. Generally speaking, the narrower the temperature measurement range, the higher the resolution of the output signal for monitoring temperature. Accuracy and reliability are easy to solve. If the temperature measurement range is too wide, the temperature measurement accuracy will be reduced
Working wavelength: According to the law of black body radiation, the change of radiation energy caused by temperature in the short-wave band of the spectrum will exceed the change of radiation energy caused by emissivity error. Therefore, it is better to use short-wave as much as possible when measuring temperature, but it must Consider the factors of emissivity in combination with the detected object:
The emissivity and surface properties of the target material determine the spectral response wavelength of the pyrometer, and for high reflectivity alloy materials, there is low or varying emissivity. In the high temperature area, the wavelength of jia for measuring metal materials is near infrared, and 0.8~1.0μm can be selected. Other temperature zones can choose 1.6, 2.2 and 3.9μm. Since some materials are transparent at a certain wavelength, infrared energy will penetrate these materials, and special wavelengths should be selected for this material, such as measuring the internal temperature of glass with wavelengths of 1.0, 2.2 and 3.9 μm (the glass to be tested should be very thick, Otherwise, it will pass through); 5.0 μm is used for measuring the surface temperature of glass; 8~14 μm is suitable for measuring low temperature area, and 3.43 μm is used for measuring polyethylene plastic film, 4.3 or 7.9 μm for polyester, and the thickness exceeds 0.4mm. 8~14μm, such as measuring CO in the flame with a narrow band of 4.64μm, measuring NO2 in the flame with 4.47μm, etc.
Spot size: The area of the measuring point of the thermometer is called "spot size". In order to obtain the best temperature reading, the distance between the thermometer and the test target must have a suitable range. The farther the distance from the target, The larger the spot size. Therefore, in the application, attention should be paid to the ratio of distance to spot size, or D:S. When determining the measurement distance, care should be taken to make the target diameter equal to or greater than the measured spot size. If the target is smaller than the spot size being measured, the thermometer will simultaneously measure the temperature of the background object, reducing the accuracy of the reading.
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 generally 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 field of view of the thermometer and interfere with the temperature reading, causing errors. 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 too small to 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 targets that are moving or vibrating, sometimes moving in the field of view, or may partially move out of the field of view, under these conditions, it is more appropriate to use a two-color thermometer. If it is impossible to aim directly between the thermometer and the target, and the measurement channel is bent, narrow, blocked, etc., it is appropriate to choose a two-color fiber optic thermometer. This is due to their small diameter, flexibility, and ability to transmit optical radiant energy over curved, blocked, and folded channels, thus enabling measurement of targets that are difficult to access, in harsh conditions, or near electromagnetic fields.
The distance factor (optical resolution) is determined by the ratio D:S, which is the ratio of the distance D between the pyrometer probe to the target and the spot diameter. If the thermometer must be installed far away from the target due to environmental conditions, and to measure small targets, a thermometer with high optical resolution should be selected. The higher the optical resolution, the greater the D:S ratio. 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 spot is small at the focal point of the optical system, and the spot will increase near and far from the focal point. There are two distance factors. Therefore, in order to measure temperature accurately at distances close to and far from the focus, the size of the target to be measured should be larger than the spot size at the focus. The zoom thermometer has a small 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.
Response time: Indicates the reaction speed of the infrared thermometer to the measured temperature change, defined as the time required for 95% of the energy after reaching the reading, which is related to the time constant of the photodetector, signal processing circuit and display system. 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. For stationary or target thermal processes where thermal inertia exists, the response time of the pyrometer can be relaxed. Therefore, the selection of the response time of the infrared thermometer should be adapted to the situation of the measured target, mainly based on the target's movement speed and the target's temperature change speed. For stationary targets or targets due to thermal inertia, or where the speed of existing control equipment is limited, the response time of the pyrometer can be relaxed.
Signal processing function: In view of the difference between discrete processes (such as parts production) and continuous processes, infrared thermometers are required to have a variety of signal processing functions (such as peak hold, valley hold, average value), such as when measuring the temperature of bottles on the conveyor belt , it is necessary to use the peak hold function to transmit the output signal of its temperature to the controller, otherwise the thermometer will read the lower temperature value between the bottles. If peak hold is used, the thermometer response time should be set slightly longer than the time interval between bottles so that at least one bottle is always under measurement.
