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Key points of infrared temperature measurement
1. Infrared temperature measurement
The surface of all objects radiates infrared light, the intensity of which changes with temperature.
Infrared temperature sensor is a temperature measurement system composed of optical system, photoelectric sensor, circuit, microcomputer, etc. The optical system focuses the light signal on the infrared photoelectric sensor, converts it into an electrical signal through photoelectric conversion, and then amplifies it. Light signal goes through the circuit, collected and processed by the microcomputer, and finally a temperature value is obtained. The biggest advantage of the infrared temperature sensor is that it can measure temperature without touching the object to be measured, can easily measure the surface temperature of moving object or an object that is difficult to reach.
1.1 Classification according to the principle of temperature measurement: monochrome infrared temperature measurement and colorimetric infrared temperature measurement (also known as dual-color infrared temperature measurement).
Monochromatic infrared temperature measurement uses a wavelength photoelectric sensor to sense the infrared signal of the measured target, and obtain the temperature value of measured target according to the size of infrared signal. For colorimetric infrared temperature measurement, two sensors with different wavelengths are used to sense the infrared signal of the measured target, and to obtain the temperature of the measured target according to the ratio of two signals.
1.2 The emissivity of the target object indicates the ability of the object to absorb and emit infrared energy.
The surface of an object with the same emissivity has different radiant energy under the same temperature and other conditions, and the higher the emissivity, the greater the radiant energy. The emissivity is usually between 0 and 1.00, for example, the emissivity of a mirror is 0.10, while the emissivity of a so-called black body can reach 1.00. In order to accurately obtain the true temperature of the measured target, the emissivity of the infrared thermometer should be accurately set. The emissivity value of most objects can be found in “IX. Appendix- Form of Material Emissivity”
2. Factors affecting material emissivity
(1) Material of substance
(2) Surface characteristics; surface quality (bright, rough, oxidized, sandblasted), geometric shape (flat, concave, convex)
2.2 Method of determining emissivity
(1) Use RTD (PT100), thermocouple or any other suitable method to determine the actual temperature of the target material. Then, measure the temperature of the object and adjust the emissivity setting until the correct temperature value is reached, in order to obtain the correct emissivity of the measured object.
(2) If possible, apply on the surface of the object a layer of black matte paint with the emissivity of 0.95. Then use an instrument with an emissivity setting of 0.95 to measure the temperature of the painted area. Finally, measure the phase of the object, adjust the temperature of the adjacent area and adjust the emissivity until the temperature is the same, so as to obtain the correct emissivity of the measured object.
3. Factors affecting accurate temperature measurement:
Whether the measurement angle is within the allowable range;
Whether to aim and focus correctly, and inaccurate aiming may cause abnormal temperature measurement;
Whether the appropriate spectral range (wavelength) is appropriately selected, when measuring the target in a container with glass windows, the spectral range cannot be selected about 1 μm other than between 8 and 14 μm;
Regardless of whether there is a window glass, the window glass will attenuate the infrared energy, so the monochromatic temperature measurement value is lower (can be corrected by adjusting the emissivity value). Smoke, water vapor, and dust basically have no effect on the colorimetric infrared temperature measurement.
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