Contact temperature sensor

　　The detection part of the contact temperature sensor has good contact with the measured object, also known as a thermometer. The thermometer achieves thermal equilibrium through conduction or convection, so that the value of the thermometer can directly indicate the temperature of the measured object.

　　 Generally, the measurement accuracy is higher. Within a certain temperature measurement range, the thermometer can also measure the temperature distribution inside the object. But for moving objects, small targets or objects with small heat capacity, larger measurement errors will occur. Commonly used thermometers include bimetallic thermometers, glass liquid thermometers, pressure thermometers, resistance thermometers, thermistors and thermocouples.

　　 non-contact temperature sensor

　　 Its sensitive components and the measured object do not touch each other, also known as non-contact temperature measuring instrument. This kind of instrument can be used to measure the surface temperature of moving objects, small targets and objects with small heat capacity or rapid temperature change (transient), and can also be used to measure the temperature distribution of the temperature field. The most commonly used non-contact temperature measuring instruments are based on the basic law of black body radiation and are called radiation temperature measuring instruments. Radiation thermometry includes luminance method (see optical pyrometer), radiation method (see radiation pyrometer) and colorimetric method (see colorimetric thermometer). All kinds of radiation temperature measurement methods can only measure the corresponding luminosity temperature, radiation temperature or colorimetric temperature. Only the black body (object that absorbs all radiation and does not reflect light) is the true temperature.

　　 If you want to measure the true temperature of an object, you must correct the surface emissivity of the material. The surface emissivity of a material not only depends on temperature and wavelength, but also on the surface state, coating film and microstructure, so it is difficult to accurately measure. In automated production, it is often necessary to use radiation temperature measurement to measure or control the surface temperature of certain objects, such as the steel strip rolling temperature, roll temperature, forging temperature and the temperature of various molten metals in smelting furnaces or crucibles in metallurgy . Under these specific circumstances, the measurement of the surface emissivity of an object is quite difficult.

　　 For the automatic measurement and control of the solid surface temperature, an additional mirror can be used to form a black body cavity together with the surface to be measured. The influence of additional radiation can increase the effective radiation and effective emission coefficient of the measured surface. Use the effective emission coefficient to correct the measured temperature through the meter, and finally get the true temperature of the measured surface. The most typical additional mirror is a hemispherical mirror. The diffuse radiation energy of the measured surface near the center of the sphere is reflected back to the surface by the hemispherical mirror to form additional radiation, thereby increasing the effective emission coefficient where ε is the surface emissivity of the material and ρ is the reflectivity of the mirror. As for the radiation measurement of the true temperature of gas and liquid media, a method of inserting a heat-resistant material tube to a certain depth to form a black body cavity can be used.

　　The effective emission coefficient of the cylindrical cavity after reaching thermal equilibrium with the medium is calculated by calculation. In automatic measurement and control, this value can be used to correct the measured cavity bottom temperature (that is, the temperature of the medium) to obtain the true temperature of the medium.