The thermometer achieves thermal equilibrium by conduction or convection, so that the indicated value of the thermometer directly represents the temperature of the measured object. Generally, the measurement accuracy is high. In a certain temperature measurement range, the thermometer can also measure the temperature distribution inside the object. However, for moving bodies, small targets or objects with small heat capacity, there will be a large measurement error, commonly used thermometers are bimetallic thermometers, glass liquid thermometers, pressure thermometers, resistance thermometers, thermistors and thermocouples. They are widely used in industrial, agricultural, commercial, and other sectors. People also use these thermometers in daily life. With the wide application of cryogenic technology in national defense engineering, space technology, metallurgy, electronics, food, medicine and petrochemical and other departments and the research of superconducting technology, cryogenic thermometers measuring temperatures below 120K have been developed, such as cryogenic gas thermometers, vapor pressure thermometers, acoustic thermometers, paramagnetic salt thermometers, quantum thermometers, low-temperature thermal resistance and low-temperature thermocouples. Low temperature thermometers require small temperature sensing elements, high accuracy, good reproducibility and stability. The carburized glass thermal resistance made of carburizing and sintering of porous high silica glass is a temperature sensing element of low temperature thermometer, which can be used to measure the temperature in the range of 1.6~300K.
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Various radiometric temperature measurement methods can only measure the corresponding photometric temperature, radiation temperature or colorimetric temperature. Only the temperature measured for a black body (an object that absorbs all radiation and does not reflect light) is the true temperature. To determine the true temperature of an object, a correction of the emissivity of the material surface must be performed. The surface emissivity of a material depends not only on temperature and wavelength, but also on surface state, coating film, and microstructure, so it is difficult to measure accurately. In automated production, it is often necessary to use radiation temperature measurement to measure or control the surface temperature of certain objects, such as steel strip rolling temperature in metallurgy, roll temperature, forging temperature and the temperature of various molten metals in smelting furnaces or crucibles. In these specific cases, the measurement of the emissivity of the surface of the object is quite difficult. For automatic measurement and control of solid surface temperature, additional mirrors can be used to form a blackbody cavity with the measured surface. The influence of additional radiation can increase the effective radiation and effective emission coefficient of the measured surface. The effective emission coefficient is used to correct the measured temperature accordingly through the instrument, and the true temperature of the measured surface can finally be obtained. The most typical additional mirror is a hemispheric mirror. The diffuse radiation of the measured surface near the center of the ball can be reflected back to the surface by the hemispherical mirror to form additional radiation, which increases the effective emissivity formula ε where the emissivity of the material surface and ρ is the reflectivity of the mirror. For radiometric measurement of the true temperature of gaseous and liquid media, the method of inserting a heat-resistant material tube to a certain depth can be used to form a blackbody cavity. The effective emission coefficient of the cylinder cavity after reaching thermal equilibrium with the medium is calculated by calculating. In automatic measurement and control, this value can be used to correct the measured cavity bottom temperature (i.e. medium temperature) to obtain the true temperature of the medium.
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