Contact type: Contact temperature sensors have good contact between the sensing part and the object being measured, and are also called thermometers.
Thermometers achieve thermal equilibrium through conduction or convection, allowing the thermometer reading to directly represent the temperature of the object being measured. They generally have high measurement accuracy. Within a certain temperature range, thermometers can also measure the internal temperature distribution of an object. However, they can produce significant measurement errors for moving objects, small targets, or objects with very small heat capacity. Commonly used thermometers include bimetallic thermometers, glass liquid thermometers, pressure thermometers, resistance thermometers, thermistors, and thermocouples. They are widely used in industry, agriculture, commerce, and other sectors. People also frequently use these thermometers in daily life. With the widespread application of cryogenic technology in defense engineering, space technology, metallurgy, electronics, food, medicine, and petrochemical industries, and with research into superconducting technology, cryogenic thermometers for measuring temperatures below 120K have been developed, such as cryogenic gas thermometers, vapor pressure thermometers, acoustic thermometers, paramagnetic salt thermometers, quantum thermometers, cryogenic resistance thermometers, and cryogenic thermocouples. Low-temperature thermometers require sensing elements that are small in size, highly accurate, reproducible, and stable. Carburized glass resistance thermometers, made by carburizing and sintering porous high-silica glass, are one type of sensing element in low-temperature thermometers and can be used to measure temperatures in the range of 1.6–300K.
Non-contact thermometers, also known as non-contact temperature measuring instruments, have sensing elements that do not contact the object being measured. These instruments can be used to measure the surface temperature of moving objects, small targets, and objects with small heat capacity or rapidly changing (transient) temperatures. They can also be used to measure the temperature distribution of a temperature field.
The most commonly used non-contact temperature measuring instruments are based on the fundamental law of blackbody radiation and are called radiation thermometers. Radiation thermometry includes the luminance method (see optical pyrometer), the radiation method (see radiation pyrometer), and the colorimetric method (see colorimetric thermometer). Each radiation thermometry method can only measure the corresponding photometric temperature, radiation temperature, or colorimetric temperature. Only the temperature measured for a blackbody (an object that absorbs all radiation and does not reflect light) is the true temperature. To determine the true temperature of an object, corrections must be made for the material's surface emissivity. The emissivity of a material surface depends not only on temperature and wavelength, but also on surface condition, coating, and microstructure, making it difficult to measure accurately. In automated production, radiation thermometry is often used to measure or control the surface temperature of certain objects, such as the rolling temperature of steel strips, rolls, forgings, and the temperatures of various molten metals in furnaces or crucibles in metallurgy. In these specific cases, measuring the surface emissivity is quite challenging. For automatic measurement and control of solid surface temperature, an additional reflector can be used to form a blackbody cavity with the surface being measured. The effect of the additional radiation increases the effective radiation and effective emissivity of the measured surface. By using the effective emissivity to correct the measured temperature with an instrument, the true temperature of the measured surface can be obtained. The most typical additional reflector is a hemispherical reflector. Diffuse radiation from the surface near the center of the sphere is reflected back to the surface by the hemispherical mirror, forming additional radiation and thus increasing the effective emissivity. In the formula, ε is the emissivity of the material surface, and ρ is the reflectivity of the reflector. For the radiation measurement of the true temperature of gaseous and liquid media, a method can be used to insert a heat-resistant material tube to a certain depth to form a blackbody cavity. The effective emissivity of the cylindrical cavity after reaching thermal equilibrium with the medium is calculated. In automatic measurement and control, this value can be used to correct the measured cavity bottom temperature (i.e., the medium temperature) to obtain the true temperature of the medium.
Advantages of non-contact temperature measurement: The upper limit of measurement is not limited by the temperature resistance of the sensing element, therefore, in principle, there is no limit to the highest measurable temperature. For high temperatures above 1800℃, non-contact temperature measurement methods are mainly used. With the development of infrared technology, radiation thermometry has gradually expanded from visible light to infrared light, and is now used for temperatures below 700℃ up to room temperature, with very high resolution.