Temperature is a basic physical quantity, and all processes in nature are closely related to temperature. Temperature sensors are the earliest developed and most widely used sensors. The market share of temperature sensors greatly exceeds that of other sensors. From the beginning of the 17th century, people began to use temperature for measurement. With the support of semiconductor technology, this century has developed semiconductor thermocouple sensors, PN junction temperature sensors and integrated temperature sensors. Correspondingly, acoustic temperature sensors, infrared sensors and microwave sensors have been developed one after another according to the law of interaction between waves and matter.
Two conductors of different materials, if connected to each other at a certain point, heating this connection point, a potential difference will appear in the part where they are not heated. The value of this potential difference is related to the temperature of the measuring point of the unheated part and the material of these two conductors. This phenomenon can occur in a wide temperature range. If the potential difference is accurately measured, and then the ambient temperature of the unheated part is measured, the temperature of the heating point can be accurately known. Because it must have two conductors of different materials, it is called a thermocouple. Thermocouples made of different materials are used in different temperature ranges, and their sensitivities are also different. The sensitivity of a thermocouple refers to the amount of change in the output potential difference when the temperature of the heating point changes by 1 ° C. For most thermocouples supported by metallic materials, this value is between 5 and 40 μV / ° C.
The thermocouple sensor has its own advantages and disadvantages. It has low sensitivity, is easily affected by environmental interference signals, and is also affected by the temperature drift of the preamplifier, so it is not suitable for measuring small temperature changes. Since the sensitivity of the thermocouple temperature sensor has nothing to do with the thickness of the material, a very fine material can also be used to make the temperature sensor. Also because the metal material of the thermocouple has good ductility, this tiny temperature measuring element has a very high response speed and can measure the process of rapid changes.
The temperature sensor is the most commonly used among all kinds of sensors. The modern temperature sensor has a very small shape, which makes it widely used in various fields of production practice and also provides countless conveniences and functions for our lives. .
There are four main types of temperature sensors: thermocouples, thermistors, resistance temperature detectors (RTD) and IC temperature sensors. IC temperature sensor includes two types of analog output and digital output.
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 indication of the thermometer can directly represent the temperature of the measured object. Generally, the measurement accuracy is high. Within a certain temperature measurement range, the thermometer can also measure the temperature distribution inside the object. But for moving bodies, small targets or objects with small heat capacity, large measurement errors will occur. Commonly used thermometers include bimetal thermometers, glass liquid thermometers, pressure thermometers, resistance thermometers, thermistors, and thermocouples. They are widely used in industry, agriculture, commerce and other sectors. People often use these thermometers in their daily lives. With the wide application of cryogenic technology in defense engineering, space technology, metallurgy, electronics, food, medicine, petrochemical and other departments and the research of superconducting technology, cryogenic thermometers measuring temperatures below 120K have been developed, such as cryogenic gas thermometers, steam Pressure thermometer, acoustic thermometer, paramagnetic salt thermometer, quantum thermometer, low temperature thermal resistance and low temperature thermocouple. Low temperature thermometers require small temperature sensing elements, high accuracy, good reproducibility and stability. The carburized glass thermal resistance made by 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 to 300K.
The sensitive element of the non-contact temperature sensor is not in contact with the measured object, 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 instrument is based on the basic law of black body radiation and is called a radiation temperature measuring instrument. Radiation temperature measurement method includes brightness method (see optical pyrometer), radiation method (see radiation pyrometer) and colorimetric method (see colorimetric thermometer). Various radiation 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. If you want to determine the true temperature of an object, you must correct the emissivity of the material surface. The emissivity of the material surface not only depends on the temperature and wavelength, but also depends on the surface state, coating film and microstructure, etc., 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 the temperature of steel strip rolling in metallurgy, the temperature of rolls, the temperature of forgings, 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 the automatic measurement and control of the solid surface temperature, an additional reflector can be used to make a black body cavity together with the measured surface. The effect of additional radiation can improve the effective radiation and effective emission coefficient of the measured surface. The effective emission coefficient is used to make corresponding corrections to the measured temperature through the meter, and finally the true temperature of the measured surface can be obtained. The most typical additional mirror is a hemispherical mirror. The diffuse radiation of the measured surface near the center of the sphere can be 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 Ï 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 tube of heat-resistant material 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 obtained by calculation. In automatic measurement and control, you can use this value to correct the measured cavity bottom temperature (that is, the medium temperature) to obtain the true temperature of the medium.
Advantages of non-contact temperature measurement: The upper measurement limit is not limited by the temperature resistance of the temperature sensing element, so there is no limit to the maximum measurable temperature in principle. For high temperatures above 1800 ° C, the non-contact temperature measurement method is mainly used. With the development of infrared technology, radiation temperature measurement has gradually expanded from visible light to infrared light. It has been adopted below 700 ° C until normal temperature, and the resolution is very high.
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