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Thermocouple knowledge _ instrument

by:JVTIA     2020-10-29
A, thermocouple/probe is one of the industry is relatively commonly used temperature detecting element. Advantages are: (1) high accuracy of measurement. Due to the thermocouple/probe direct contact with the object being measured, not affected by intermediate medium. (2) the measurement range. The commonly used thermocouple/probe from - 50 ~ + 1600 ℃ can renew edge measurement, some special thermocouple/probe low measurable to - 269 ℃ thermocouple probe thermocouple/probe ( Such as gold iron nickel chrome) High, up to + 2800 ℃ ( Such as tungsten-rhenium thermocouples) 。 3. Simple structure, easy to use. Thermocouple/probe is usually composed of two different kinds of metal wire, but is not limited by the size and the beginning of, outside has protective casing, which are very convenient. Second, what is the thermocouple thermocouple is a temperature sensing element, belongs to the contact temperature measurement in the temperature measurement. It can convert temperature signal into thermoelectric potential signals through electrical measuring instrument, can measure the temperature being measured. Three, the basic principle of thermocouple/probe temperature will be two different welding material A conductor or semi-conductor A and B to form A closed loop, when the conductor A and B two persistent point temperature difference between 1 and 2, occurs between the electromotive force, and thus form the size of A current in the circuit, this kind of phenomenon called the thermoelectric effect. Is the so-called seebeck effect. Thermocouple/probe is to work with this effect. Conductor A and B is called hot electrode. A higher temperature side ( T > call work side ( Usually welded together) ; One end of the low temperature ( The To > call free end ( Usually under a constant temperature >. According to the thermoelectric potential function relation with temperature. Thermocouple indexing table can also be made. Indexing table is at the free end To = 00 c temperature conditions. Different thermocouples with different indexing table. Access in the thermocouple loop when the third metal material, the two contacts at the same temperature as long as the material, produced by the thermocouple thermoelectric is set to remain the same, which is not affected by the third metal access in the loop. Therefore, when the thermocouple temperature measurement, can be connected to the measuring instrument, measured after the thermoelectric potential, can know the temperature of the measured medium. In theory, any two conductors can be made into thermocouple, but in fact is not all materials can make thermocouple, the hot electrode materials must meet the following several points: ( 1) Thermocouple materials subjected to temperature after can produce high thermoelectric potential and thermoelectric potential and the relationship between the temperature better linear or approximate linear single valued function relation; ( 2) Can measure high temperature, and applied in a wide temperature range, after long-term use of physical, chemical properties and pyroelectric properties remained stable; ( 3) Requirements for the temperature coefficient of resistance material is small, high resistivity, good conductivity, heat capacity is small; ( 4) Better repeatability, easy to mass production and exchange, to formulate unified indexing table; ( 5) Good mechanical performance, material evenly; ( 6) Rich in resources, the price is cheap. Four, thermocouple/probe type and structure formation ( 1) Thermocouple/types of commonly used thermocouple probe and the probe can be divided into: 1, 2 standard thermocouple/probe, non-standard thermocouple/probe two kinds big. Use standard thermocouple/probe refers to the national standard specifies its thermoelectric potential and the relationship between temperature, permissible error, and a unified standard score table of thermocouple/probe, it has with matching display instrument to choose from. Non-standard thermocouple/probe or on the order of magnitude less than the scope of use of standardized thermocouple/probe, generally no unified indexing table, mainly used for the measurement of some special occasions. Standardized thermocouple/probe in China from January 1, 1988, thermocouple/probe and thermocouple according to IEC international standard production, and specify S, B, E, K, R, J, T seven standardization thermocouple/probe for our country unified design type thermocouple probe. ( 2) The structure of thermocouple/probe thermocouple/probe to ensure reliable, steady work, the structure of its requirement is as follows: (1) of thermocouple/probe two hot electrode welding must be strong; (2) two hot electrode should be well insulated between each other, in case of short circuit; (3) compensation conductor should be connected to the thermocouple/probe free end should be convenient and reliable; (4) protect casing thermal electrodes should be able to make sufficient isolation and harmful medium. ( 3) Cold end temperature compensation due to the thermocouple/probe materials are generally more expensive ( Especially when using precious metals) And temperature measuring points are far away the distance to the instrument, in order to save thermocouple materials, reduce cost, usually adopt compensating conductor of the cold junction of the thermocouple/the probe ( The free end) Reaches to the relatively stable control indoor temperature and connected to the meter terminals. It must be pointed out that the thermocouple/probe the role of the compensating conductor only extend thermal electrode, making the cold end of thermocouple/probe to the meter terminals on the control room, which itself does not eliminate the cold end temperature change on the influence of temperature, no compensation. So, still need to take some of the other correction method to compensate of the cold end temperature t0 indicates influence on measuring temperature 0 ℃. Must pay attention to in using thermocouple/probe compensating conductor model match, cannot be wrong polarity, compensation conductor should be connected to the thermocouple/probe the temperature should not exceed 100 ℃
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