A thermocouple is a commonly used temperature measuring element in temperature measuring instruments. When two conductors of different components are connected to a composite circuit at both ends, when the temperature of the two junction thermocouples is different, a thermal current will be generated in the circuit. If there is a temperature difference between the working end of the thermocouple and the reference end, the display instrument will indicate the temperature value corresponding to the thermoelectric potential generated by the thermocouple. The thermoelectric heat of the thermocouple will increase as the temperature of the measuring end increases. Its size is only related to the temperature of the thermocouple material and both ends, and has nothing to do with the length and diameter of the thermoelectrode. The appearance of various thermocouples is often very different due to needs, but their basic structure is roughly the same, usually composed of main parts such as thermode, insulating sleeve protection tube and junction box, usually with display instrument, recording instrument and electronic adjustment Used in conjunction with the device. The material used to make thermocouple temperature measurement components. Mainly metallic materials, but also non-metallic materials, such as graphite. In 1821, T.J. Seebeck discovered that when two different conductors are connected at the ends and the junctions of the two ends are at different temperatures, an electromotive force will be generated in the circuit composed of the two conductors. A thermocouple is a component that measures temperature made by applying this thermoelectric effect principle. (Thermocouple composed of platinum alloy with 10% rhodium and platinum) The thermocouple was first developed in 1885 and was put into practical use around 1890. Fe/(Cu-43Ni) thermocouples began to be used in 1910, and (Ni-10Cr)/(Ni-3Mn-2Al-1Si) thermocouples gained industrial application during World War II. The thermoelectricity of metals is related to chemical composition and organizational structure. Thermocouple materials usually require uniform chemical composition (including the uniformity between materials produced in different batches and the length of a single wire), stable performance during use, and no environmental pollution, a wide temperature range of use, and good high-temperature strength. It is easy to process and form, and the price is cheap. The widely used thermocouples include platinum rhodium/platinum thermocouples, nickel chromium/nickel silicon (aluminum) thermocouples, nickel chromium/constantan thermocouples, iron/constantan thermocouples and copper/constantan thermocouples. These varieties have excellent comprehensive performance and have been listed as standard thermocouple products by China and other countries. In addition to the above-mentioned standard thermocouples, non-standard thermocouples for use under special conditions have also been developed. Tungsten rhenium thermocouple is a kind of high temperature thermocouple developed in the 1960s. There are W/(W-26Re), (W-3Re)/(W-25Re), (W-5Re)/(W-26Re) and ( W-5Re)/(W-20Re) etc. The long-term use temperature is 2000-2400℃, and the short-term use temperature is as high as 3000℃. In the low-temperature thermocouple, (Au-2.1Co)/Cu thermocouple, the thermoelectric potential rate above 10K is greater than 10μV/K; gold contains a trace amount (about 0.07 atomic percent) of iron alloy and nickel-chromium alloy thermocouple , The thermoelectric potential rate at liquid helium temperature (4K) is better than 10μV/K.
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