Thermocouple probe knowledge
thermocouple probe knowledge: first, thermocouple probe is one of the most commonly used temperature detection elements in industry. Its advantages 4. The error caused by friction resistance. The friction resistance of the host part is: ① high measurement accuracy. Because the thermocouple probe is in direct contact with the measured object, it is not affected by the intermediate medium. ② Wide measuring range. Commonly used thermocouple probes can be continuously measured from -50~+1600 ℃, and some special thermocouple probes can measure as low as -269 ℃. Thermocouple probes (such as gold iron nickel chromium), up to +2800 ℃ (such as tungsten rhenium). ③ The structure is simple and easy to use. The thermocouple probe is usually composed of two different kinds of metal wires, which are not limited by the size and beginning. There is a protective sleeve outside, which is very convenient to use
second, what is thermocouple
thermocouple is a kind of temperature sensing element, which belongs to contact temperature measurement in temperature measurement. It can convert the temperature signal into thermal potential signal, and the measured temperature can be measured through the cooperation of electrical measuring instruments
third, the thermocouple probe temperature measuring silicone oil will be under the action of its own gravity. The basic principle
the conductors or semiconductors A and B of two different materials will be welded together to form a closed loop. When there is a temperature difference between the two attachment points 1 and 2 of conductors A and B, electromotive force will be generated between them, so a large and small current will be formed in the loop, If the number of bits of AD converter, that is, the resolution is low, then the measurement result will not be accurate, which is called thermoelectric effect. It is called Seebeck effect. Thermocouples/probes use this effect to work
conductors A and B are called thermoelectrodes. The higher temperature end (T) is called the working end (usually welded together); The lower temperature end (to) is called the free end (usually at a constant temperature).
according to the functional relationship between thermal potential and temperature, the thermocouple graduation table can be made. The graduation table is obtained under the condition of free end temperature to=00c. Different thermocouples have different graduation tables.
when the third metal material is connected to the thermocouple circuit, as long as the temperature of the two contacts of the material is the same, the thermoelectric potential generated by the thermocouple will remain unchanged, that is, it is not affected by the third metal Influence in the access circuit. Therefore, when the thermocouple measures temperature, the measuring instrument can be connected. After measuring the thermoelectric potential, the power supply: AC 220V 50Hz, the temperature of the measured medium can be known. Theoretically, any two kinds of conductors can be made into thermocouples, but in fact not all materials can be made into thermocouples, so the Thermoelectrode materials must meet the following points: (1) thermocouple materials can produce high thermoelectric potential after being affected by temperature, and the relationship between thermoelectric potential and temperature is preferably a linear or approximately linear single value function; (2) It can measure higher temperature and be applied in a wide temperature range in China. After long-term use, the physical and chemical properties and thermoelectric characteristics remain stable; (3) It is required that the resistance temperature coefficient of the material should be small, the resistivity should be high, the conductivity should be good, and the heat capacity should be small; (4) Good reproducibility, convenient for mass production and exchange, and easy to formulate a unified graduation table; (5) Good mechanical performance and uniform material; (6) Rich resources and cheap prices
IV Types and structure formation of thermocouple probes (1) types of thermocouple probes commonly used thermocouple probes can be divided into: 1, standard thermocouple probes 2, non-standard thermocouple probes
the so-called standard thermocouple probe refers to the thermocouple probe whose national standard stipulates the relationship between thermal potential and temperature, allowable error, and has a unified standard graduation table. It has its supporting display instrument for selection. The non standardized thermocouple probe is inferior to the standardized thermocouple probe in the scope of use or order of magnitude. Generally, there is no unified graduation table, which is mainly used for measurement in some special occasions
standardized thermocouple probes since January 1st, 1988, China has produced thermocouples/probes and thermal resistors in accordance with IEC international standards, and designated seven standardized thermocouple probes s, B, e, K, R, J and t as thermocouple probes of unified design in China
(2) structural form of thermocouple probe in order to ensure the reliable and stable operation of the thermocouple probe, its structural requirements are as follows: ① the welding of the two thermoelectrodes constituting the thermocouple probe must be firm; ② The two thermoelectric poles should be well insulated from each other to prevent short circuit; ③ The connection between the compensation wire and the free end of the thermocouple probe should be convenient and reliable; ④ The protective sleeve shall be able to ensure that the Thermoelectrode is fully isolated from harmful media. (3). Temperature compensation of the cold end
because the material of the thermocouple probe is generally expensive (especially when precious metals are used), and the distance from the temperature measuring point to the instrument is very long, in order to save thermocouple materials and reduce costs, compensation wires are usually used to extend the cold end (free end) of the thermocouple probe to the control room with relatively stable temperature and connect it to the instrument terminal. It must be pointed out that the function of thermocouple/probe compensation wire is only to extend the Thermoelectrode and make the cold end of thermocouple probe move to the instrument terminal in the control room. It itself cannot eliminate the influence of cold end temperature change on temperature measurement and has no compensation function. Therefore, other correction methods need to be used to compensate for the influence of cold end temperature t0 ≠ 0 ℃ on temperature measurement
when using the thermocouple probe compensation wire, you must pay attention to the matching of the model, the wrong polarity cannot be connected, and the temperature at the connection end of the compensation wire and the thermocouple probe cannot exceed 100 ℃
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