Principle of measurement of the thermocouple and index number and temperature compensation

Ahead of us have been specifically the thermocouple are introduced in detail, now continue to tell everyone about the measuring principle of thermocouple,

1, the principle of thermocouple measuring thermocouple is a temperature sensing element, it can convert temperature signal into thermoelectric potential signals, by electrical measuring instrument to cooperate, can measure the temperature being measured. The basic principle of thermocouple temperature measurement is a thermoelectric effect. By the two different materials of the conductor of A and B, closed loop as A and B two contacts at different temperature T and the To, can produce thermoelectric potential in the loop. This is the so-called seebeck effect. Conductor A and B is called hot electrode. One end of the higher temperature ( T > Call work side ( Usually welded together) ； One end of the low temperature ( 来> Call the free end ( Usually under a constant temperature & gt; 。 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.

2, and dividing Numbers of thermocouple standardized thermocouple, according to IEC international standard production. Thermocouple index number mainly S, R, B, N, K, E, J, T wait for a few kinds. S, R, B belong to precious metal thermocouple, N, K, E, J, T belong to cheap metal thermocouple.

S index number is characterized by strong antioxidant properties, appropriate continuous use in oxidation, the inert atmosphere, the long-term use of temperature 1400 ℃, 1600 ℃ in the short term. In all thermocouples, S degree, the accuracy of the highest grade, often used as a standard thermocouple; R degree, compared with S degree, in addition to the thermoelectric emfs big by about 15%, other properties are almost identical; B dividing minimal thermoelectric emfs at room temperature, so the general without compensation conductor when measuring. Its long using temperature is 1600 ℃, 1800 ℃ in the short term. Can be used in the oxidation or neutral atmosphere, and also can be used under vacuum conditions in short term. N the characteristics of the index number is 1300 ℃ high temperature oxidation resistance is strong, the long-term stability of the thermoelectric emfs and short-term thermal cycle of good repeatability, nuclear radiation resistance and low temperature resistance, can partially replace # S dividing thermocouple; K index number is characterized by strong antioxidant properties, appropriate continuous use in oxidation, the inert atmosphere, the long-term use of temperature 1000 ℃, 1200 ℃ in the short term. The most widely used in all thermocouple; E the characteristics of the index number is in commonly used thermocouple, the thermoelectric emfs, the largest is the highest sensitivity. Appropriate continuous use in oxidation, the inert atmosphere, using temperature 0 800℃； # $vmeJ dividing is characteristic of both can be used for oxidizing atmosphere ( Using the maximum temperature 750 ℃) , also can be used in a reducing atmosphere ( Using the maximum temperature 950 ℃) And resist the corrosion of gas H2 and CO, used in refining and chemical industry; T index number in all cases precision metal thermocouple is the feature of the highest rank, usually used to measure the temperature under 300 ℃.

3, thermocouple cold end temperature compensation as a result of the thermocouple 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 the compensating conductor (of the cold junction of the thermocouple The free end) Reaches to the relatively stable control indoor temperature and connected to the meter terminals. It must be pointed out that the role of the thermocouple compensation wire only extend thermal electrode, make moves to the control room of the cold junction of the thermocouple meter terminal, it itself does not eliminate the cold end temperature change on the influence of temperature, no compensation. So, still need other correction method is used to compensate the cold end temperature t0 indicates influence on measuring temperature 0 ℃. Must pay attention to when using thermocouple compensating conductor model match, cannot be wrong polarity, compensation conductor should be connected to the thermocouple temperature should not exceed 100 ℃. The cold end temperature compensator model should be consistent with the model of thermocouple, and used in the specified temperature range. When the cold end temperature compensator is connected to the thermocouple polarity not wrong; Adjust balance temperature of compensator meter starting point, makes the pointer instructions in equilibrium temperature; With automatic compensating mechanism of display instrument installation compensator; Compensator must be regular inspection and verification.