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Thermocouple cold junction compensation _ instrument

by:JVTIA     2020-10-28
Why thermocouple cold junction compensation because of the thermocouple thermoelectric potential hot end and cold end is relative, if you don't know the cold end temperature and the measurement of thermoelectric potential doesn't mean what as benchmark, so to be cold junction compensation. So, according to the measurement of thermoelectric potential and the cold end temperature corresponding electric potential can be the hot end of the corresponding temperature. Thermocouple cold junction compensation 1, mechanical zero compensation, what method is used for moving coil table, such as fixed compensation, the error is big, not used to the cold end temperature changes and the accuracy is not high. 2, compensation conductor, the general should cooperate and other compensation method, is used to compensate for the difference in temperature thermocouple cold end and digital display instrument position. 3, compensation bridge, generally for the thermocouple ( Copper) , the existing use a diode temperature characteristic, a compensation in the secondary instrument. 4, water containers, ice water, generally as a standard 0 degrees Celsius, for accurate measurement, used in laboratory equipment. Ice bath method commonly used in the laboratory, that is, with the reference temperature constant at 0 degrees, but high cost, difficult to do. The cold end temperature correction method commonly used in the request is not high, when the cold end can't constant temperature of 0 degrees, it need to modify indicating value of the instrument. Easy to do but the error is bigger. Compensation bridge method is less used alone, is the use of electric potential generated by unbalanced bridge to compensation thermocouple cold junction temperature change caused by the thermoelectric potential value. Compensation bridge has a separate product, also have to do within the instrument. Compensation conductor method which is commonly used method, namely, extended the thermocouple cold end temperature leads to a stable place ( Usually a control room) And then adjusted by artificial cold end temperature, namely the instrument zero to room temperature, or by the instrument for automatic compensation circuit. For platinum rhodium thermocouple extended the thermocouple is impossible, because the price is not high, just use the thermoelectric properties of base metals for extended conductor, middle temperature rule is applied the theoretical foundation of the compensating conductor. Compensation conductor does not automatically compensate the change of the thermocouple cold end temperature, only the thermocouple cold end temperature leads to a stable place, compensation by artificial and instrument. Therefore compensation conductor should be called thermocouple extension, so as not to cause the wrong understanding to the person.
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