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Cold junction compensation principle of thermocouple temperature transmitter _ instrument

by:JVTIA     2020-10-29
Thermocouple temperature measuring instrument is commonly used in temperature measuring element, it directly measure temperature, and the temperature signals into a thermoelectric emfs, through electrical instrument ( Secondary instrument) Convert the temperature of the measured medium. The appearance of all kinds of thermocouple is often due to the need to vary, but they are roughly the same basic structure, usually by the hot electrode, insulation sleeve protecting tube and junction box, etc. Mainly grouped into temperature transmitter, through the compensating conductor and display meter, recording meter and electronic regulator. Thermocouple measuring temperature to the cold end ( Measure the end for the hot end, by the end of the lead connected to the measurement circuit is called cold junction) Temperature remains constant, the size of thermoelectric potential and measured temperature in certain proportion relationship. When measuring, the cold end ( Environment) Changes in temperature, will be affected the accuracy of measurement. Take action at the cold end compensation due to the impact of the cold end temperature change is called the thermocouple cold junction compensation. Thermocouple cold junction compensation is usually used in a composed of thermocouple cold end series bridge. The three bridge arms for standard resistance, in addition, there are a bridge arm by ( Copper) Thermocouple. When the cold end temperature change ( Rise, for example) Thermocouple thermoelectric potential will change (to produce Reduce) The thermocouple resistance in series, while the bridge will be change and make the change at the ends of the bridge voltage also ( Higher) 。 If the parameter is a good choice and wiring is correct, produced by the bridge voltage as it happens and thermoelectric potential changes over temperature equal to the amount, the thermocouple measurement circuit, the total output voltage ( Potential) Exactly reflect the measured temperature value. This is the cold junction compensation principle of thermocouple temperature transmitter. Thermocouple thermometer is the widely used a thermometer, thermocouple thermometer is a kind of temperature electric measuring instrument, it is usually by thermocouple, thermocouple cold junction compensation device ( Or element) And display instrument of three parts, among with wires connected. The thermocouple temperature measurement end compensation is how to calculate the beginners are into the train of thought, because they are not thoroughly understand the 'law of intermediate temperature'. Intermediate temperature law refers to: two contact thermocouple loop ( Temperature T, T0) Thermoelectric potential between two nodes is equal to the thermocouple in the thermoelectric potential when temperature T, Tn and when the temperature of Tn, T0 algebra and thermoelectric potential. Tn say middle temperature. The application of law of intermediate temperature: when the cold end temperature of 0 degrees Celsius, cannot use the known loop real thermoelectric potential E ( t, t0) Look-up table directly calculate the temperature value of hot end. More can't use the known loop real thermoelectric potential E ( t, t0) Direct look-up table to calculate the temperature value, plus the cold end temperature measured temperature value of hot end, for sure, but need to be done according to the laws of the intermediate temperature correction. Thermocouple temperature measurement, the cold end compensation calculation method, sum up a word: 'first summation, then look-up table temperature'.
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