The compensation wire can be used to extend the cold end of the thermocouple to a place where the temperature is relatively stable. However, the temperature of the extended end is generally not 0 ° C. The indexing of the thermocouple is obtained when the cold end temperature is 0 ° C. The matching instrument used for the thermocouple is also scaled at a cold junction temperature of 0 °C. In order to ensure the accuracy of the measurement, when using the thermocouple, only the cold junction temperature is kept at 0 ° C. Or the correction can be made to obtain the determined measurement result. In doing so, it is called the cold junction temperature compensation of the thermocouple. Commonly used cold junction temperature compensation methods are:
1 ice bath method. The cold ends extended by the compensation wires are respectively inserted into the test tubes containing the transformer oil, and the test tubes are placed in a container filled with the ice water mixture to maintain the cold end temperature at 0 °C. This method is not suitable for practical production and is mostly used in laboratories.
2 formula correction method. According to the formula (3-2), the measured thermoelectric potential EAB(t, to). and the thermoelectric potential EAB(t, to) obtained by the checklist are added to obtain the thermoelectric potential EAB of the inter-text temperature F ( t, to). Check the degree table again to find the measured temperature t. This method is only suitable for experimental empty or temporary temperature measurement, and it is not practical for continuous measurement.
3 Correct the instrument zero point method. Generally, when the instrument is displayed, the pointer refers to the zero position (mechanical zero point). If the cold junction temperature to (the room temperature) of the thermocouple is relatively constant, the test circuit can be disconnected before the temperature measurement, and the mechanical zero point of the display meter is adjusted to to, which is equivalent to pre-applying the thermoelectric potential correction On the instrument. When the measurement circuit is turned on, the indication of the display meter is the actual measured temperature.
This method is simple and easy to use. It is often used in industry. If the room temperature of the control room changes frequently, there will be some measurement error, which is usually used when the temperature measurement is too high.
4 compensation bridge method. When the temperature of the cold junction of the thermocouple fluctuates greatly, a compensation bridge (unbalanced bridge) can be connected behind the compensation wire to generate an unbalanced voltage Δu to automatically compensate for the temperature change of the cold junction of the thermocouple. The thermoelectric potential changes.
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