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Uses of compensation wires
2011-5-27 17:12:54

To understand the temperature compensation of thermocouples , we must start with the principle of thermocouples. Now we will only talk about the total thermoelectric potential and the intermediate temperature rule of the closed loop of the thermocouple . The former illustrates that for the selected thermocouple , when the reference junction temperature is constant, the total thermoelectromotive force becomes a single-valued function of the measurement junction temperature. That is, a certain thermoelectric potential corresponds to a certain temperature, and in the index table of the thermocouple, the reference junction temperature is all 0 degrees. However, at the application site, the temperature of the reference junction varies widely, and it is impossible to keep it constant at 0 degrees. This will cause measurement errors, which is why thermocouples need to perform temperature compensation. The reference junction of a thermocouple is often referred to as the cold junction in practical applications.

The methods of thermocouple cold junction temperature compensation are:
1. The cold-junction temperature correction method is commonly used in low-requirement sites, that is, when the cold-junction temperature cannot be constant to 0 degrees, the indicator value of the instrument needs to be corrected. It's easy to do but the error is large.
2. The ice bath method is commonly used in the laboratory, that is, the temperature of the reference end is constant at 0 degrees, but it is costly and difficult to do.
3. The compensation bridge method is rarely used alone. It uses the potential generated by the unbalanced bridge to compensate for the change in the thermoelectric potential caused by the temperature change at the cold end of the thermocouple . Compensation bridges are available as separate products or as built-in instruments.
4. Compensation wire method This is the most commonly used method, that is, extending the thermocouple to lead the cold end to a place with a stable temperature (usually the control room), and then manually adjusting the cold end temperature, that is, adjusting the zero point of the meter to room temperature, Or automatic compensation by the circuit in the instrument. It is impossible for the precious metal thermocouple to extend the thermocouple, because the price is too high to work, and base metals with similar thermoelectric characteristics are used to extend the wire. The intermediate temperature is the theoretical basis for applying the compensation wire. The compensation wire does not automatically compensate for the change in the temperature of the cold junction of the thermocouple . It only just leads the cold junction of the thermocouple to a place where the temperature is relatively stable. The compensation must be performed manually and by an instrument. Therefore, the compensation wire should be called a thermocouple extension line, so as not to cause misunderstanding.