Cold-junction compensation methods for thermocouples

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To understand the temperature compensation issue of thermocouples, one must start with the principle of thermocouples. Here, we will only discuss the total electromotive force of the thermocouple’s closed circuit and the intermediate temperature rule related to this. The former shows that for a selected thermocouple, when the temperature of the reference terminal remains constant, the total electromotive force becomes a single-valued function of the temperature at the measuring terminal. That is, a certain thermoelectromotive force corresponds to a certain temperature, and in the calibration table of thermocouples, the temperature at the reference terminal is always 0 degrees. However, in practical applications, the temperature at the reference end varies greatly and cannot remain constant at 0 degrees, which leads to measurement errors; this is why thermocouples need to be temperature-compensated. In practical applications, the reference terminal of a thermocouple is often referred to as the cold junction. Methods for compensating the cold junction temperature of thermocouples include: 1. The ice bath method, which is commonly used in laboratories to maintain the reference terminal temperature at 0 degrees; however, it is costly and difficult to implement. 2. Cold-end temperature correction method: This is commonly used in applications where high precision is not required; that is, when the cold-end temperature cannot be kept at 0 degrees, it is necessary to adjust the reading displayed by the instrument. It’s easy to do, but the errors are large. 3. Compensation bridge method: It is rarely used on its own; it utilizes the potential generated by an unbalanced bridge to compensate for the changes in thermoelectric potential caused by variations in the cold junction temperature of the thermocouple. Compensation bridges are available as separate products, as well as built into instruments. 4. Compensation wire method: This is a commonly used approach. It involves extending the thermocouple so that its cold end is placed in a location with a more stable temperature (usually the control room). The cold-end temperature is then adjusted manually, by setting the instrument’s zero point to room temperature, or through automatic compensation by the circuit within the instrument. It is also impossible to extend precious metal thermocouples, as the high cost makes it unfeasible; instead, base metals with similar thermoelectric properties are used as extension wires. The intermediate temperature rule forms the theoretical basis for the use of compensation wires. Compensation wires cannot automatically compensate for changes in the temperature at the cold end of the thermocouple; they merely move the cold end of the thermocouple to a location with a more stable temperature. Compensation still needs to be carried out manually using instruments. Therefore, the compensation wire should be called a thermocouple extension cable, so as to avoid misleading people.