Classification and working principle of compensation wire
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Classification and working principle of compensation wire
When we use compensation wire with K graduation to match thermocouple with N graduation, it will cause over-compensation and display temperature is too high; on the contrary, using compensation wire with N graduation to match thermocouple with K graduation will cause under-compensation and display temperature is too low.
Classification and working principle of compensation wire
Within a certain temperature range, the wire with thermoelectric properties very similar to those of thermocouple is called compensation wire of thermocouple.
According to the intermediate temperature rule of thermocouple, the total potential value of thermocouple temperature measurement circuit is only related to the temperature of hot end and reference end, and is not affected by the change of intermediate temperature.
Classification of compensation wire
According to the compensation accuracy, it is divided into ordinary grade and precision grade. The error after compensation of precision grade is roughly only half of that of ordinary grade, and it is usually used in places with high measurement accuracy requirements. For example, the tolerance of the compensation wire of S and R graduation is ±2.5℃ for precision grade and ±5.0℃ for ordinary grade; the tolerance of the compensation wire of K and N graduation is ±1.5℃ for precision grade and ±2.5℃ for ordinary grade. In the model, the ordinary grade is not marked, and the precision grade is indicated by 'S'.
In principle, it is divided into extended type and compensated type. The nominal chemical composition of the alloy wire of the extended type is the same as that of the thermocouple used, so the thermoelectric potential is also the same, which is indicated by 'X' in the model. The nominal chemical composition of the alloy wire of the compensated type is different from that of the thermocouple used, but within its working temperature range, the thermoelectric potential is close to the nominal value of the thermoelectric potential of the thermocouple used, which is indicated by 'C' in the model.
In terms of working temperature, it is divided into general use and heat-resistant use. The working temperature of general use is 0 ~ 100℃ (a few are 0 ~ 70℃); the working temperature of heat-resistant use is 0 ~ 200℃.
In addition, the compensation wires can be divided into single-strand and multi-core (soft wire) according to the number of cores, and ordinary and shielded compensation wires according to whether they have a shielding layer. There are also compensation wires specially used for intrinsically safe circuits in explosion-proof occasions.
Several issues in application
Matching of compensation wires and thermocouples
Compensation wires of various graduations can only be used with thermocouples of the same graduation, otherwise they may be under-compensated or over-compensated. The thermoelectric potential values of commonly used thermocouples at 100℃ and 200℃ that need to be compensated are shown in
Thermoelectric potential values of commonly used thermocouples at 100℃ and 200℃, so compensation wires that match the thermocouple materials can be used to replace the precious thermocouple materials that need to be extended, and the reference end can be extended from the thermocouple junction box to the digital display terminal, and the compensation wire can compensate the original reference end temperature. In addition to reducing measurement errors, compensation wires also have the following advantages: they can improve the physical and mechanical properties of the thermocouple temperature measurement circuit. For example, the use of multi-strand wire cores or small-diameter compensation wires can increase the flexibility of the line, making it easier to connect and shield external interference; they can reduce the cost of the measurement line.
