FAQ (Frequently Asked Questions)

Thermocouples/Compensating Cables

Since the distance from the thermocouple to the measuring instrument is far away, we use a compensating cable, but what is the influence on distance restriction and accuracy?

At some sites such as power plants and large-scale chemical plants, there may be a distance of several kilometers from the temperature measuring place to the instrument room. What are the problems in that case?

With a thermocouple circuit, the generated thermo-electromotive force is determined in principle only by the thermocouple materials used and the temperatures at both ends and the characteristics are not changed by the wire diameter and length of the thermocouple wire. Therefore, even if the measuring part is thin, and thick in the middle, there is no impact, and even with a thermocouple with a thin plate, there is no problem with measurement accuracy.
However, the measurement instrument which measures the thermo-electromotive force always has a restriction of resistance input on the measurement instrument. Therefore, for large-scale plants such as power plants, we design the extension lead wire diameter to be thick for long distance transmission. As for temperature adjusters from a certain manufacturer, the allowable signal source resistance of the thermocouple is 100 Ω or lower and the allowable wiring resistance for the resistance thermometer sensor is 10 Ω or lower per wire.
Recently, the need for sheathed thermocouples with a thin outer diameter has increased, and only the thermocouple part has a high resistance in some cases. We had a problematic case where it was expected that measurement errors would increase in a reactor in a chemical plant since φ0.5 sheathed thermocouples of several meters long was used in the plant and it was assumed the resistance would exceed over 1kΩ. The solution method was that turning off the burn-out detection circuit within the measuring instrument so that the allowable resistance of the conducting wire on the thermocouple side increased.

Technical Data

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