FAQ (Frequently Asked Questions)
Thermocouples/Compensating Cables
What is the error when a thermocouple is connected with a terminal or others?
With an instrumentation by a general process, the terminals of a thermocouple are collected into a J. BOX or others and reconnected to the extension lead wires with a general purpose terminal block in many cases. That is a dissimilar metal connection. What is the error?
With a general measurement method, to reduce the cost, the temperature is measured with a thermocouple and an extension lead wire is used up to a distant measurement instrument. Even though base metal thermocouples (K, E, J and T) are cheap, there are differences if the cost for thermocouples is compared to ones for extension lead wires; therefore, extension lead wires are used.
Thermocouples are connected to extension lead wires with terminal boxes and commercially available standard terminal panels are used for connection in many cases. Extension lead wires are conducting wires whose thermo-electromotive force characteristics are almost equivalent to those of thermocouples; however, metal plates used for terminal plates are copper alloy in most cases and the + side and - side have the same metal type. Therefore, dissimilar metals exist in a thermocouple circuit and the occurrence of an error may be cause for concern.
Strictly speaking, dissimilar metals exist in a thermocouple circuit and when temperature differences are generated at the both ends, errors are generated. Therefore, if temperature differences are not generated within ten-odd mm of the thermocouple connection side and the extension lead wire connection side of the terminal part, we can assume there will be no errors. It is difficult to imagine a use method with a large temperature difference for both distances under normal use methods, and the product is used with the assumption that the temperature of the terminal part is uniform.
However, in places where the installation environment is extremely severe, the temperature of the terminal part is always high and a large temperature change is expected, temperature differences may be generated at the connection part. We may provide support in that case by manufacturing terminal plates, crimped terminals for the thermocouple terminal, etc. with materials the same as those of the thermocouple.
Technical Data
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