Thermocouple Cables
It is often desirable to connect a thermocouple probe as part of a very long circuit from the sensor itself to a remote reference unit and/or measuring instrumentation. Yet, we would rather avoid the expense of high specification thermocouple cables on a long run. Connecting cheaper cable would be ideal - but we need to do so without having to take particular care that the temperature where the connection is made is known and taken into account. We only want to concern ourselves with the hot junction and remote reference junction temperatures - in the usual way.

For this to be possible, the thermoelectric properties of the additional conductors must not differ too much from those of the thermocouple itself. Extension and compensating cables provide convenient, economic solutions - each with its pros and cons.

Extension cables use wires of nominally the same conductors as the thermocouple itself, which thus inherently possess similar thermopower characteristics, and present no connection problems. Miss-match errors arising from high connecting box temperatures are likely to be relatively small. These cables are less costly than thermocouple wire, although not cheap, and are usually produced in a convenient form for carrying over long distances - typically as flexible wiring or multi-core cables. They are recommended for best accuracy.

Compensating cables, on the other hand, are less precise, but cheaper. They harness quite different, relatively low cost alloy conductor materials whose net thermoelectric coefficients are similar to those of the thermocouple in question, but which do not match them as faithfully as do extension cables. Thus, the combination develops similar outputs to those of the thermocouple, but the operating temperature range has to be restricted to keep miss-match errors acceptably small.

Obvious examples where the use of compensating cables can save cost - and at the same time be a lot more convenient to install - include the situation where rare (say platinum) metal thermocouples, or alternatively heavy gauge base metal thermocouples (say on an industrial furnace or in a nuclear reactor) are being used. Here the cost of the thermocouple material is clearly quite high, and in the latter case far from ideal for weaving long distance around an industrial plant.

Much lower cost compensating cables can provide an instant economic solution to extending the thermocouple circuit. Also, since it is available as relatively light, multi-stranded wires, the practical issues are resolved. In practice there would be a junction box close to the measuring point, probably in the thermocouple head to allow the coupling of the different wires. It must, however, be borne in mind that if the temperature of the connector is allowed to deviate beyond an acceptable band, the output from the compensating cables will diverge progressively from that of the thermocouple (since their thermopowers are not identical), and errors in the temperature reading will result.

Another example of the use of compensating cables is in situations where Type K thermocouple is closely matched at low temperatures by the combination of Cu vs Cu-Ni conductors. As one conductor is already copper, the number of reference junctions is halved - which provides a distinct advantage, especially with large multi-thermocouple projects. Further, the loop resistance of this cable is also somewhat less than the equivalent Type K conductors.

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