Thermocouple extension wires has approximately the same characteristic as thermocouple wires but it’s accuracy is guaranteed over a more limited range of temperature. It is important that along the entire length of cable between a sensing point and either an instrument, or a transmitter, the leads used replicate the thermocouple material in terms of the EMF generated, through temperature gradients. It is clear however that for platinum sensors, to continue the circuit in this precious metal would be expensive and commercially vulnerable. Extension and compensating cables are used to reduce the cost.

Extension cables use the actual thermocouple materials, but in cheaper forms. This is achieved by using a cheaper insulations, wider tolerance alloys and thinner conductors (as they will see little thermal stress in their lifetime). They are produced in multi-stranded forms for ease of insulation. The thermoelectric performance is clearly very similar to the actual sensor.

Compensating cables are use completely different alloys that happen to exhibit very similar thermoelectric properties up to a limited temperature (usually 200°C). Great care should be taken to control the temperature of the junction between the compensating cable and the actual thermocouple material to keep it below the acceptable maximum. Compensating cables come in convenient cabling forms for site installations.