Suppose that we have to measure an oven temperature (800 °C) with a thermocouple and that the measuring equipment is located at a distance of dozens of meters from the measuring point. Usually a few meters are sufficient to bring the temperature down to 200 °C or less. From that point, in order to reduce the costs, it is possible to use either extension or compensating cable to connect the thermocouple to the measuring device.
Extension and compensating cables are used between open ends of the thermocouple and the reference junction in those installations where the conductors of the thermocouple are not directly connected to the reference junction.
The thermoelectric properties of extension and compensating cables shall be close to the properties of the corresponding thermocouple.
Extension cables are manufactured from conductors having the same nominal composition as those of the corresponding thermocouples. They are designated with the letter «X» preceded by the designation of the thermocouple, i.e. JX for thermocouple type J.
Compensating cables are manufactured from conductors having a composition different from that of the corresponding thermocouple. They are designated with the letter «C» preceded by the designation of the thermocouple, i.e. KC for thermocouple K. In some cases different tolerances apply for the same thermocouple type over different temperature ranges. They will then be identified with an additional letter such as A or B. For example, compensating cables for thermocouple K will be identified as KCA and KCB.