A thermoelectric measuring device is built up with a sensor, the thermocouple or the mineral insulated, thermocouple), an internal or external reference junction with a known temperature, a voltmeter or a temperature controller or a thermograph as well as the suitable connection wires between the above mentioned elements.

The thermocouple with the measuring point (hot junction) is located inside the thermocouple assembly. The thermoelectric voltage being produced at the measuring point depends on the temperature difference between the measuring point and the reference junction.

For technical and economic reasons, the thermocouple is not always directly connected to the reference junction. The open ends of the thermocouple are usually connected to a plug or ceramic terminal block inside the connection head. The open ends of the thermocouple are extended to the reference junction with the aid of a compensation or extension cable. Fig. 1 shows a measuring system with a separate (external) reference junction. At an external reference junction the temperature ist kept on an constant level, usually (0°,20° or 50°C).

A copper cable connects the external reference junction to a temperature measuring device, which is adjusted to the constant reference temperature.

In practice, temperature measuring devices with integrated (internal) reference junctions are increasingly used. In this case the compensation or extension cable is directly connected to the binders of the measuring instrument, see Fig.
2. The temperature at the binders of the measuring instrument is internally measured and transformed in millivolts to correct the measured thermoelectric voltage.

Thermocouples with integrated transmitters in the connection head can be connected directly to the measuring device with a copper cable, here is no need for extension or compensation cable.

Compensation cables

Compensation cables are manufactured as solid or stranded wires in different materials with various insulations. In compliance with DIN 43 722 they are identified with a “C” written behind the code letter of the thermocouple (e.g.“KC”).

The wires (flexible strands or solid wires) are made of substitute materials and therefore the chemical composition differs from the corresponding thermocouple material. Different alloys may be used for the same thermocouple type. They are distinguished in this case by additional letters (e.g. “KCA” and “KCB”).

The substitute material and the corresponding thermocouple have the same thermoelectric characteristics within the allowed temperature range. Compensation cables are used to extend the thermocouple types K, N, R, S and B because the substitute materials are cheaper than the corresponding thermocouple material, particularly for types R, S and B.

T/C extension cables

Extension cables for thermocouples are manufactured as stranded or solid wires in different materials with various insulatiions. In compliance with DIN 43 722 they are identified with an ”X” behind the code letter of the thermocouple (e.g. “KX”).

The wires (flexible strands or solid wires) are made of substitute materials. These conductor materials and the corresponding thermocouples have the same nominal structure and chemical composition. However, the thermoelectric voltage of the conductor materials have to correspond to the basic values of the corresponding thermocouple only up to 200 °C (exception: type “TX” to 100°C). This is the characteristic difference between extension cables for thermocouples and insulated thermo wires.

Extension cables for thermocouples are available for thermocouples types K, N, J, T and E (all according to DIN EN 60 584).