In particular used for temperature measurement in industrial processes in order to achieve accurate measuring values even at high temperatures and harsh ambient conditions.
It consists of two different metals which are connected (welded) at their ends. Due to the so-called Seebeck effect, voltage is generated along the conductor in case of temperature differences. This voltage value is used as measurand. These thermal voltages are measured by means of sensitive millivoltmeters which are incorporated in the conductor circuit.
It must be noted that, as a matter of principle, any measurement with thermocouples is a differential measurement between the temperature at the thermocouple head (of the welding or soldering point) and the transfer point (connection point of the measuring instrument also referred to as reference point) of the thermal and compensation line to the connecting line (copper) or plug. Thus, thermal voltage results from the temperature difference between hot soldering point and the cold end of the element (reference point). (Figure 1).
Therefore, the temperature must also be measured at the reference point. Since it is not always possible to mount the measuring instrument directly at the reference point (at the cold end), the elements are extended to the reference point (zone of known and constant temperature) by special compensation lines.
Up to a temperature of 200 °C, compensation lines have the same thermoelectric properties as the thermocouple. Therefore, no additional voltage is generated. Certain material combinations were selected from the variety of possible thermocouples which are specified as standard thermocouples in DIN EN60584-1 and IEC 584-1, respectively. For correct measurement with thermocouples, they must be connected to the appropriate compensation line.
Figure 2 shows the international color coding for compensation lines and depicts the most common thermocouples.