Siemens SIMATIC Manual page 251

Operating principle of thermocouples
Any temperature difference between the measuring point and the free ends of the
thermocouple (point of connection) generates a thermoelectric voltage. The thermoelectric
voltage is a function of the temperature difference between the measuring point and the free
ends, and is also determined by the material composition of the thermal elements.
As thermocouples always sense temperature differences, it is essential to maintain the free
ends at the known temperature of a reference junction in order to be able to determine the
temperature at the measuring point.
The thermocouples can be extended from their point of connection to the reference junction
by means of compensating lines. These compensating lines are made of the same materials
as the thermocouple wires. The supply lines from the reference junction to the module are
made of copper.
Note
Make sure of the correct polarity in order to avoid serious measuring errors.
Compensation for the reference junction temperature
You can compensate for the influence of temperature fluctuation at the reference junction by
means of a compensating circuit.
You have several options of measuring the reference junction temperature in order to obtain
an absolute temperature value as a function of the temperature difference between the
reference junction and the measuring point.
You can use either an internal or an external compensating circuit, depending on the
required location of the reference junction.
S7-300 Module data
Manual, 06/2017, A5E00105505-AJ
Principles of analog value processing
4.6 Wiring and connecting thermocouples
251