Siemens SIMATIC S7 System Manual page 161

Basic operation of a thermocouple
Thermocouples are formed whenever two dissimilar metals are electrically bonded to each
other. A voltage is generated that is proportional to the junction temperature. This voltage is
small; one microvolt could represent many degrees. Measuring the voltage from a
thermocouple, compensating for extra junctions, and then linearizing the result forms the
basis of temperature measurement using thermocouples.
When you connect a thermocouple to the TC analog input module, the two dissimilar metal
wires are attached to the module at the module signal connector. The place where the two
dissimilar wires are attached to each other forms the sensor thermocouple.
Two more thermocouples are formed where the two dissimilar wires are attached to the
signal connector. The connector temperature causes a voltage that adds to the voltage from
the sensor thermocouple. If this voltage is not corrected, then the temperature reported will
deviate from the sensor temperature.
Cold junction compensation is used to compensate for the connector thermocouple.
Thermocouple tables are based on a reference junction temperature, usually zero degrees
Celsius. The cold junction compensation compensates the connector to zero degrees
Celsius. The cold junction compensation restores the voltage added by the connector
thermocouples. The temperature of the module is measured internally, and then converted to
a value to be added to the sensor conversion. The corrected sensor conversion is then
linearized using the thermocouple tables.
For optimum operation of the cold junction compensation, the thermocouple module must be
located in a thermally stable environment. Slow variation (less than 0.1 °C/minute) in
ambient module temperature is correctly compensated within the module specifications. Air
movement across the module will also cause cold junction compensation errors.
If better cold junction error compensation is needed, an external iso-thermal terminal block
may be used. The thermocouple module provides for use of a 0 °C referenced or 50 °C
referenced terminal block.
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
6.1 Configuring the operation of the PLC system
PLC device configuration
161