Siemens SIMATIC ET 200AL, System Manual

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Thermocouples
5.2 Structure and operating principle of thermocouples
Analog value processing
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Function Manual, 06/2014, A5E03461439-AC
Operating principle of thermocouples
Temperature differences between the measuring point and the free ends of the pair of
thermal sensors (connection point) generate a thermal voltage at the reference junction.
The value of the thermal voltage is determined by the temperature difference between the
measuring point and the free ends, as well as by the material composition of the pair of
thermal sensors.
Thermocouples always acquire a differential temperature, which means that the temperature
at their free ends (reference junction) must be known to determine the temperature at the
measuring point.
The thermocouple can be extended with compensating lines from the connection point.
This allows the reference junction to be located at a specific point, for example, where the
temperature can be kept constant or the temperature sensor is easy to install. The
compensating lines and thermocouple wires are made of the same material. Copper cables
are used to connect the reference junction to the module.
Note
Always observe the polarity, as incorrect wiring will result in unacceptable measuring errors.