Structure And Operating Principle Of Thermocouples - Siemens SIMATIC ET 200AL System Manual

5.2

Structure and operating principle of thermocouples

Thermocouple structure
A thermocouple consists of a pair of thermal sensors and the respective installation and
wiring elements. Two wires made of different metals/alloys are welded together at their ends
(hot end). The welding point forms the so-called measuring point, while the free ends of the
thermocouple form the reference junction.
The free ends are interconnected with the evaluation device (e.g. analog input module) via
insulated wires or cables.
The different thermocouple types, e.g. K/J/N, are derived from different material composition;
the same measuring principle is applied to all thermocouples, independent on their type.
①
②
③
④
⑤
⑥
⑦
Figure 5-1
Analog value processing
Function Manual, 06/2014, A5E03461439-AC
Thermal voltage acquisition point
Copper supply cable, for example
Reference junction
Compensating line (same material as thermocouple)
Connection point
Thermocouple with positive and negative thermal elements
Measurement point
Thermocouple
5.2 Structure and operating principle of thermocouples
Thermocouples
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