Connecting Thermocouples - Siemens SIMATIC S7-400 Reference Manual

5.13

Connecting thermocouples

Thermocouple design
Thermocouples consist of a pair of thermal probes, and all necessary installation and
connecting parts. The thermocouple consists of two wires of dissimilar metals or metal alloys
soldered or welded together at the ends.
There are different types of thermocouple, depending on the composition of the material used,
for example, K, J, N thermocouples. The measuring principle of all thermocouples is the same,
irrespective of their type.
°C
(1)
(2)
(3)
(4)
(5)
(6)
(7)
Figure 5-15
Operating principle of thermocouples
If the measuring point is subjected to a temperature different from that of the free ends of the
thermocouple (point of connection), a voltage, the thermo e.m.f., occurs at the free ends. The
magnitude of the thermo-e.m.f. generated depends on the difference between the temperature
at the measuring junction and the temperature at the free ends, as well as on the material
combination used for the thermocouple.
Since a thermocouple always measures a temperature difference, the free ends must be kept
at a known temperature at a reference junction in order to determine the temperature of the
measuring junction.
S7-400 Automation System Module Data
Reference Manual, Ausgabe 11/2016, A5E00850736-08
1
3
4
5
6
7
Measuring junction
Thermocouple with positive or negative limbs
Connecting point
Compensating leads
Reference junction
Leads
Measurement input
Thermocouple design
Analog modules
5.13 Connecting thermocouples
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