Connecting Thermocouples; Structure Of Thermocouples - Siemens Metro 21 M216KA Operator's Manual

11.6 Connecting Thermocouples

Structure of
Thermocouples
How
Thermocouples
Work
ET 200L, ET 200L-SC and ET 200L-SC IM-SC Distributed I/O Device
EWA 4NEB 780 6009-02c
Thermocouples are used to measure temperature.There are various types of
thermocouple, which differ with regard to their temperature range and output
voltage, depending on the material of their wires.
A thermocouple assembly consists of:
The thermocouple itself (sensor)
The required built-in and connected components.
The thermocouple itself consists of two wires made of different metals or metal
alloys, the ends of which are soldered or welded together. Thermocouples are
categorized into different types (e.g. R, J, K) according to the combination of
materials used. The measurement principle is the same for all thermocouple
types.
C
Figure 11-1 Structure of Thermocouples
If the measurement point is subjected to a different temperature to the free
end of the thermocouple, a potential difference develops between the free
ends, which is referred to as the thermal e.m.f.
The thermal e.m.f. depends on the difference between the temperature of the
measurement point and the temperature at the free ends of the reference junc-
tion, as well as the combination of materials used in the thermocouple. A
thermocouple always measures a temperature difference, so to determine the
temperature of a measurement point the free ends at a reference junction
must be kept at a known temperature.
SC Analog Electronic Modules – Parameters
Measurement point
Thermocouple with plus and mi-
nus components
Connecting point
Equalizing conductor
Reference junction
Supply conductor (Cu)
Trimming resistor
Measurement point for thermal e.m.f.
11-9