Siemens SIMATIC S5-90U, Системное руководство

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Module Spectrum S5-90U/S5-95U
Function
The module 6ES5 464-8MA11 is suitable for connection of thermocouples and for voltage
measurement.
Design and Mode of Operation of Thermocouples
A thermocouple consists of
•
the thermocouple wires (sensors) and
• the necessary installation and connection components.
The thermocouple consists of two wires of different metals or alloys, the ends of which are soldered
or welded to each other. Since it is possible to have different combinations of materials, there are
various types of thermocouples, e.g. types K, J and L. The measuring principle is the same for all
types.
Figure 15-1. Thermocouple
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6
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1 Trimming resistor
2 Supply conductor
3 Reference junction
4 Equalizing conductor
5 Point of connection
6 Thermocouple with plus and minus
thermo limbs
7 Measuring junction
If the measuring junction is subject to a different temperature than the free ends of the
thermocouple, a thermal emf is generated between the free ends.
The value of the thermal emf depends on the difference between the temperature of the measuring
junction and the temperature at the free ends, as well as on the material combination of the
thermocouple. Since one temperature difference is always sensed with one thermocouple, the free
ends must be kept at a known temperature at a reference junction in order to determine the
temperature of the measuring junction.
From their point of connection, the thermocouples can be extended via equalizing conductors up
to a point with, as far as possible, constant temperature (reference junction).
The equalizing conductors are of the same materials as the wires of the thermocouple. Make sure
that the equalizing conductors are connected with the right polarity, as polarity reversal can cause
considerable measuring errors.
The influence of temperature fluctuations at the reference junction can be compensated by an
equalizing circuit, e.g. by a compensating box.
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EWA 4NEB 812 6115-02b