Siemens SIMATIC ET 200S Manual page 408

Analog Electronic Modules
Use of a compensating box
The influence of the temperature on the reference junction of a thermocouple (for
example, terminal boxes) can be adjusted with a compensating box.
The compensating box contains a bridge circuit that is adjusted for a certain
reference junction temperature (compensating temperature). You connect the
thermocouples or their adjustment lines to the compensating box. The
compensating box then forms the reference junction.
If the actual reference temperature differs from the compensating temperature, the
temperature-dependent bridge resistance changes. A positive or negative
compensation voltage occurs that is added to the thermo-electromotive force.
Compensating boxes with a reference junction temperature of 0 5C must be
used for the compensation of the analog input modules.
Please note:
• The compensating box must be supplied on an isolated basis.
• The power supply unit must have adequate interference filtering (by means of a
grounded shielding winding, for example).
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ET 200S Distributed I/O System
EWA-4NEB780602402-09