Siemens SIMATIC ET 200S Manual page 428

Analog Electronic Modules
Extension to a reference junction
The thermocouples can be extended from their connection side by compensation
cables as far as the reference junction (transition to copper wiring). The reference
junction can also be an ET 200S terminal module.
The compensation cables are made of the same material as the wires of the
thermocouple. The connecting cables are made of copper. Make sure that you do
not reverse polarity.
Use of a compensation unit
The influence of the temperature on the reference junction of a thermocouple (e.g.
terminal box) can be compensated with a compensation unit.
The compensation unit contains a bridge circuit that is compensated for a specific
reference junction temperature (compensation temperature). The thermocouples or
their compensation wiring are connected to the compensation unit. The
compensation unit then forms the reference junction.
If the actual reference temperature deviates from the compensation temperature,
the temperature-dependent bridge resistance changes. A positive or negative
compensation voltage is generated and added to the thermal voltage.
Compensation units with the reference junction temperature of 0 °C must be
used for compensation of the analog input module.
Note:
• The compensation unit must have an isolated power supply.
• The power unit must have sufficient interference filtering, e.g. by a grounded
shield winding.
12-22
ET 200S Distributed I/O System
EWA-4NEB 780602402-12