Siemens Metro 21 M216KA Operator's Manual page 214

SC Analog Electronic Modules – Parameters
Non-Insulated
Thermocouples
Extension to a
Reference
Junction
Equalizing Circuit
Use of a
Compensating Box
11-12
If you use non-insulated thermocouples, you must be careful to comply with
the permitted common-mode voltage.
The thermocouples can be extended from their connecting point via equaliz-
ing conductors to a point with as constant a temperature as possible (refer-
ence junction).
The equalizing conductors are made of the same material as the wires of the
thermocouple. The supply conductors are of copper. In this case, internal
compensation must not be set. Ensure that the polarity of the connection is
correct, since otherwise serious measurement errors occur.
The effect of temperature fluctuations on the reference junction can be com-
pensated for by an equalizing circuit (e.g. by means of a compensating box
for a reference junction outside the terminal block or by means of internal
compensation for a reference junction in the terminal block). In this case, the
equalizing conductors must lead to the terminal block.
The effect of temperature on the reference junction of a thermocouple (e.g.
terminal box) can be equalized by means of a compensating box.
The compensating box contains a bridge circuit, which is compensated for a
specified reference junction temperature (compensating temperature). The
connections for the ends of the equalizing conductor of the thermocouple
form the reference junction.
If the actual reference temperature differs from the compensating tempera-
ture, the temperature-dependent bridge resistance changes. The result is a
positive or negative compensation voltage, which is added to the thermal
e.m.f.
To compensate the analog input modules, compensating boxes with the ref-
erence junction temperature of 0 C must be used.
Note:
The compensating box must be supplied potential-free.
The power supply unit must have sufficient interference filtering (e.g. by
means of a grounded shielding winding).
ET 200L, ET 200L-SC and ET 200L-SC IM-SC Distributed I/O Device
EWA 4NEB 780 6009-02c