II - DCE FEL ČVUT v Praze

SIMATIC S7 Ex Analog Modules
Operating
principle of
thermocouples
Extension to a
reference junction
Use of
thermostatically
controlled terminal
boxes
Compensation of
thermocouples
External
compensation
3-26
If the measuring junction is subjected to a different temperature than at the
free ends of the thermocouple (connection point), a voltage, i.e. the thermal
e.m.f. is produced between the free ends.
The value of the thermal e.m.f. depends on the difference between the
temperature at the measuring junction and the temperature at the free ends as
well as on the type of material combination used for the thermocouple. Since
a temperature difference is always recorded with a 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.
Thermocouples can be extended from their connection point by equalizing
conductors up to a point with known temperature (reference junction).
The material of the equalizing conductors has the same thermal e.m.f. as the
wires of the thermocouples. The conductors leading from the reference
junction up to the analog module are made of copper.
It is possible to use temperature-compensated terminal boxes. Use boxes with
reference junction temperatures of 0 C or 50 C when using
thermostatically controlled terminal boxes.
External or internal compensation can be adopted depending on where
(locally) you require the reference junction.
In the case of external compensation, the temperature of the reference
junction for thermocouples is taken into consideration by means of a
compensation box or thermal resistor.
In the case of internal compensation, the internal terminal temperature of the
module is used for the comparison.
The temperature of the reference junction can be compensated by means of a
compensating circuit, e.g. by a compensation box.
The compensation box contains a bridge circuit which is calibrated for a
certain reference junction temperature (compensating temperature). The
terminal connections for the ends of the equalizing conductor of the
thermocouple form the reference junction.
If the actual reference temperature deviates from the compensating
temperature the temperature-dependent bridge resistance will change. A
positive or negative compensation voltage is produced which is added to the
thermal e.m.f.
Compensation boxes with a reference junction temperature of 0 C must
be used for the purpose of compensating the analog input modules.
A further external compensation option is to record the reference junction
temperature with a thermal resistor in the climatic range (e.g. Pt 100).
I/O Modules with Intrinsically-Safe Signals
C79000-G7076-C152-04