II - DCE FEL ČVUT v Praze

Thermocouples
with direct
looping-in of
compensation box
Equaalizing conductor
(material with same
thermal e.m.f.
as thermocouple)
Thermocouple
I/O Modules with Intrinsically-Safe Signals
C79000-G7076-C152-04
.
.
.
When all thermocouples are wired floating, it is possible to loop the
compensation box directly into the measuring circuit.
The compensation channel CH7 which is not required can now be used as an
additional measurement input.
The measurement mode ”thermocouples with linearization and
compensation to 0 o C” must be set for all channels. The thermocouples
which use a compensation box must all be of the same type.
Supply
conductor
(copper)
Reference junction
.
.
.
+
-
CH0
.
.
.
CH6
CH7
U v
M+
M-
M+
M-
M+
M-
ADU
Compensation box with
reference junction temperature of 0
o C
SIMATIC S7 Ex Analog Modules
P5V
M
Logic
Backplane
bus
Fig. 3-5 Connection of floating thermocouples to a compensation box and measurement mode ”Compensation to
0 o C” with the analog input module SM 331; AI 8 x TC/4 x RTD
Advantages: – When using a compensation box with a reference junction temperature of 0 oC, the
voltage corresponding to the reference junction temperature is subtracted directly.
– Channel 7 can be used as an additional measuring channel in this circuit variant.
– The number of connection lines between the compensation box and analog input
module is reduced.
– Faults which are attributed to isolated compensation measurement do not occur.
Condition: The thermocouples which are routed to the same compensation box must only be
grounded once at one point.
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