Siemens SIMATIC ET 200iSP Operating Instructions Manual page 302

Analog electronic modules
13.9 Fundamentals of analog value processing
Compensation of the reference junction temperature
There are various ways of obtaining the reference junction temperature in order to get an
absolute temperature value from the temperature difference between the reference junction
and the measuring point.
Table 13-29 Compensation of the reference junction temperature
No compensation
Use of a Pt100 Cli‐
matic Range resist‐
ance thermometer to
record the reference
junction temperature
(best method)
Internal compensa‐
tion 4 AI TC
Extension to a reference junction
From their point of connection, thermocouples can be extended using equalizing cables as far
as the reference junction (transition to copper wiring). The reference junction can also be an
ET 200iSP terminal module.
The equalization lines are made of the same material as the wires of the thermocouple. The
supply lines are made of copper. Ensure correct polarity when connecting.
Compensation by means of a resistance thermometer at the 4 AI RTD
If thermocouples that are connected to the inputs of the 4 AI RTD have the same reference
junction, compensate by means of a 4 AI RTD.
302
Option
You record not only the temperature of
the measurement point. The tempera‐
ture of the reference junction (transition
from Cu line to thermocouple line) also
affects the thermo-electromotive force.
The measured value then includes an
error.
You can record the reference junction
temperature using a resistance ther‐
mometer (Pt100 Climatic Range). If par‐
ameterized accordingly, this tempera‐
ture value is distributed to the 4 AI TC
modules in the ET 200iSP where it is off‐
set against the temperature value ob‐
tained at the measuring location.
Number of reference junctions: 2
The TC sensor module (temperature
sensor) is mounted onto the terminals of
terminal module EM 4 AI TC. The tem‐
perature sensor reports the temperature
of the terminals to the 4 AI TC. This value
is then calculated together with the
measured value from the channel of the
electronic module.
Explanation
Operating Instructions, 11/2017, A5E00247483-07
Reference junction parameters
None
The parameter assignment of the IM
152 and the 4 AI TC must be coordi‐
nated:
● 4 AI RTD assigned parameters
for Pt100 climatic range in correct
slot;
● 4 AI TC: Reference junction :
"yes"; select reference junction
number "1" or "2"
● IM 152-1:Assignment of the
reference junction to a slot with 4
AI RTD; channel selection;
● 4 AI TC: Reference junction
number "internal"
ET 200iSP