Siemens SIMATIC S7-400 Reference Manual page 216

Analog modules
5.13 Connecting thermocouples
The thermocouples can be extended from their point of connection to the reference junction
by means of compensating wires. These compensating wires are made of the same materials
as the thermocouple wires. The supply lines are made of copper.
Note
Make sure that the polarity is correct, otherwise significant measuring errors will occur.
Compensation of the reference junction temperature
There are several options for you to choose from for acquiring the reference junction
temperature in order to obtain an absolute temperature value from the difference in
temperature between the reference junction and measuring point.
You can use internal or external compensation, depending on where you want the reference
junction to be.
The last column of the following table lists the feature you must set for the "Reference Junction"
parameter in STEP 7. The reference temperature value is a separate parameter in STEP 7.
Options for compensation of the reference junction temperature
Option
No compensation
Internal compensation
External compensation with a compensat‐
ing box in leads of an individual thermo‐
couple
External compensation with a resistance
thermometer to obtain the reference junc‐
tion temperature (recommended method)
External compensation with a resistance
thermometer when thermocouples with
the same reference junction are divided
amongst several modules
Constant reference junction temperature
(thermometer, ice bath)
Principle of operation of internal compensation
Internal compensation allows you to form the reference point at the terminals of the analog
input module. In this case, route the compensating lines directly to the analog module. The
internal temperature sensor measures the module's temperature and returns a compensation
voltage.
Note: internal compensation is not as accurate as external compensation.
216
Explanations
When you want to acquire only the difference in tempera‐
ture between the measuring point and the reference junction
If you employ internal compensation, the internal tempera‐
ture of the module is used for comparison purposes.
You have already acquired and compensated the reference
junction temperature using a compensating box, which you
have looped into an individual thermocouple.
Further processing at the module is not required.
You can acquire the reference temperature by means of a
resistance thermometer (pt 100) and have it calculated by
the module for any thermocouple.
Use a resistance thermometer on a module that measures
the reference junction temperature. Read in the climatic
temperature value to the CPU and transfer the value to the
other modules using SFC55.
If the reference junction temperature is constant and known,
you can specify this value in parameter assignment in STEP
7.
S7-400 Automation System Module Data
Reference Manual, Ausgabe 11/2016, A5E00850736-08
Reference junction
None
Internal
None
RTD on Channel 0
RTD on Channel 0
Reference temper‐
ature value