Siemens S7-400PLC Spezifikationen Seite 232

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Analog modules
5.13 Connecting thermocouples
S7-400 Automation System Module Data
232 Reference Manual, Edition 09/2009, A5E00850736-06
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 Explanations Reference junction
No compensation When you want to acquire only the difference in
temperature between the measuring point and the
reference junction
None
Internal compensation If you employ internal compensation, the internal
temperature of the module is used for comparison
purposes.
Internal
External compensation with a
compensating box in leads of an
individual thermocouple
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.
None
External compensation with a resistance
thermometer to obtain the reference
junction temperature (recommended
method)
You can acquire the reference temperature by means of a
resistance thermometer (pt 100) and have it calculated by
the module for any thermocouple.
RTD on Channel 0
External compensation with a resistance
thermometer when thermocouples with
the same reference junction are divided
amongst several modules
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.
RTD on Channel 0
Constant reference junction temperature
(thermometer, ice bath)
If the reference junction temperature is constant and
known, you can specify this value in parameter
assignment in STEP 7.
Reference
temperature value
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