Priniples of analog value processing
4.6 Wiring thermocouples
S7-300 Automation System Module data
4-14 Manual, 08/2006, A5E00105505-04
Operating principle of thermocouples
Any temperature difference between the measuring point and the free ends of the
thermocouple (point of connection) induces a thermoelectric voltage whch is tapped at the
terminating ends. The thermoelectric voltage induced on the thermocouple is a function of
the temperature difference between the measuring point and the free ends, and is also
determined by the material factor.
Thermocouples always sense a temperature difference. It is therefore essential to hold the
free ends at the known temperature of a reference junction, in order to be able to determine
the temperature at the measuring junction.
The thermocouples can be extended from their point of connection to the reference junction
by means of compensating cables. These compensating wires are made of the same
materials as the thermocouple wires. The supply lines between the reference junction and
module are made of copper.
Note
Make sure of the correct polarity, for the device will otherwise return significant measuring
errors.
Compensation of the reference junction temperature
You can compensate for the influence of temperature fluctuation at the reference junction by
means of a compensating circuit.
You have several options of measuring the reference junction temperature, in order to
calculate the absolute temperature value as a function of the temperature difference
between the reference and measuring junction.
You can use either an internal or an external compensating circuit, depending on the
required location of the reference junction.
Options of compensating the reference junction temperature
Table 4-1 Options of compensating the reference junction temperature
Option Explanations
No compensation To record only the temperature difference between the measuring
point and reference junction.
Internal compensation
(for the wiring, see
Connecting thermocouples with
internal compensation box to electrically isolated
analog inputs
)
The internal compensation is based on a comparison using the
internal temperature (thermocouple internal comparison) of the
module.
External compensation with compensation box in
the feed lines of each thermocouple (the wiring is
shown in the figure
Connecting thermocouples with
compensation box to electrically isolated analog
inputs
and
connecting thermocouples with
reference junction (order no. M72166-xxx00) to
electrically isolated analog inputs
)
You have already measured and compensated the reference
junction temperature (thermocouple external comparison) using an
interconnected compensating box in the feed lines of each
thermocouple.
Further processing at the module is not required.
Only for SM 331; AI 8 x TC:
External compensation with resistance thermometer
for recording temperature at reference junction
You can measure the reference temperature using a (platinum or
nickel) resistance thermometer, and compute it in the module for any
thermocouple.
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