Operation With An External Cold Junction - Beckhoff ELX3312 Operating Manual

5.4

Operation with an external cold junction

For temperature measurement with an external reference point/cold junction, the value 2 (external process
data) must be set in object 0x80n0:0C [} 74]. The thermocouple is not connected directly to the terminal in
this case (the cold junction compensation would take place internally in the case of direct connection), but
rather it is coupled to the terminal via a connecting cable.
In this case, the temperature T
process via the fieldbus master and fieldbus as a variable (external) (see fig. External cold junction). The
reference data are written into index 0x70n0:11 [} 77].
Alternative to cold junction measurement
As an alternative to the procedure described above, the cold junction can be maintained at a de-
fined temperature through ice water (0°C), for example. In this case, the temperature is known with-
out measurement of the cold junction temperature (Fig. External cold junction) and can be reported
to the ELX331x via the process data.
Ensure measurement accuracy
The user has to ensure, that an application of external cold junction measurement have no negative
effect to the measurement accuracy.
Fig. 44: External cold junction
Take explosion protection precautions for temperature sensors in explosion-hazardous ar-
eas!
The external temperature sensor for the reference temperature may be installed in explosion-hazardous ar-
eas. In this case, appropriate explosion protection precautions must be taken for this temperature sensor.
ELX3312 and ELX3314
is sensed by a temperature sensor at the cold junction and fed to the
v
WARNING
Version: 1.3.0
Parameterization and programming
61