Siemens SIMATIC Distributed I/O ET 200eco PN Operating Instructions Manual page 174

Reference junction for thermoresistor (TC)
A difference in temperature between the measuring point and the free ends of the
thermocouple (terminal point) generates a voltage between the free ends, namely the
thermoelectric voltage. The value of this thermoelectric voltage is determined by the
temperature difference between the measuring point and the free ends, and by the type of
material combination of the thermocouple. Since a thermocouple always measures a
temperature difference, the free ends at the reference junction must be maintained at a
known temperature in order to determine the temperature of the measuring point.
In the case of the I/O device 8 AI 4 U/I + 4 RTD/TC 8×M12 and 8 AI RTD/TC 8xM12, the
following compensation settings are possible:
• No / external compensation: The reference junction temperature is measured outside the
module, e.g., via a compensation slot.
With this compensation type, the reference junction temperature of the thermocouples is
measured outside the two I/O devices. For this purpose you can, for example, connect a
compensation slot to the thermocouple.
With this compensation type, the temperature of the reference junction is specified at 0 °C
.
The temperature of 0 °C is reached when a compensation slot is used. One compensation
slot is required per thermocouple.
• Internal: The temperature of the measuring point is measured in the housing of the I/O
device.
With this type of compensation the reference junction temperature is determined with an
internal temperature sensor. One temperature sensor is integrated into each I/O device.
The reference junction temperature is detected by an internal temperature sensor.
The same reference junction temperature is assigned to all the channels of the I/O devices
that you selected for this type of compensation.
With the I/O device 8 AI RTD/TC 8xM12, the following compensation settings are also possible
:
• RTD (0): The reference junction temperature is determined by means of a thermal resistor
(Pt1000).
With this compensation the reference junction temperature is determined by measuring
the resistance value of a Pt1000 at the terminal point of the M12 compensation
connector. The resistance measurement may only be carried out on the round socket X1
(Channel 0).
The reference junction temperature is determined by the resistance value Pt1000.
The same reference junction temperature is assigned to all the channels of the I/O device
8 AI RTD/TC 8xM12 that you selected for this type of compensation.
The cycle time increases by 1 x the conversion time.
If the compensation RTD (0) is selected on a channel of the I/O device 8 AI RTD/TC 8xM12,
the measurement type for Channel 0 (X1) is limited; that is, only the measurement types
"Thermocouple" or "±80 mV" are possible.
A wire break at Pt1000 cannot be detected and is reported with the diagnosis "Reference
channel error".
• The M12 compensation connector is provided for the temperature compensation "RTD (0)"
(prerequisite: "Reference junction" parameter set to "RTD (0)". For all other temperature
compensations, no M12 compensation connector is required.
The M12 compensation connector has an integrated resistance thermometer Pt1000 (with
α = 0.003851) for compensating the reference junction temperature of the
ET 200eco PN
Operating Instructions, 06/2022, A5E01250250-AN
I/O device analog input/analog output
10.1 I/O device analog input
175