Siemens SIMATIC ET 200AL, System Manual

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Parameters/address space
4.3 Explanation of the parameters
Analog input module AI 4xRTD/TC 4xM12 (6ES7144-5KD50-0BA0)
Equipment Manual, 08/2021, A5E48753535-AB
27
Reference junction
If the measuring point is subjected to a different temperature than the reference junction
(free ends of the thermocouple at the connection point), a thermal voltage arises between
the free ends. The level of the thermal voltage depends:
•
On the temperature difference between the measuring point and the free ends
• On the type of material combination of the thermocouple
Since a temperature difference is always detected with a thermocouple, the temperature of
the reference junction has to be determined in order to determine the temperature of the
measuring point.
The possible compensation types that can be parameterized via the parameter "reference
junction" are set out in the table below.
Table 4- 5 Parameterizable compensation types for the "reference junction" parameter
Compensation
type
Explanation
Fixed reference
temperature
Properties
With this type of compensation, the reference junction temperature is stored as a fixed value.
Principle of operation
The reference junction temperature is specified in the "fixed reference temperature" parameter.
The possible value range is displayed in the Table 4-4 Value ranges for the "reference temperature" for
thermocouples. (Page 26).
The configured reference junction temperature applies to all channels of the module that you have
selected for this type of compensation.
Wiring
Connect the thermocouples to terminals 3 and 4 from the reference junction with copper cables.
If you detect the reference junction temperature directly at the M12 connector of the
AI 4xRTD/TC 4xM12, you can also connect the thermocouples directly or with compensating lines.
Dynamic reference
temperature
Properties
With this compensation, the reference junction temperature can be measured via an external module
of another station.
Principle of operation
The reference junction temperature is transferred by means of SFB 53 by the data records 192 - 195
from the CPU to the AI 4xRTD/TC 4xM12 electronic module.
Each channel that you select for this type of compensation can have its own reference junction tem-
perature assigned to it via the user program. For more information, see section Structure of a data
record for dynamic reference temperature (Page 31).
Wiring
Connect the thermocouples to Terminals 3 and 4 from the reference junction with copper cables.
If you detect the reference junction temperature directly at the M12 connector of the
AI 4xRTD/TC 4xM12, you can also connect the thermocouples directly or with compensating lines.