Siemens ET 200pro Operating Instructions Manual page 430

Analog electronic modules
15.8 Overview of parameters of the analog electronic modules
Reference junction
A difference in temperature between the measuring point and the reference junction (free
ends of the thermocouple at the terminal point) generates a voltage between the free ends,
namely the thermoelectric voltage. The level of the thermoelectric voltage depends on:
● The difference between the temperature at the measuring point and the temperature at
the free ends
● The combination of materials used to make the thermocouple
Because a thermocouple always detects any temperature difference, the temperature at the
reference junction must be determined in order to establish the temperature at the
measuring point.
The following parameters enable you to switch between the following compensation types:
Compensation Explanation
type
None Properties
With this compensation type, the reference junction temperature for thermocouples is measured
outside the 4 AI TC High Feature electronic module. You can, for example, connect a compensating
box to the thermocouple for this purpose.
Principle of operation
With this compensation type, the temperature at the reference junction is defined as 0 °C.
This can be achieved using a compensating box. A compensating box is required for each
thermocouple.
Connection
Use copper cables to connect the compensating box to the CM IO 4 x M12 of the 4 AI TC High
Feature (see the example in the Appendix, Connecting thermocouples to analog inputs (Page 520)).
Note: Type B thermocouples do not need a compensating box.
Internal Properties
With this compensation type, the reference junction temperature is established using an internal
temperature sensor integrated in the 4 AI TC High Feature electronic module.
Note: Please note the response time when the ambient temperature changes.
Principle of operation
The temperature at the reference junction is recorded using an internal temperature sensor.
All the 4 AI TC High Feature channels you select for this compensation type obtain the same
reference junction temperature.
Connection
Connect the thermocouples to the CM IO 4 x M12 of the 4 AI TC High Feature either directly or using
compensating lines (see the example in the Appendix, Connecting thermocouples to analog inputs
(Page 520)).
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ET 200pro distributed I/O system
Operating Instructions, 03/2013, A5E00335544-08