Thermocouples
5.3 Compensation for the reference junction temperature
Analog value processing
98 Function Manual, 06/2014, A5E03461439-AC
"None" or external compensation
Function principle
The reference junction temperature of the thermocouples is measured outside the analog
input module, e.g., by means of a compensating box on the thermocouple. The temperature
of the reference junction for this type of compensation is specified as 0° C.
The compensating box includes a bridge circuit that is calibrated for a specific reference
junction temperature (compensation temperature). The connectors for the ends of the
compensating line of the thermocouple form the reference junction. If the actual comparison
temperature deviates from the compensation temperature, the temperature-dependent
bridge resistor changes. A positive or negative compensation voltage is generated and
added to the thermal voltage.
Note
Thermocouples of type B do not require a compensating box.
Configuration steps:
1. Open the project in STEP 7.
2. Select the required analog module in the Device view.
3. Select the "Inputs" tab in the Inspector window of the selected module and the respective
channel.
4. Select the "Thermocouple" value as "Measurement type" in the "Measuring" area.
5. As "Reference junction", select the value "None" or "External compensation" depending
on the used module.
Figure 5-15 No reference junction
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