Issue of mismatch between range in PLC and range indicated on nameplate for K-type thermocouples
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As the title suggests, in a certain project, the range indicated on the label of the K-type thermocouple is not -40°C to 800°C; instead, the range set in the PLC is 50–500°C. If the PLC displays 300°C, what should be the actual temperature?
I’m not sure what specific question you have. If the configuration is correct and there are no issues with any of the hardware components, then the value displayed by the PLC represents the actual temperature
1. Actual temperature: (300–50)/(500–50) = (X–(-40))/(800–(-40)); thus, X = 426.7. 2. The range of the transmitter should be set to match the range of the PLC, which is one of the key aspects in circuit inspection.
There shouldn’t be a temperature transmitter on site; instead, it goes directly to the PLC’s thermocouple module, right? With such a configuration, this temperature is the actual temperature.
Thermocouple temperature measurement outputs a temperature value corresponding to mV values; if the PLC receives mV signals, it has little to do with the range setting. If it receives 4–20mA signals from a temperature transmitter, then the PLC’s range setting must be consistent with that of the temperature transmitter.
If there is no safe deletion or conversion of the temperature variation into a 4-20mA signal, then 300 represents the actual temperature.
If the thermocouple does not come with a transmitter, then it should represent the actual temperature. The input/output values of the thermocouple are directly corresponding to each other; the relationship between temperature and millivolt value is not affected by the range ； The range of a thermocouple is more about the allowable value range.
The PLC should match the range of the safety barrier; if there is no safety barrier, a compensation cable should be used, with one cable corresponding to the range indicated on the thermocouple’s label