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Hello everyone,

I read data from Merlin Gerin PM9C using MODBUS protocol, but the voltage readings make no sense. I supose there is some kind of formula to convert those raw values to engineering units, but I can't find how.

I'm pretty sure that I used the correct reading function, etc. because the rest of the readings are ok.

The voltage readings are in mV so you will need to divide by 1000 to get volts. I assume it is register 1009 - 1019?

John

The input should be calibrated such that full scale is some known value (volts or millivolts), and there is a known relationship between volts and whatever the desired engineering units are.

You also need to find out what full scale reading for that device is. For example, if the reading is such that 10 volts full scale is 1024, then the voltage scaling factor is 1 count is 9.76 millvolts. Any additional engineering scaling can be calculated similarly (e.g. to convert metres to millvolts). You also have to add in any desired reading offset.

After that, the formula is just y = mx + b, where x is volts (or millivolts), y is the result in engineering units, m is the scale factor, and b is the offset (if any).