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I know, but for what? Just because you calibrate in a different range doesn't change the accuracy of the result. Only the ADC values are determined during calibration, nothing more.
It is much more...

I don't really understand your problem, you can measure your RO water safely.

Regards Pit

You use the 1000 uS calibration solution for the second calibration value.
When determining the first calibration value, the probe is in the air, and determines the value 0
The measuring range is...