"Arjan Bakker" wrote in message ...
I don't think it matters,
in my opinion you calibrate the slope mv/ph.
If the ph value is below 7 the mv/ph is negative,
above 7 is positive. But the slope stays the same.
At least this is what I remember from chemistry-lessons.
(a long long time ago!!!)

In an ideal world - maybe... :-)

But real probe does not have linear characteristic
so it is a curve. If you want acurate measurements
you are interested in calibrating the meter as close
to the measured value as possible...