I have checked the calibration of the probe
with 10.0 test solution and it checks O.K.

Most probes use two point calibration, get a 7.0 solution and calibrate using
the two. I cant find the article at the moment, but I believe it was on
reefkeeping.com or one of the monthly online mags. It was for pH probe
maintenance and goes into pretty heavy detail as to how they work.
~John