Calibration issues with Watermatic C660

[MachineShop]

Does anyone have experience with a Watermatic C660? My chem test says the pool is at 7.2, but the controller reads it at an 7.9. The obvious thought would be to calibrate the controller, but therein lies the issue. The calibration won't go down past an 7.8. The web hasn't been a whole lot of help and so I was hoping someone here would have an answer or at least know where to look for one.

Thanks

 

[Bama Rambler]

Have you cleaned the pH sensor according to the manual?
How old is the probe?
Have you checked the probe and/or your tester with a buffer solution?