I used the Oakton PC Testr for a peatland study. The pH meter worked fine for about a week, then I started getting pH readings of ~12.0. I tried to recalibrate, but the sensor would not read anything remotely close to the pH value on the calibration solutions. A tech support representative told me I need to replace the sensor module for $77. I replaced it and the exact same thing happened. I ended up replacing it 3 times and ultimately decided I could not trust any of the pH measurements unless I calibrated before and after each field session, which was impractical. But at least I got Specific Conductivity!

I would appreciate it if anyone could explain why this happened. The tech support at Oakton would probably like to know too.

Good luck Junwei.

http://www.4oakton.com/proddetail.asp?parent=2&prod=364&seq=1&TotRec=15