My Redox probe was not measuring the Redox Value on my Profilux 4, I re calibrated it and still no reading, I purchased a new one thinking I had a bad probe, after calibrating it, I was getting a reading, within 5mins my PH probe went from showing 8.1 to constant 3 and the value has not moved for 3 days. Anyone got any ideas why this would happen?

Steve