if you ever let the sensor tip dry out, it's ruined, at least with most brands ... I leave mine sitting in tapwater, when not using :thumbsup: ... I called the Milwaukee pH pen company, they said not to leave it in either distilled water, or reverse-osmosis water (as I had been doing :wtf: ), but plain tapwater is best ... he also said, do not store tip in calibration fluid, either ... I understand little of this, only relaying the Milwaukee manufacturer's instructions ... good luck :smokin: