So, I brewed a pale beer yesterday. It had 9lbs pils, .5lbs Caramel Pilsner (9L crystal), .25lbs special roast (50L), and .25lbs acidulated malt. So, that's 2.5% acid malt. I calibrated my meter and checked the pH at about 15 minutes in to the mash. Somehow I got a reading of 5! My water was basically RO (Poland springs with 3 grams of calcium chloride in a thin mash (roughly 2 quarts/lb)). As a little background, I haven't used the meter for ~4-5 months. The probe has been sitting with storage fluid around the glass part of the probe for that time (at least it still seemed to be submerged...didn't look very closely). When I calibrated it the readings were way off initially (4 seemed to be around 4.5...7 maybe around 7.3). Would a cleaning with cleaning solution help? Are my buffers perhaps too old (they're near the end of the bottle and about a year old)? I heard bacteria can change the buffers' pH. I'm not especially worried that my mash was actually at 5, since I tasted the wort and it tasted quite good. Plus, from what I've read on here it's not likely that my mash could actually go that low (unless I accidentally added the wrong amount of acidulated malt, which I don't believe I did.). Anyways, what say you HBT?