Sorry to yell, but I've brewed at least two 25g batches at unknown mash temps. I have Ranco digital controllers on a More Beer recirculating system. There are also analog thermometers on the HLT and Mash Tun. I'd noticed that the analog and digital temps weren't quite jiving, but it was always during mashing, and by the time I finished brewing and clean-up, I'd forget to check back into it. So today I decided to get some ice water out there and calibrate. The analog was off by 2*. The Ranco was off by 15*!!!
Shame on me for not noticing that big of a discrepancy. But I'd started paying attention to just the digital since it controlled the pump. Lazy...
Turns out that a hidden section of the cord to the probe had melted nearly through. Not enough to give me an error reading, but enough to somehow skew the temp. I replaced the probe, adjusted the analog, and all is well. But I can't help but wonder how this will affect my last two beers. They both hit their numbers (OG and FG), and are tasting okay. But I can't help but think that mashing 15* too cool would cause some issues.
Please calibrate your thermometers. It's like backing up a hard drive. You know you should, but don't always follow through. Ugh.