After some startlingly thin negatives, my suspicion grows that my thermometer is the culprit. I throw one into my sink full of water which water jackets the bottles, and another I use to check temps out of the faucet and in the tank. They've always agreed so I never looked at the one at the bottom of the sink anymore. Now, they seem to disagree by close to two degrees. They're both mercury/glass. So I decided to try the freezing point calibration... for the first time ever. And here's where it gets fun... I had a blast setting up the experiment, with the ice/salt/water and I witnessed super-cooling for the first time. Bear in mind, I'm a pilot and we're taught that icing is bad, real bad. But what and how super-cooled water makes it so bad never enters the picture. Here, instead, I witnessed the water drop to -5.5 C and remain perfectly liquid. Then, very suddenly, the temperature shoots up to zero and, BAM, ice crystals everywhere. That was so fascinating I had to do it three times! But more to the point, at what point of the process am I supposed to be calibrating my thermometer on? Right after the heat discharge and the state of ice I presume? If that's the case, couldn't I just mix ice and water (no salt) and use that as my zero degrees?