So it took a while to actually do the testing on the thermapen. I did two rounds of testing one before I recently brewed and one after. In between it took temps of a kegerator, strike water, sparg water, and some food. Both times I started by putting water on the stove and while it gets up to a boil preparing a proper ice bath. I started by going back and forth from hot to cold after getting a stable reading.
Test #1 showed the first few readings were off. Started with the boil, it was off low. Freezing was off low as well. After a few back and forth between hot and cold readings things seemed to even out and boil reading was about where it should be for my elevation. Freezing started to come in dead on. Maybe 5-8 cycles of hot/cold readings to get accurate measurements.
For test #2 the setup was the same, going from boil to freezing tests again. This time boil started low and freezing was high. Freezing was really high actually,didn't get near freezing for the first couple hot cold cycles. Took a few rounds but freezing finally would go to freezing. Boiling for my altitude was achieved quicker. Took around the same cycles to get accurate readings the second time, around 5-8.
Going from boil to freezing seems to have adjusted the thermapen. I did not take apart or attempt to calibrate the thermapen other than make it go from boiling to freezing. Not sure if this would qualify as a calibration technique but it seems to have helped...if only for a week. Test your thermapens weekly apparently.