For my latest brew session, I made a disc of reflectix, the same size as the pot and sat it on top of the mash. I was quite alarmed as when I looked at the thermo on the kettle, I had lost 15 degrees in 10 minutes. It turns out the kettle thermo is off, because when I measured the temp with a handheld thermo, it was bang on where it should have been.
I'm guessing that the probe is too short, and the bag/grain get some separation from the side of the pot. Any other ideas?