Quote Originally Posted by bcr229 View Post
Paper towels are light enough that they can float a bit over the tub and leave an air gap, which would allow air around the temperature probe and cause a low reading. Try running the setup for a while with a folded up towel, shirt, etc. on top of the paper towel over the UTH and thermometer probe, and then check the thermometer reading.
I have done this with a towel and the temperature drops