I have never took the temps directly on the UTH the reason I have so much substrate is the tank he is in is sitting on top of my homemade enclosure that I use lights to heat. And with out that much substrate his tank gets a little to hot. I don't think he can get down to the UTH because I use coco coir (like the bed a beast of eco earth stuff), which stays fairly compact cuz I keep it damp. I have never seen him close enough to it to get burnt... And it didn't seem to hurt him when I touched him. Also my corn use to borrow down to it and never burnt himself