As pointed out, if placed inside the snake can sit on it, push it, spill water or poop or pee on it, all of which will change the temp reading. Also you want it outside on the UTH as that will give you the most accurate reading. Then you use a temp gun and get the reading directly from the glass as that will be the hottest. On top of the substrate is pointless as the snake can burrow down and sit right on the glass.