Think about the temp of the glass when the thermometer is on top of the substrate. It's going to be 5 to 10 degrees warmer than the temp your getting on top of the substrate (maybe even more depending on how thick your substrate layer is). Now some BP's like to burrow or push aside the substrate and get to the bottom of the tank. This could potentially be a health hazard and cause burns. If you put the sensor on top of substrate just make sure that the snake isn't burrowing at all. Mine likes to.