The reason you don't want the probe inside is if the snake moves it away from the heat source, your tstat is going to compensate by throwing more power. You'll end up burning your snake if you're lucky, killing it if you're a little less lucky, or burning the house down on a bad day. If you attach it to the heat source on the outside, where it can't be moved, then you can adjust the tstat until you have the temps you require on the inside.