The probe should sit in the middle of the heat pad. If you don't like the bubble (I couldn't handle the different temps. Drove me batty) I use styrofoam sheets. So my set up goes
snake
substrate
Glass
uth
probe
styrofoam.
Ill find ttaylorrs drawing.
Oh great mine isnt in the middle its near the edge does this mean i have to pull it apart and re do it? Or is it fine like this ?