I have one cage with it on the outside and one with it on the inside, because I wanted to expirement with what was more accurate. The one on the inside is hot glued in place, the one on the outside is held on with Kapton tape. The temp in the room that I keep my reptiles in doesn't fluctuate more than a few degrees and I have found no difference on where the probe was placed. Once I dial in the temp it stays.