A lot of people suggest putting it between the mat and the glass but I don't feel like that gives accurate reading on the temps inside. I have mine on top of the glass under his hide. His hide is has a bottom on it so I do periodically open it and check the temp inside the hide with my heat gun. I usually run the mat a couple of degrees hotter than the hide needs to be to get the correct temp. I do suggest you get some digital thermometer/hygrometers because those little round analog ones are notoriously inaccurate.

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