The device itself is what will measure the humidity. The probe will moniter the temp. You will have 3 reading on the screen. An indoor temp (the device), humidity (the device), outdoor temp (the probe). This is why you put the whole thing in your tank. The device on the cool end to measure humidity and cool temp. The probe on the hot end, under substrate, on UTH, to measure hot side. If you have enough wetness under the substrate to make the probe go bad then something is very wrong. The slight dampness after a snake goes pee or spills a little water, will not harm anything as long as its not 100% humidity for days and days.