UTH's do not change the air temperature in the cage, all they do is heat the floor. This means that if your thermometer doesn't have a probe, or if the probe isn't on the floor of the cage directly over the UTH you won't know how hot it is.
The general rule is that you want the UTH to cover about 1/3rd of the bottom of the tank.
As was stated above, a thermostat is a must when using an UTH. Here are 2 good ones.
http://www.amazon.com/gp/aw/d/B000NZZG3S
http://spyderrobotics.com/
This thread shows you everything you need to set up a tub: http://ball-pythons.net/forums/showt...Basics...*DUW*Quote:
what kind of tubs?