You would be much better off measuring the temperatures at both ends of the tanks. You can get thermometers that have a probe coming out as well as a sensor on the base unit. This way, you can monitor both ends at what and have a more accurate read on your temperature gradient. Just monitoring the middle relies a bit too much on guesswork, and won't tell you if either end goes beyond the acceptable range.

Also, thawing is covered in another thread:
http://www.ball-pythons.net/PNphpBB2...ic-t-3837.html