One of the early lessons that I learned in my two months of snake ownership is that the adhesive-backed, liquid crystal thermometers that they sell to stick on the sides of aquariums or cages really don't give useful information about the conditions inside the cage where the snake lives. If you correlate the temperature reading on one of these with temperature readings taken in the cage by other means and your correlation accounts for different room temperatures, then these "thermometers" might help you understand a little of what the snake is seeing. However, the main point is that these thermometers aren't that good.
I found one way to make these thermometers better. Take a piece of inch-and-a-quarter to maybe two-inch PVC piping and cut it just longer than the thermometer. Then, cut the piece lengthwise to make two little half-pipes. A thermometer can be put on the outer diameter of each of them. These pieces can then be put on the cage floor.
Because the pipe was sectioned lengthwise, there's no longer any "inside" in which a snake could get caught. A very little snake could even use one as a half-log hide. The PVC will eventually come to the same temperature as the bedding on which it sits, and the thermometer will read a pretty stable value that represents the PVC and maybe the air temperature above the PVC to some extent. The accuracy of the thermometer still won't be great, but the measured temperature won't be greatly affected by the temperature of the room.
Admittedly, this idea won't save a great deal of money. These thermomters are still about a dollar each and the low-end digitals are only five to ten dollars. On the other hand, the low-end digitals may not be quite as accurate as some people like to claim. They read to tenths of a degree, but that doesn't mean that any of the numbers that they show are correct. The digitals are also sensitive to probe placement. I've seen the temperature drop from 90 to 80 in half an hour. When I started investigating, I realized that my snake had bumped the probe so that instead of reading the bedding temperature it was reading air temperature just above a damp spot. A PVC pipe with a liquid crystal thermometer wouldn't be as subject to this kind of variation.
Bill
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It's not how many snakes you have. It's how happy and healthy you can keep them.