(Skipped through most the thread, so bare with me if I'm a bit too behind on this reply.)
Concerning the under tank heater--it is a BAD idea unless you have a thermostat or rheostat to hook it to. Reptile heat pads (like the Zoomed Reptitherms) get far too hot to be safe... I've measured them reaching 140*F. A thermostat or a rheostat will control the heat output of the heat mat so it doesn't get too hot.
On the same note, if you can get a heat pad specifically designed not to get very hot (the seedling tray heat pads like Yvonne mentioned earlier) or a thermostat/rheostat to control that one, I don't see where having very light underbelly heating (as in low to mid 80s) would be a bad idea.. It would warm up the substrate a bit and encourage evaporation, and therefore humidity. Good things for your little sullie.
As for it melting the plastic... Plastic has a very high melting temperature. If the heat pad is nearly high enough to melt the plastic, it's definitely more than hot enough to kill your tortoise. If it's cool enough to be safe for your tortoise, it's cool enough that it's not going to melt the enclosure.
By the way, a rheostat is basically a lamp dimmer. The downfall of rheostats compared to thermostats is that rheostats just limit how much electricity gets to the unit, where a thermostat actually turns the unit on/off according to the temperature at the probe. As long as the room temperatures don't fluctuate a great deal, a rheostat would be fine for your usage. Just don't forget to always have a DIGITAL thermometer around to measure temperatures. It should be buried deep in the substrate, so you know just how hot your little tortie is able to get if he decides to dig deep.
Hope this is helpful to you.
(after reading through the thread)
Another thought that occurred to me. How are you measuring his temperatures? Do you have digital thermometers at the substrate (bedding) level to measure the temperatures, or is it something like the little dial thermometers?
Dial thermometers are incredibly inaccurate. Digital thermometers will give you an accurate reading wherever you put the probe (or, in some cases, the unit). You should have one digital thermometer with the probe sitting directly under the heat lamp, and another on the cool side. You can get an indoor/outdoor thermometer from Walmart or the like for this. These usually consist of a unit and a probe, where the "outdoor" temperature is measured at the probe (this would be at your basking site), and the "indoor" temperature is measured at the main unit (which you would just set on the cool side).
Also, your enclosure looks a little too small for something as high wattage as 160 watts. You need enough room that the tortoise has its 100F+ basking spot, but that the cool side is still only in the 80s... a 20 degree drop across the enclosure, which can be impossible to accomplish in a small space. Once you've been able to get good readings with good digital thermometers both under the basking light and on the cool side, you should be able to figure out whether you need something bigger or not.
Concerning the under tank heater--it is a BAD idea unless you have a thermostat or rheostat to hook it to. Reptile heat pads (like the Zoomed Reptitherms) get far too hot to be safe... I've measured them reaching 140*F. A thermostat or a rheostat will control the heat output of the heat mat so it doesn't get too hot.
On the same note, if you can get a heat pad specifically designed not to get very hot (the seedling tray heat pads like Yvonne mentioned earlier) or a thermostat/rheostat to control that one, I don't see where having very light underbelly heating (as in low to mid 80s) would be a bad idea.. It would warm up the substrate a bit and encourage evaporation, and therefore humidity. Good things for your little sullie.
As for it melting the plastic... Plastic has a very high melting temperature. If the heat pad is nearly high enough to melt the plastic, it's definitely more than hot enough to kill your tortoise. If it's cool enough to be safe for your tortoise, it's cool enough that it's not going to melt the enclosure.
By the way, a rheostat is basically a lamp dimmer. The downfall of rheostats compared to thermostats is that rheostats just limit how much electricity gets to the unit, where a thermostat actually turns the unit on/off according to the temperature at the probe. As long as the room temperatures don't fluctuate a great deal, a rheostat would be fine for your usage. Just don't forget to always have a DIGITAL thermometer around to measure temperatures. It should be buried deep in the substrate, so you know just how hot your little tortie is able to get if he decides to dig deep.
Hope this is helpful to you.
(after reading through the thread)
Another thought that occurred to me. How are you measuring his temperatures? Do you have digital thermometers at the substrate (bedding) level to measure the temperatures, or is it something like the little dial thermometers?
Dial thermometers are incredibly inaccurate. Digital thermometers will give you an accurate reading wherever you put the probe (or, in some cases, the unit). You should have one digital thermometer with the probe sitting directly under the heat lamp, and another on the cool side. You can get an indoor/outdoor thermometer from Walmart or the like for this. These usually consist of a unit and a probe, where the "outdoor" temperature is measured at the probe (this would be at your basking site), and the "indoor" temperature is measured at the main unit (which you would just set on the cool side).
Also, your enclosure looks a little too small for something as high wattage as 160 watts. You need enough room that the tortoise has its 100F+ basking spot, but that the cool side is still only in the 80s... a 20 degree drop across the enclosure, which can be impossible to accomplish in a small space. Once you've been able to get good readings with good digital thermometers both under the basking light and on the cool side, you should be able to figure out whether you need something bigger or not.
