Reptile & Amphibian Forums

Ok I took some cinder blocks and put them on the corners/sides of a tank of mine and used it so I could put a 6'x2'x1" wood board on it and attached 2 4" flexwatt strips for some extra cages useing IRIS MCBL tubs. I need some advise on adjusting thermostat.

Normaly on my cages I use tile as substrate. I cant have loose substrate and I like it better than paper towels. I tried useing tile with the tubs and had to set the thermostat so high and it would not reach target temp of 89 degrees. Only got to 85 so I decided I needed to use paper towels. The tubs have little legs so they sit a little bit above the flexwatt and only the legs touch it. I changed to paper towels so the tubs have 2 or 3 layers of towels depending on where they were layed out. I put my thermostat probe directly on the flexwatt and a thermometer probe inside the tub underneath the papertowels and ontop the plastic of the tub.

Now I set the thermostat to 89 and the thermometers read 94 . I have to turn the thermostat way below target temp. On my regular tanks & cages I always have to set the thermostat at the temp I want or a degree or two above my target temp but never had to set it below.

Is this normal? Should I place the thermometer probe ontop the towels?I was thinking of cutting off the legs so it sat directly on the flexwatt. Would that change the way its acting?

Help and advise would be appreciated.

You actually answered your own question when you said, "I put the thermostat probe on the Flexwatt and the thermometer in the box ontop of the plastic and under the paper towels.."

If you want the controller setpoint to match closely to what your thermometer reads, then the controller probe must be placed where the thermometer is placed. Anywhere else and there may be a discrepency. Depending on the physics of the setup, as in substrate, height of box over Flexwatt, surface Flexwatt rests on, ambient air exposure, heat capacity of the air (humidity), the discrepency can be large or small. In an enclosed rack, the discrepency is usually small. In an open setup like yours, the discrepency is often larger.
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Mark

I understand the discrepency. I have been told its better to put the thermostat probe directly on the flexwatt and use a thermometer to measure temps inside cage & then calibrate. My question is why on this tubs setup do I set the thermostat to 82 degrees yet get a temp inside tubs of 89-90 when on my normal tanks & cages I normaly would set the thermostat to 89 or higher to have to achieve the desired temps inside teh cage. One time I had it set to 78 degrees and the thermometer probe measured 90.

One possibility is where you have the probe attached to the Flexwatt. If that area rests snuggly against a thermal mass (something that absorbs the heat) then the temp at that one spot will be lower than the temp elsewhere. I've seen this happen too. Plus, paper is an insulator, so heat gets trapped under paper towels and builds up, as opposed to having ceramic tile which absorbs alot of heat, or glass which conducts it to the substrate (unless the substrate is paper or wood shavings, then it builds up and the glass gets way hot.)

In the end, the most direct method is to have the controller probe in the box exactly where you want a certain temperature to be. I often used one slot in a rack for just that purpose. Anything else becomes guesswork unless the rack is well insulated and enclosed.

BTW, these issues are why I use radiant heat panels in many cages. And where I do use Flexwatt or UTH, I use a substrate like pebbles or course sand or ceramic tile. This is for discrete cages.
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Mark