I have a homemade incubator. The heat and fan come on when the air temperature drops to about 99.5 and turn off when the air is heated to 101.5 or so. I'm using a cheap reptile tank heating regulator with no digital readout, so I get these temperatures with a separate thermometer -- I'm currently using a digital cooking thermometer, the kind you stick in a roast to get the internal temperature. But, air temperature doesn't tell us what the egg temperature is. Egg temperature surely does not fluctuate as much as the air temperature -- the mass of the egg holds heat better than the light, circulating air. So, I stuck the end of my temperature probe into a glob of playdough. It is a bit smaller than an egg and weighs a bit less too, but I thought it would give some idea of how the internal temperature of the egg fluctuates. After being in a up-to-temp incubator for 24 hours, the internal temp of the playdough is just 95! I've run successful hatches in this incubator in the past with the same 99.5-101.5 cycle. But, given the very slow rate at which the playdough is getting up to temp, I am feeling a lot better about any temperature fluctuations in the incubator. Even if it happened to cycle up to 105 for a while, I doubt the eggs would be harmed. And, opening the incubator to turn the eggs (or cracking it to get a peek at the hatch) shouldn't have much effect either. But, I'm now wondering if I haven't been running the incubator at too low a temperature. If 24 hours of 99.5-101.5 has only gotten the internal temperature of the playdough to 95 degrees, that suggests that any eggs in the incubator would also be slow to get to 99.5. I guess my question is should the environmental temperature be 99.5 or should the internal temperature of the eggs be 99.5? In a closed system, they should eventually be equal...in theory...right? But, there will also tend to be cooler and warmer spots in an incubator, without perfect air circulation, the equilibrium may never be met, at least not for all of the incubator. This is sort of rambling, sorry. But, I recently borrowed a still air incubator and noticed a significant difference in temperature depending upon where I placed the thermometer probe. When it was 97 under the egg turner, it could be 105 on top of the eggs (closer to the heating element). Who knows what the internal temperature of the eggs in this incubator might be?