Regarding eggs: colubrid eggs can experience quite wide temp differences and be fine. Done this too many times to say otherwise. If you keep them between 75 and 85, that is great. They can handle lower and higher than that too for limited time easily.

Back to my favorite subject..
Insulated cages always make for better temp control. That was a good point by LarryS. This is true with any temperature controller that uses feedback (a probe).

What controllers cannot do is be perfect in every circumstance when the physics just isn't there to make it possible, as others have mentioned. For example, if I have a 4 watt bulb in a 20 gal screen top glass tank, even the best controller in the world couldn't maintain 85 deg if the ambient room temp is cool and the probe is well away from the bulb.

Similarly, if I have a 150 watt bulb in a 5 1/2 gallon tank and the probe is right near the bulb, it would be very difficult to maintain a perfect 80 deg without swing. Heater in that case is sized way too large and the probe can't even respond that fast as the bulb heats up.

Which brings up a point: the probe can only react so fast to temperature changes, especially these rubber or vinyl coated probes often used by herp temp controllers. If my cage goes from 80 to 100 deg in 15 seconds, the probe may only show 85 or so at 15 seconds. It takes time. So probe placement must be where the eggs are, not by the heater. Again, just giving theory, not saying you are doing one thing or another. This scenario can account for why a desired temp is actually higher than the setpoint.

Even with proportional controllers, one needs to size the heater to a reasonable level for the intended results. The proportional controller is always trying to find the "sweet spot" where the power to the heater is just right to hold temperature. There is a range where that sweet spot functionality works. Outside that range, the proportional acts more like an ON/OFF. So if for example a heater was sized much too small or much too large, or the ambient air temp is far too warm or far too cool for the intended results, the proportional controller is really just an ON/OFF controller as far as results are concerned. Not saying this is true for you (I'm sure it isn't in a house), but if it was, then the proportional controller will not hold a tight temperature.

So to sum it up, good insulation, proper probe placement coupled with a heater that is not too large or too small for the intended results will result in very good control when using a proportional controller.

Or else your controller is malfunctioning.. lol
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Mark

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