This is incorrect, they do in fact have a very basic thermostat circuit, based around a thermistor... The problem is that the circuit is bare bones and based on the teeter-totter balancing act between the resistance of the dials variable resistor and the thermistor... An action as basic as taking off the lid can ever so slightly turn the dial/variable resistor and then the teeter-totter is whacked... Plus the termistor is inside the electronics housing up top and not actually down by the eggs so it's not in the best place to get the an accurate reading... I don't know if it was this thread or another, but I have suggested removing the circuit board and enlarging the hole for the dial, the shaft on mine was rubbing the foam, thus it's very likely the foam was nudging that dial just a bit here and there...
Now since these foam incubator circuits are made in Asia with a cost driven goal the actual boards will vary by model and production date or what not, but they are all basically the same circuit... If you look closely at the circuit inside you will see the thermistor, it's generally floating on the PC board over a hole to better allow air to flow around it, but being in a housing up top it's still not exactly getting decent air flow, especially in a still air incubator...
I think this is why numbers don't mean anything with the Little Giant incubators and there are no number settings anywhere. The goal is to get the temp where you want it at the egg level and not worry about the temp in other parts of the incubator. If you measure above the eggs it will read too hot, if you measure below the eggs it will read too cold. It is the core egg temperature that needs to be at about 99.5 without getting too hot or too cold.
If the room temperature is warmer than usual, the cold air entering the incubator is not as cold and the heater does not cycle on as much, which will keep the upper air pocket relatively cooler even though the overall temperature inside the incubator may be warmer. However if the room temperature is colder than usual the cold air entering the incubator will take longer to warm up, causing the heater to work harder and produce higher temperatures at the top of the incubator in order to mix with the colder air at the bottom of the incubator. As long as the temp in between the high pocket and low pocket is adequate to maintain the egg core temperature at an appropriate temperature, the various measurements only serve to frustrate people and make them think something is wrong with the incubator.
The hen turns eggs over in the nest to warm up the cold side and keep the internal egg temperature somewhat close to her own body temperature. When she gets off the nest, her eggs cool off and she has to warm them again so eggs can survive a range of temperatures as long as they are mostly kept at the proper incubation temperature.
The hen does not have the ability to overheat her eggs while an incubator running on a thermostat that measures air temperature can get heat spikes in the air temperature near the heater in order to mix that hotter air with the cooler air near the sensor. When the eggs are getting cooled off during the removal of the lid, the heater is getting hotter to compensate for the cooler internal temperatures and it is putting out more heat, which will cause higher temperature readings at the top of the incubator closest to the heating element and cooler temperature readings at the bottom of the incubator farthest from the heating element. If you are looking at this higher or lower reading of the air temperature and adjusting the thermostat as a result, you have just changed your overall temperature setting instead of allowing the warmer and cooler air pockets to mix and maintain the original temperature setting.
It is necessary to let the air mix fully inside the incubator and wait until the heater has cycled off for awhile to get a more accurate overall temperature reading. That is why it takes small adjustments over a long time to get the dial set where it needs to be and then it needs to be left alone. There will always be warmer and cooler temperatures in the incubator so temps should be taken in the same location rather than taking them in many different locations and then trying to match up the numbers by playing with the settings.
The hen is not distressed when the far sides of her eggs become cooler to the touch. She warms one side, then she turns the eggs to warm the other side. The embryo grows as long as the core temperature stays in a safe range without getting too hot or too cold. The hen is not capable of keeping all sides of her eggs warmed uniformly at all times but she does keep the core temperature of her eggs steady enough for the embryos to develop properly.
Edited by Duck Drover - 5/21/16 at 9:40pm