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       Still air incubators use the force of hot air rising to pull fresh air in the bottom vents.

But because this proses is very slow the air in the incubator settles in to thermal layers. The hottest air forms at the top an the coldest air forms at the bottom. The cold air at the bottom will be just above the temperature of the out side air. as you move up threw the incubator the air will get warmer. Because the thermostat is at the very top it is reading the hottest air in the incubator not the air temperature at the egg.

Lets say you set up a still air incubator in your house. Lets also say that you keep your house at 70*F.

The first thing you would do is put a thermometer in the incubator at about the level of the top of the egg. You would then adjust the thermostat till the thermometer reads about 101*F. But remember that is the temperature at the top of the egg. Your actually setting the thermostat at around 120*F. If you could look in the incubator with a thermal camera it would look something like this.

Here's the big problem. The egg temp is about mid way between the outside air temperature an the thermostat temperature. That means that if the room temperature goes up then the egg temperature goes up too. Not as much as the room but by a percentage of what the room went up. So if your room goes up to say 80* the inside of the incubator starts looking something like this.

!!!! COOKED EGGS !!!!

But why has the thermometer failed me? Its doing its job just fine. Its keeping the air around it at the 120*F you set it to. An that's why still air incubators suck!

So how do we fix this problem? Ya put a fan in it. Ya need a fan to break up the thermal layers. With a fan in the incubator all the air in the incubator stays pretty much the same temperature. So if the outside air temp changes the thermostat feels the change just as much as the eggs an can compensate for it with more or less heat.

So should you go stick a fan in the incubator your running now? Not if you have egg in it. Remember your thermostat is set at 120* so putting a fan in it now would bring your whole incubator up to 120*. Wait till you don't have any eggs in it so you can readjust the thermostat without worrying about cooking your eggs.

If you still have fluctuations in a incubator with a fan then your thermostat sucks.