Lets say I built an incubator.(Incubator A) An I put enough vent holes so that the air in the incubator replaced its self every hour. After trial an error I figure out that I have to run my humidity at 40% to get a good hatch rate. Now lets say I built two more almost identical. Incubator B: I put in enough vent holes to let the air replace its self every two hours. Incubator C: I put in enough vent holes to let the air replace its self every 30 minutes. Now I set all three up side by side at a humidity of 40%. Three weeks later incubator A starts hatching but B an C has almost none hatch. Why? Opening eggs from incubator B you would see very wet drowned chicks. Opening eggs from incubator C you would find chicks dried out an stuck. Whats going on here? When cold air enters the incubator an starts to warm up it starts pulling moisture from anywhere it can find it. Humidity trays an eggs. Everyone knows that bigger humidity trays means less moisture loss from the eggs. In incubator B the air replaces its self ever 2 hours. As it does it as to absorb X amount of moisture from the eggs. In incubator A the air is replace every hour. The same X amount of moisture is absorbed each time. So you have twice as much evaporation over 3 weeks as incubator B. Incubator C replaces the air every 30 minutes. It has twice as much evaporation as incubator A an 4 times as much as incubator B. I could run the humidity up to something like 60% in incubator C an I may get good hatch rates. But I'm not fixing the problem I'm compensating for it. I could also drop the humidity in incubator B to 25% but again I'm not fixing the problem I'm compensating for it. If you have to run really high humidity to get good hatches your ventilation is to good. If you have to run really low humidity then you need more ventilation. Yes outside humidity has an effect on it but not as much as ventilation. That's why no one can agree on the right humidity. Everyone has different ventilation levels. Even with manufactured incubators the difference between having one plug out or two is doubling the ventilation level.