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I was also VERY confused about humidity levels. Temp recommendations are quite consistent, but humidity recommendations are all over the place. Plus, everyone warns that if the humidity is wrong your chicks die. But I finally figured it out (I think -- those of you with lots of experience, please let me know if this is right or not). The reason humidity is so important is that humidity determines how much water evaporates from the egg white during incubation. When water evaporates out of the egg, the air sac in the egg enlarges. If the air sac is too small at hatch time, the chick can't breath after its internal pip, so it suffocates or drowns. If the air sac is too large, then the inner membrane has shrunk down too much and the chick can't pip properly, which some people call shrink wrapped. So the humidity has to be low enough to allow the excess water to evaporate out of the egg, but not so low that too much evaporates. But different egg shells and membranes have different porosities, so they evaporate differently. So for some eggs, the right humidity will be 20%, others 30%, others even higher. To determine how to adjust your humidity, you can either weigh your eggs or candle your eggs regularly. Weighing determines how much water they've lost, and candling determines how big the air sac is. There are references in the Hatching 101 article on BYC (and lots of other places) that tell you how much weight the eggs should lose per week or what the air sac should look like every week. If your eggs aren't on schedule, then you need to increase or decrease your humidity accordingly, I've been told that most people start out with a humidity between 20-30% the first week, then "let the eggs tell you what to do." At lock down, I've been told to use 65% humidity to make it easier for the chicks to reposition themselves in the egg -- otherwise it gets too sticky to move around.
A "dry hatch" isn't really dry. That's just what people call it when you don't have to add water to the incubator to get the humidity where you want it. People who live in very humid areas of the world will often have enough moisture in the air without needing to add extra. People who live in very dry areas of the world can't do this successfully. Often it's a seasonal option, all depending on your climate, and where you keep the incubator. People talk about winter and summer incubating being totally different.
Do I have it right, or are some of the details wrong?
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