I just completed another successful hatch with a still air incubator. For the first 18 days my humidity is at 40-50%, and for lockdown my humidity is 65%. At lockdown the humidity will fluctuate significantly when the chicks are hatching, that is why the caps are removed, also so the chicks can dry faster. I have left to do errands and come back to find my humidity at 90% due to so many chicks hatching at one time and have never had any ill effects on the hatched or unhatched chicks.
I was a nervous Nelly my first hatch and worried about any fluctuation in Temperature and humidity, now that I feel a bit more seasoned I would only concern myself if the temperature is fluctuating (which hasn't happened yet). Maybe I am too laid back now but my first hatch was 97% and my second was 94% but this was due to infertility not to incubator issues. I have a forced air incubator that I am trying for the first time that I will be locking down tomorrow, keeping the humidity up in this one is much more of a challenge than the still air incubator. 3 different people have tried with no success in hatching with this incubator. So far my chicks have been developing fine.I don't know where they had the incubator placed or any other pertinent information or environmental conditions which may or may not been a factor in the unsuccessful hatch. I hope I have a good outcome so I can add a shiny star to my achievement list lol. Good luck on your hatches, sometimes you have to learn the hard way, and sometimes everything goes perfect! Lets hope for perfect! God Bless