What is the correct relative humidy for an incubator?
Maintain humidity at 60% to 75% during the first week(increases growth). Lower the humidity to 35% to 45% during the middle week(reduces growth speed) . Maintain above 55 from day 18. Don't open the incubator during hatching so that wetness from hatching can increase humidity for softening shells.
If it is too much increasing and reducing humidity, maitain it 50% to 60%
Explanation is this: when humidity is elevated, the embryo grows very fast and very big. This can cause two problems. The chick occupies the air sack and suffocates, or during hatching the chick cannot turn into a position for pecking the shell cause its too tight.