For those having trouble with humidity whilst in the last days of hatch day 18 -22 opening the incubator can drop humidity instantly, chills the eggs and dries out the membranes giving the fatal shrink wrap effect.
To prevent this set up the incubator correctly early on day 18.
Add wicking pads (I use clean face washer towels dampened with warm water or puppy pads cut to fit the space) Place these up the sides of the inc with a little corner dipping into the water tray or in rows beside the eggs but don't let them touch the eggs as you could inadvertently drown or suffocate the egg. Buy a couple of hygrometer/thermometers with a sensor on a long long lead (see ebay and try a couple of different types). Place the sensors in different areas of the inc beside eggs, the leads are thin enough that you can put the lid back on the inc and have the screen sit on top so it's easy to read, then take an average of the two (or 3 if your inc has a digital hyrgrometer).
Then we go into total lock down which means no opening at all until all have hatched and dried into little fluff balls.
If you find the humidity is low you can add water without opening the inc. I purchased a large 60ml syringe from a vet with a long nozzle then I pushed some irrigation tubing on the end long enough and thin enough to push through the air vent hole. (4mm tube would suit). I'm sure a turkey baster, sauce bottle or other similar tube like bottle would work the same way. Add water warmed to your syringe (body temp or 37 degrees celcius) and squirt it over the washers, puppy pads or other wicking pad you have devised. It will instantly up the humidity whilst retaining the precious humidity and warmth inside. Just add 60mls at a time and let it settle for 30 mins then add more until you reach your chosen humidity. (If like my inc I have to repeat this 1-3 times daily during hatch to retain moisture) PS As more chicks hatch naturally the humidity goes up as they release moisture from their feathers and discarded eggs so just keep an eye on the hygrometers. If the chicks take longer than 2-3 hours to dry out stop adding water.
PPS If you absolutely must open the inc, do so quickly, use a blanket over your arms and the open section to prevent cold draughts, then to compensate slowly add nice warm slightly steamy water to get the temp and humidity up to normal level again.
Good luck, Mine are due to hatch on the 21st of April- Fingers crossed!