IT should hold fine. The main times you have to worry about opening the bator is if you have chicks pipped. If nothing has pipped yet then you did no harm at all opening the bator to add the sponge. It took us all time and mistakes to get it a little better at hatching and we all still have those inevetable problems that stump us. Relax and dont worry. It truely can take 24 hour or more after an egg pipps to hatch so dont panic. Jenn