I once had a problem with my eggs not hatching. After day 24 I threw them out. Several of the eggs had developed some, but not a lot. I ended up thinking it was the temperature. I took the five incubator thermometers I had and placed them in the now clean incubator. I had five vastly different temperatures. Upon closer inspection I noticed small air bubbles in the mercury. I closed the gap with 120-degree water, but the problem would come back later on. Not wanting to repeat my past failure, I settled on digital indoor/outdoor thermometers you get from Wal-Mart. I set it on outdoor and leave the unit outside the incubator and run the little cable through the vent hole about 1/2" above the floor of the incubator (securing the cable with the vent plug). No more problems.
