It may not be the thermostats, but their location. If your incubator is in the barn and there is a draft, the thermostats will react to the breeze. I bet in still air they all pretty much come in at 99 degrees plus or minus a degree. The biggest trick isnʻt whether its 99 or 101 degrees as much as its the SAME temperature constantly. Plus 50 - 60 % humidity and turning, of course.
Sorry I'm not going to be any help with your problem, I never dealt with incubators, I've never wanted more additions than a broody hen could provide, but...
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thanks chicken lady found a good article on how to compare a thermometer to one you take ur own temperature with, im going to try this method to see what actual temp of mine are