I use one of those hand held thermometers that is digital and you just point it at the egg and a red dot shines on the egg .I don't know the name they are called.
Chick eggs moving in incubator but still no hatch!!!! - Page 3
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If you get your incubator back up to temp the live eggs should be at temp within a hour. By then you will feel a difference in temp between the ones that are alive and the ones that are not doing the hand method. Don't forget to raise the humidity up to 65-70% on the last 3 days so the chicks won't get stuck to the shells. 99.5-99.9 degrees will be the best final temp in a circulated air incubator.