No matter what thermometer you buy or use, it is totally useless unless it is calibrated to an accepted accurate thermometer. I calibrate ALL of my thermometers to 100*F before each incubation, even if they were all calibrated the previous season. For my gold standard, I use a good medical grade thermometer. The digital ones are guaranteed to be accurate to (+/-) .2*F. I also have 2 old fashioned mercury thermometers that have proven very accurate.
IMO, calibrating to 32* is a useless piece of information. While a thermometer may be accurate at 32*, that's not going to help a bit when you need that accuracy at 99 - 102*.
Before committing eggs to the incubator, I calibrate the thermometers, check for any high/low temp areas and either correct those by altering air flow, or make note of them so I can shift eggs through the high/low spots. I run the bator for a few days using filled water bottles to approximate the volume of the eggs. I can have as many as 5, and no less than 3 thermometers in my bator to help me track the temps. (a bit OCD???!!!)
So i set it up, let it run for 6 hours. All seemed good. Put the 12 eggs in at about 10 pm. Got home from work at 1 the next day and both thermometers read different readings and we're both over 101 degrees.
Did the bator have a fan? The standard is 99.5 for forced air, and for still air, it is 102*!
So, if your thermometer was accurately calibrated, you should be fine.
I strongly advise all folks who are committing to hatching eggs, (either with a broody or an incubator) to read the entirety of "hatching eggs 101" in the learning center. I always review this excellent reference every season before I plug in my bator!
