Robin, I used 3 thermometers to compare temps. I kept two in the same location and one with the probe in the egg box. They varied up to about 2 degrees from each other but everything went smoothly. At least you would know if one of them was way off from the other.

Edit: One was an AcuRite, and two were Exo Terra I think? Then I also started using a Zilla thermo/hygro combo.