Check this one out on Fleabay
https://www.ebay.com/itm/Control-Company-Digital-Thermometer-Traceable-4305-monitor-refrigerator-freezer/143155369760?epid=1901689165&hash=item2154b99720:g:mpoAAOSw-H1a3KC2
The one I got was fischer scientific and used, but I've never had any temperature problems...
a traceable thermometer is a calibrated thermometer whose indications are traceable to NIST. I got mine on fleabay for about the same cost as a standard one. This is what I go by only, I have a Grumback incubator and its foolproof for hatching.