X2...
I always tell people you can not calibrate a thermometer using just the ice water method and expect absolute accuracy at incubation temps. I work at a dairy plant, all calibrations for the HTST's are done with state certified glass mercury thermometers, pasteurization temps are done with a hot water bath for the probes, the cold side is done with ice bath, BOTH with a state certified glass mercury thermometer to boot. Calibrations if possible should be done as close to the temp desired as possible, IMHO of course. Kinda hard to do at incu temp though.
A medical thermometer is close to incubation temperature.