Duggy, I'm just a noob here, but I think you'd calibrate your thermo by measuring the thermo against a known value. Icewater=32 F......boiling water = ~212F (make sure you adjust for your local altitude)what do you mean by calibrate your thermometer every time??
Either boiling water , which boils @ 212 degrees or ice water bath . 32 degrees or so. check any thermo you use this way .That way you know if it is off and how much and you know the difference in multiple thermos.