(Please note I am the Operations Director and Deputy Quality & Technical Manager for ISO/IEC 17025:2005 at Tempsys which makes the CheckPoint Wireless Monitoring System) As has been mentioned, having a new or re-calibrated NIST traceable probe sent to you is the simplest solution and definitely worth $40. Calibrating probes in an ice bath for 0C is fine however I don't recommend trying to perform comparative measurements for -70C unless you're familiar with measurement uncertainty and calibration procedures. Our probes are spec'd at +/- 0.5C although Dan's +/- 1.0C should be fine since most departments set their low/high alarms at -80/-60.
