Digital reference thermometer requirements
Digital reference thermometers may be used in accredited microbiology laboratories under the following conditions:
- They must be certified by the National Institutes of Standards and Technology (NIST), traceable to NIST, or conform to NIST specifications
- The stated accuracy must be ± 0.05 or better
- They must be checked by ice point calibration with each use. If ice point temperature is off, they must be sent out for recalibration/recertification before use
- They must be recalibrated/recertified as recommended by the manufacturer or as on the calibration certificate; this must be every five years, at a minimum
- They must be recalibrated/recertified at the temperatures used for annual thermometer calibration checks for in-use thermometers
Mercury-in-glass thermometers are becoming more difficult to find or to have shipped into Washington, even though they are legal to use for laboratory calibration, and digital thermometers lose calibration much more easily through rough handling. Accredited microbiological laboratories must follow the guidelines above when using digital thermometers and should handle all thermometers with great care.