Every refrigerator log, incubator check and vaccine temperature record in your lab is only as good as the thermometer behind it. A probe that reads two degrees off can hide a failing freezer for weeks. Here is how labs check their thermometers, and why a check is not the same as a calibration.
The ice-point check
The simplest check uses the fact that a properly made ice bath sits at 0 °C.
- Fill an insulated container with crushed ice made from distilled water.
- Add just enough cold distilled water to fill the gaps, without the ice floating.
- Stir, wait a few minutes, then insert the probe into the center, away from the sides.
- Wait for the reading to stabilize and record it.
A reading close to 0 °C tells you the thermometer is working at that point. It does not tell you how it reads at -20 °C, 37 °C or -80 °C, which is where your lab actually uses it. That is the limit of a single-point check.
The comparison method
Professional calibration compares your thermometer against a reference thermometer, one that is itself calibrated and traceable to NIST, at the temperatures you use. Both sit in a stable bath or dry-block at each point, and the difference is recorded. For a lab that might mean 2–8 °C for refrigerators, -20 °C for freezers, 37 °C for incubators and -80 °C for ultra-low freezers.
The result is a correction, or confirmation that the thermometer is within tolerance, at every point that matters.
When you need a certificate
- CLIA and CAP labs need thermometers checked against a traceable reference on a defined schedule, with records
- VFC vaccine storage: the CDC requires digital data loggers with a current certificate of calibration
- After a drop or damage, or if two thermometers in the same unit disagree
- New thermometers, unless they arrive with a valid certificate
What a good certificate shows
- The test points and the reading at each one
- The reference standard used, with its own calibration date
- The measurement uncertainty
- Pass or fail against your tolerance, and the next due date
Our thermometer calibration service calibrates thermometers and data loggers on-site at your lab's temperatures. For vaccine storage, see VFC refrigerator requirements.