A melting mixture of ice and water gives a constant reference at 0 °C; in the field it shows within ten minutes whether a thermometer has drifted. It does not replace calibration; it is a one-point intermediate check.
- Fill an insulated vessel to the brim with finely crushed ice; add water to about one centimetre below the top of the ice and wait a minute.
- Immerse the probe in the middle of the mixture, touching neither the side nor the bottom, to its own immersion depth, and stir gently.
- Read once the value settles. If the deviation exceeds the instrument tolerance, decide on adjustment or service; food safety guidance asks for ±0.5 °C.
- Write down the date, device ID, probe position and result.
A vessel that is mostly water misleads: a cup of ice water alone can read more than 6 °C above the reference. Guides use distilled water.
This method checks 0 °C; it does not show another point such as -18 °C. A traceable certificate needs a calibration laboratory. Infrared devices read a surface by its emissivity, and this method is for immersion probes.
