Calibrating Instant-Read Thermometer

To calibrate an instant-read thermometer, it’s common to temp either ice-water or boiling water. At sea-level, this is 0 C and 100 C, respectively.

However, I’m here to advocate the using ice-water exclusively. The reason is that ice-water is effectively 0 C. See the phase diagram for water, it’s a vertical line across typical pressures of human habitat.

Steam has a gentler slope. What this means is that the boiling point of water will vary a non-negligible amounts. There are many high-altitude cities in South America, where the altitude is higher than 2400 m. At this altitude, the boiling point of water is ~92 C.

So it’ll be more universal to calibrate on ice-water at 0 C. You can confirm measurement quality by checking boiling water afterwards.

The reasoning for the phase diagram is due to how compressible gases are, and how incompressible liquids and solids are. Near freezing, pressure has little influence on either ice or water. Near vaporization, lower pressure encourages gaseous form, while higher pressures keep the molecules intact in the liquid phase.