Reaumur to Triple point of water

The Réaumur scale was proposed by René Antoine Ferchault de Réaumur in 1730. Réaumur was a French scientist primarily known for his contributions to natural history, particularly entomology. His thermometer used dilute alcohol and was calibrated between the freezing and boiling points of water, with the scale divided according to the actual expansion of the alcohol.

Réaumur calibrated his scale based on the physical expansion of his thermometric fluid — a specific dilute alcohol mixture. Between water's freezing and boiling points, the fluid expanded by exactly 80 parts in 1000 of its volume at freezing. He used this empirical measurement directly as the degree count, making 80°Ré the boiling point. The number 80 was not chosen arbitrarily but measured.

The French Revolution's embrace of the metric system in the 1790s demanded decimal-friendly units. Celsius's 0-to-100 scale fit the decimal philosophy perfectly; Réaumur's 0-to-80 did not. The French Academy of Sciences standardized Celsius for scientific work, and it spread through Napoleonic Europe. Réaumur lingered in German-speaking regions and Russia into the mid-19th century but eventually yielded everywhere. The irony is that Réaumur was French — his own country was the first to abandon his scale.

Multiply the Réaumur value by 5/4 (or 1.25). For example, 16°Ré × 1.25 = 20°C. To convert Celsius to Réaumur, multiply by 4/5 (or 0.8). The two scales share the same zero (0°C = 0°Ré); they differ only in degree size, since 100°C = 80°Ré.

Classic French confectionery texts list sugar syrup stages in Réaumur: "thread" stage at 80°Ré (100°C), "soft ball" at 94°Ré (117°C), "hard crack" at 124°Ré (155°C). Pastry chefs memorized these benchmarks and tested with a thermometer or by dripping syrup into cold water. Some vintage French and Swiss recipe books still reference Réaumur temperatures — modern reprints add Celsius equivalents in brackets. It is the last practical domain where Réaumur had a lingering foothold.

The triple point of water is the unique combination of temperature and pressure (273.16 K / 0.01°C and 611.657 Pa) at which water can coexist as solid, liquid, and gas simultaneously. It is a fixed thermodynamic point that cannot vary — any change in temperature or pressure causes one phase to disappear.

The triple point is a perfectly reproducible, invariant temperature — it occurs at exactly one pressure and temperature. From 1954 to 2019, the kelvin was defined as 1/273.16 of the thermodynamic temperature of the triple point of water, providing a stable international calibration reference accessible to any metrology lab.

The freezing point of water at standard atmospheric pressure is 273.15 K (0.00°C), while the triple point is 273.16 K (0.01°C) at 611.657 Pa. The triple point is 0.01°C warmer and occurs at much lower pressure than normal atmospheric conditions. Both are distinct and precisely defined reference points.

In the 2019 redefinition of SI units, the kelvin was redefined by fixing the value of the Boltzmann constant (k = 1.380649 × 10⁻²³ J/K) exactly. This makes the kelvin independent of any physical substance, more stable, and consistent with other SI redefinitions that fixed fundamental constants rather than relying on material artifacts.

The triple point requires a pressure of about 611 Pa — roughly 0.6% of standard atmospheric pressure. On Earth's surface this does not occur naturally. On Mars, where atmospheric pressure is around 600–700 Pa at the surface, conditions near the triple point of water can occur, meaning liquid water, ice, and water vapor can briefly coexist on the Martian surface under the right conditions.