Triple point of water to Delisle
TPW
°De
Conversion History
| Conversion | Reuse | Delete |
|---|---|---|
| No conversion history to show. | ||
Quick Reference Table (Triple point of water to Delisle)
| Triple point of water (TPW) | Delisle (°De) |
|---|---|
| 0 | 559.725 |
| 0.9999 | 150.0259734247825666884 |
| 1 | 149.98499942472503919231 |
| 1.073 | 120.07397938272996705336 |
| 1.136 | 94.260359346487644522465 |
| 1.366 | 0.020159214174403536705 |
About Triple point of water (TPW)
The triple point of water is a fundamental thermometric reference: the unique temperature and pressure (273.16 K, 611.657 Pa) at which water coexists simultaneously as solid, liquid, and vapor. When used as a temperature unit, one triple-point unit (TPW) equals 273.16 K, so temperatures are expressed as multiples of this fixed point. This makes 0 TPW equal to absolute zero and 1.000 TPW equal to water's triple point exactly. The freezing point (273.15 K) is 0.9999 TPW — just below 1 — while boiling (373.15 K) is approximately 1.366 TPW. This unit served as the defining reference for the kelvin from 1954 until the 2019 SI revision.
Used in metrology laboratories to calibrate precision thermometers. A sealed triple-point cell containing ultra-pure water held at exactly 273.16 K (0.01°C) serves as a primary temperature standard.
About Delisle (°De)
The Delisle scale (°De) is a historical temperature scale with an inverted direction: higher Delisle values mean colder temperatures. Created by French astronomer Joseph-Nicolas Delisle in 1732, it sets 0°De at the boiling point of water and counts upward as temperature falls; the freezing point is 150°De. This inversion arose because Delisle calibrated his mercury thermometer so that mercury contracted with cooling, measuring degrees of cooling from the boiling point rather than degrees of warmth above a cold reference. The scale was used extensively in Russia for most of the 18th century, notably by Mikhail Lomonosov, before being replaced by Celsius. Today it is an educational curiosity with no practical use.
Boiling water is 0°De; freezing is 150°De. A typical room at 20°C is 120°De. Absolute zero is approximately 559.73°De.
Etymology: Named after Joseph-Nicolas Delisle (1688–1768), a French astronomer who worked in St. Petersburg at the invitation of Peter the Great from 1726. He devised the scale in 1732 while in Russia, where it was adopted and remained in scientific use until the late 18th century when Celsius became standard.
Triple point of water – Frequently Asked Questions
What is the triple point of water?
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.
Why was the triple point of water used to define the kelvin?
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.
What is the difference between the triple point and the freezing point of water?
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.
What replaced the triple point of water in the 2019 SI redefinition?
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.
Can the triple point of water occur naturally?
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.
Delisle – Frequently Asked Questions
Why is the Delisle scale inverted?
Delisle calibrated his thermometer by observing how mercury contracted as it cooled from boiling water. He measured degrees of cooling rather than degrees of warmth — the scale counted how far the temperature had dropped from the boiling point. This made 0°De the starting reference (boiling) and larger numbers represent lower temperatures, the opposite of every other common scale.
What is the freezing point of water on the Delisle scale?
The freezing point of water is 150°De. The boiling point is 0°De. The scale spans 150 degrees between freezing and boiling — contrast with 100 degrees in Celsius or 180 degrees in Fahrenheit. To convert: °De = (100 − °C) × 1.5, so 0°C gives (100 − 0) × 1.5 = 150°De.
Who invented the Delisle scale and where was it used?
Joseph-Nicolas Delisle invented the scale in St. Petersburg in 1732, where he had been invited to work by Peter the Great. The scale was adopted by Russian scientists and used throughout the 18th century, particularly by Mikhail Lomonosov. It was later standardized by Joseph-Nicolas's colleague Joseph-Adam Braun, who fixed the two reference points.
How do you convert Delisle to Celsius?
Subtract the Delisle value from 100, then multiply by 2/3. Formula: °C = 100 − (°De × 2/3). For example, 150°De → 100 − (150 × 2/3) = 100 − 100 = 0°C (freezing). For 0°De → 100 − 0 = 100°C (boiling). Remember the scale is inverted: higher Delisle = colder temperature.
What other temperature scales run backwards like Delisle?
Delisle is the only well-known inverted temperature scale, but the concept appears elsewhere. Astronomical magnitude runs backwards (brighter stars have lower numbers). The Scoville scale for chilli heat is not inverted but is logarithmically compressed in a way that confuses people similarly. In thermometry, Delisle stands alone — every other scale (Celsius, Fahrenheit, Kelvin, Rankine, Newton, Rømer, Réaumur) increases with heat. The inversion made Delisle unintuitive, which is a key reason it lost out to Celsius.