Triple point of water to Rankine
TPW
°R
Conversion History
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Quick Reference Table (Triple point of water to Rankine)
| Triple point of water (TPW) | Rankine (°R) |
|---|---|
| 0 | 0 |
| 0.9999 | 491.63883189026091997392 |
| 1 | 491.688000690329952969228 |
| 1.073 | 527.581224740724039535968 |
| 1.136 | 558.557568784214826573042 |
| 1.366 | 671.645808942990715755954 |
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 Rankine (°R)
Rankine (°R) is an absolute temperature scale that uses Fahrenheit-sized degrees. Like the kelvin it starts at absolute zero (0°R), but its degree intervals match Fahrenheit rather than Celsius. This makes it useful in US customary engineering thermodynamics — particularly older American aerospace, HVAC, and mechanical engineering literature — where Fahrenheit-based calculations must account for absolute temperature. Water freezes at 491.67°R, boils at 671.67°R, and body temperature is 558.27°R. The Rankine scale is rarely used today outside legacy US engineering calculations; SI units with kelvin have largely replaced it internationally and increasingly within the US engineering community as well.
Jet engine combustion temperatures around 2,500°F (1,371°C) equal approximately 2,960°R in thermodynamic calculations. Cryogenic oxygen (−183°C) is about 163°R.
Etymology: Named after Scottish engineer William John Macquorn Rankine (1820–1872), who proposed the scale in 1859. Rankine made major contributions to thermodynamics, steam engine theory, and civil engineering. The Rankine cycle — the theoretical model for steam power plants — is also named after him.
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.
Rankine – Frequently Asked Questions
What is the Rankine scale used for?
Rankine is used in US customary thermodynamic calculations where absolute temperature is needed but Fahrenheit-scale degrees are preferred. It appears in older American engineering standards for steam power, HVAC, and aerospace — disciplines where engineers needed an absolute scale compatible with Fahrenheit-unit formulas without converting to kelvin.
How does Rankine differ from kelvin?
Both start at absolute zero (0 K = 0°R), but their degree sizes differ. One kelvin equals 1.8 Rankine degrees, matching the 1.8 ratio between Celsius and Fahrenheit degree sizes. So 273.15 K = 491.67°R. To convert: °R = K × 1.8.
How do you convert Rankine to Celsius?
Divide by 1.8 to get kelvin, then subtract 273.15. Formula: °C = (°R / 1.8) − 273.15. For example, 491.67°R ÷ 1.8 = 273.15 K; 273.15 − 273.15 = 0°C. Alternatively: °C = (°R − 491.67) / 1.8.
What is absolute zero in Rankine?
Absolute zero is 0°R, the same as 0 K and −273.15°C (−459.67°F). Because Rankine starts at absolute zero, it has no negative values — a property shared with kelvin. This is what makes it useful in thermodynamics: equations requiring absolute temperature work directly without offset corrections.
Is Rankine still used in modern engineering?
Rankine use has declined significantly. Most modern engineering, including US aerospace and HVAC standards, has shifted to SI units (kelvin). Rankine persists mainly in legacy documents, some US university thermodynamics courses that teach both systems, and niche industries still working from older US customary standards. New engineering work rarely specifies Rankine.