Overview

The Rankine scale is an absolute temperature scale used in thermodynamics that starts at absolute zero and measures intervals in the size of Fahrenheit degrees. It is named after the 19th-century engineer and physicist William John Macquorn Rankine. Because it shares the same zero point as the Kelvin scale but uses Fahrenheit-sized increments, the Rankine scale provides a way to work with absolute temperatures while remaining in the Fahrenheit system of units.

Definition and relationships

Rankine is an absolute, or thermodynamic, temperature scale. The scale sets absolute zero to 0 degrees Rankine (often written 0 °R or 0 °Ra to reduce ambiguity). The relationship between Rankine and other common scales can be expressed with straightforward conversion formulas:

  • °R = °F + 459.67 (so 0 °R = −459.67 °F)
  • Kelvin and Rankine: K = °R × 5/9, or °R = K × 9/5 (since a Kelvin degree equals a Celsius degree and a Rankine degree equals a Fahrenheit degree)

Because of these relationships, a change of 1 °R equals a change of 1 °F, while a change of 1 K equals a change of 1 °C. Absolute zero is therefore 0 °R = 0 K = −459.67 °F = −273.15 °C.

Typical values and examples

  • Freezing point of water: 32 °F = 491.67 °R
  • Boiling point of water (at standard pressure): 212 °F = 671.67 °R
  • Room temperature (about 68 °F): about 527.67 °R

History and use

The scale was introduced in the mid-19th century and takes its name from William Rankine, who developed thermodynamic theory and recommended an absolute scale compatible with the Fahrenheit degree. Rankine found practical use in engineering contexts where temperatures are commonly expressed in Fahrenheit, because it lets engineers apply absolute-temperature relationships (such as in ideal-gas laws and heat-engine calculations) without converting to Celsius or Kelvin.

Practical importance and notation

Today the Rankine scale is less common than the Kelvin scale in scientific literature, where the International System of Units (SI) favors kelvin. However, it still appears in some engineering calculations, particularly in the United States where Fahrenheit-based measurements remain in use. The abbreviation for Rankine can be ambiguous: "°R" has been used for both Réaumur and Rankine; to avoid confusion, some sources use "°Ra" for Rankine. For broader context about absolute temperature and thermodynamic concepts see thermodynamic temperature and general material on Kelvin and Celsius. Historical and metrological discussions often refer to the original engineering literature and standards bodies for detailed conventions (absolute zero, Fahrenheit).

In summary, the Rankine scale combines the absolute-zero reference of thermodynamics with the Fahrenheit degree size, offering a convenient absolute scale for Fahrenheit-based engineering and historical thermodynamic sources.