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Réaumur scale

Old thermometer in a pharmacy in Vienna showing average room temperature in Réaumur

The Réaumur scale French:  (°Ré, °Re, °R), also known as the "octogesimal division",[1] is a temperature scale in which the freezing and boiling points of water are set to 0 and 80 degrees respectively. The scale is named after René Antoine Ferchault de Réaumur, who first proposed something similar in 1730.[2]

Change in scale

Réaumur’s thermometer contained diluted alcohol (ethanol) and was constructed on the principle of taking the freezing point of water as 0°, and graduating the tube into degrees, each of which was one-thousandth of the volume contained by the bulb and tube up to the zero mark. He suggested that the concentration of alcohol employed be such that it began boiling at 80 °Ré — that is, when it had expanded in volume by 8%. He chose alcohol instead of mercury because it expands more visibly, but this posed problems: his original thermometers were very bulky, and the low boiling point of alcohol made them unsuitable for many applications. Instrument-makers generally chose different liquids, and then used 80 °Ré to signify the boiling point of water, causing much confusion. In 1772 Jean-André Deluc studied the several substances then used in thermometers in the light of new theories of heat and came to the conclusion that mercurial thermometers were the best for practical use; for example, if two equal amounts of water at x and y degrees were mixed, the temperature of the result was then the average of x and y degrees, and this relationship only held reliably when mercury was used. From the late 18th century mercury was used almost without exception. These thermometers, the stems of which are graduated into eighty equal parts between the freezing and boiling points of water, are not Réaumur's original thermometers in anything but name.[3]


Charles Minard's information graphic of the 1812 French invasion of Russia. The horizontal graph on the bottom, designed to be read from right to left, depicts the temperature on the army's return from Russia, in degrees below freezing on the Réaumur scale.

The Réaumur scale saw widespread use in Europe, particularly in France and Germany as well as Russia, as referenced in works of Dostoyevsky, Tolstoy, and Nabokov.[4][5][6] By the 1790s, France chose the Celsius scale for the metric system over the Réaumur measurements,[2] but it was used in some parts of Europe until at least the mid-19th century.[7] Its only modern use is in the measuring of milk temperature in cheese production. It is used in some Italian dairies making Parmigiano-Reggiano and Grana Padano cheeses and in Swiss Alp cheeses.[8]

In the Netherlands the Réaumur thermometer is used when cooking sugar syrup for desserts and sweets.

Réaumur temperature conversion formulae
from Réaumur to Réaumur
Celsius [°C] = [°Ré] × 54 [°Ré] = [°C] × 45
Fahrenheit [°F] = [°Ré] × 94 + 32 [°Ré] = ([°F] − 32) × 49
Kelvin [K] = [°Ré] × 54 + 273.15 [°Ré] = ([K] − 273.15) × 45
Rankine [°R] = [°Ré] × 94 + 491.67 [°Ré] = ([°R] − 491.67) × 49
For temperature intervals rather than specific temperatures,
1 °Ré = 1.25 °C = 2.25 °F
Comparisons among various temperature scales

Conversion table between the different temperature units

Notes and references

  1. ^ Division octogesimale in French.
  2. ^ a b Simons, Paul (17 October 2007). "How Reaumur fell off the temperature scale".  
  3. ^ Herbert Dingle. The scientific adventure: essays in the history and philosophy of science. Sir Isaac Pitman and Sons, London, 1952. Page 131.
  4. ^ Fyodor Dostoyevsky, Crime and Punishment, p. 268 Vintage Books. Part 3, Chapter VI on Project Gutenberg.
  5. ^ Leo Tolstoy, Anna Karenina, p. 685, Wordsworth Editions Limited
  6. ^ Vladimir Nabokov, The Real Life of Sebastian Knight, p. 1. Penguin Books.
  7. ^ Draper, John William (1852). Textbook on chemistry. New York: Harper and Brothers. p. 20. 
  8. ^ Reaumur thermometer for Parmigiano-Reggiano cheese)
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