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The torr (symbol: Torr) is a unit of pressure based on an absolute scale, defined as exactly of a standard atmosphere (101325 Pa). Thus one torr is exactly (≈ ).
Historically, one torr was intended to be the same as one "millimetre of mercury", but subsequent redefinitions of the two units made the torr marginally lower (by less than 0.000015%).
The torr is not part of the International System of Units (SI). Even so, it is often combined with the metric prefix milli to name one millitorr (mTorr), equal to 0.001 Torr.
The unit was named after Evangelista Torricelli, an Italian physicist and mathematician who discovered the principle of the barometer in 1644.Devices similar to the modern barometer, using water instead of mercury, were studied by a number of scientists in the early 1640s (see History of the Barometer). Torricelli's explanation of the principle of the barometer appears in a letter to Michelangelo Ricci dated 11 June 1644.
The torr is sometimes incorrectly denoted by the symbol "T", which is the SI symbol for the tesla, the unit measuring the strength of a magnetic field. Although frequently encountered, the alternative spelling "Tor" is incorrect.
Over time, 760 millimetres of mercury at 0 °C came to be regarded as the standard atmospheric pressure. In honour of Torricelli, the torr was defined as a unit of pressure equal to one millimetre of mercury at 0 °C. However, since the acceleration due to gravity – and thus the weight of a column of mercury – is a function of elevation and latitude (due to the rotation and non-sphericity of the Earth), this definition is imprecise and varies by location.
In 1954, the definition of the atmosphere was revised by the 10th General Conference on Weights and Measures BIPM – Resolution 4 of the 10th CGPM. to the currently accepted definition: one atmosphere is equal to 101325 pascals. The torr was then redefined as of one atmosphere. This yields a precise definition that is unambiguous and independent of measurements of the density of mercury or the acceleration due to gravity on Earth.
The torr is defined as of one standard atmosphere, while the atmosphere is defined as 101325 pascals. Therefore, 1 Torr is equal to Pa. The decimal form of this fraction () is an infinitely long, periodically repeating decimal (repetend length: 18).
The relationship between the torr and the millimetre of mercury is:
The difference between one millimetre of mercury and one torr, as well as between one atmosphere (101.325 kPa) and 760 mmHg (101.3250144354 kPa), is less than one part in seven million (or less than 0.000015%). This small difference is negligible for all practical purposes.
In the European Union, the millimetre of mercury is defined as Council directive 80/181/EEC (20 December 1979)
hence
Other units of pressure include:
These four pressure units are used in different settings. For example, the bar is used in meteorology to report atmospheric pressures.Note that a pressure of 1 bar (100000 Pa) is slightly less than a pressure of 1 atmosphere (101325 Pa). The torr is used in high-vacuum physics and engineering.Cohen E. R. et al. Quantities, Units and Symbols in Physical Chemistry, 3rd ed. Royal Society of Chemistry, 2007 ( IUPAC pdf copy).DeVoe H. Thermodynamics and Chemistry. Prentice-Hall, Inc., 2001, .
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