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The torr is defined as 1 / 760 of one standard atmosphere, while the atmosphere is defined as 101325 pascals. Therefore, 1 Torr is equal to 101325 / 760 Pa. The decimal form of this fraction ( 133.322 368 421 052 631 578 947 ) is an infinitely long, periodically repeating decimal ( repetend length: 18).
The standard atmosphere was originally defined as the pressure exerted by a 760 mm column of mercury at 0 °C (32 °F) and standard gravity (g n = 9.806 65 m/s 2). [2] It was used as a reference condition for physical and chemical properties, and the definition of the centigrade temperature scale set 100 °C as the boiling point of water at this pressure.
Atmospheric pressure, also known as air pressure or barometric pressure (after the barometer), is the pressure within the atmosphere of Earth.The standard atmosphere (symbol: atm) is a unit of pressure defined as 101,325 Pa (1,013.25 hPa), which is equivalent to 1,013.25 millibars, [1] 760 mm Hg, 29.9212 inches Hg, or 14.696 psi. [2]
One millimetre of mercury is approximately 1 torr, which is 1 / 760 of standard atmospheric pressure ( 101 325 / 760 ≈ 133.322 368 421 pascals). Although the two units are not equal, the relative difference (less than 0.000 015% ) is negligible for most practical uses.
For example, IUPAC has, since 1982, defined standard reference conditions as being 0 °C and 100 kPa (1 bar), in contrast to its old standard of 0 °C and 101.325 kPa (1 atm). [2] The new value is the mean atmospheric pressure at an altitude of about 112 metres, which is closer to the worldwide median altitude of human habitation (194 m).
A torr was originally defined as 1 mmHg. The pressure is quoted as the level of the mercury's height in the vertical column. Typically, atmospheric pressure is measured between 26.5 inches (670 mm) and 31.5 inches (800 mm) of Hg. One atmosphere (1 atm) is equivalent to 29.92 inches (760 mm) of mercury.
Unlike the mmHg, the torr (with its modern definition of 1/760 1 atm = 101325/760 Pa) is a perfectly legitimate unit of pressure, derived from SI units. Its use in general is discouraged, [ 3 ] since the SI unit of pressure is the pascal .
1 atm ≡ 101 325: ≡ 1.013 25: 1.0332 — 760 14.695 948 775 5142: 1 Torr 133.322 368 421: 0.001 333 224: 0.001 359 51