The Info List - Torr

The torr (symbol: Torr) is a unit of pressure based on an absolute scale, now defined as exactly 1/760 of a standard atmosphere (101.325 kPa). Thus one torr is exactly 7005101325000000000♠101325/760 pascals (≈ 133.32 Pa). Historically, one torr was intended to be the same as one "millimeter of mercury". However, subsequent redefinitions of the two units made them slightly different (by less than 6993150000000000000♠0.000015%). The torr is not part of the International System of Units
International System of Units
(SI), but it is often combined with the metric prefix milli to name one millitorr (mTorr) or 0.001 Torr. The unit was named after Evangelista Torricelli, an Italian physicist and mathematician who discovered the principle of the barometer in 1644.[1]


1 Nomenclature and common errors 2 History 3 Manometric units of pressure 4 Conversion factors 5 See also 6 References 7 External links

Nomenclature and common errors[edit] The unit's name torr is written in lower case, while its symbol ("Torr") is always written with upper-case initial; including in combinations with prefixes and other unit symbols, as in "mTorr" (millitorr) or "Torr⋅L/s" (torr-litres per second).[2] The symbol (uppercase) should be used with prefix symbols (thus, m Torr and millitorr are correct, but mtorr and milli Torr are not). 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. The misspelled symbol "Tor" is also encountered, and is incorrect. History[edit] Torricelli attracted considerable attention when he demonstrated the first mercury barometer to the general public. He is credited with giving the first modern explanation of atmospheric pressure. Scientists at the time were familiar with small fluctuations in height that occurred in barometers. When these fluctuations were explained as a manifestation of changes in atmospheric pressure, the science of meteorology was born. Over time, 760 millimeters 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 millimeter 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 of the Earth), this definition is imprecise and varies by location. In 1954, the definition of the atmosphere was revised by the 10e Conférence Générale des Poids et Mesures (10th CGPM)[3] to the currently accepted definition: one atmosphere is equal to 7005101325000000000♠101325 pascals. The torr was then redefined as 1/760 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. Manometric units of pressure[edit] See also: Pressure
measurement § Liquid column Manometric units are units such as millimeters of mercury or centimeters of water that depend on an assumed density of a fluid and an assumed acceleration due to gravity. The use of these units is discouraged.[4] Nevertheless, manometric units are routinely used in medicine and physiology, and they continue to be used in areas as diverse as weather reporting and scuba diving. Conversion factors[edit] The millimeter of mercury by definition is 7002133322387415000♠133.322387415 Pa[citation needed] (7004135951000000000♠13.5951 g/cm3 × 7000980665000000000♠9.80665 m/s2 × 6997100000000000000♠1 mm), which is approximated with known accuracies of density of mercury and standard gravity. The torr is defined as 1/760 of one standard atmosphere, while the atmosphere is defined as 7005101325000000000♠101325 pascals. Therefore, 1 Torr is equal to 7005101325000000000♠101325/760 Pa. The decimal form of this fraction (133.322(368421052631578947)...) is an infinitely long, periodically repeating decimal (repetend length: 18). The relationship between the torr and the millimeter of mercury is:

1 Torr = 0.999999857533699... mmHg 1 mmHg = 1.000000142466321... Torr

The difference between one millimeter of mercury and one torr, as well as between one atmosphere (101.325 kPa) and 760 mmHg (7005101325014435400♠101.3250144354 kPa), is less than one part in seven million (or less than 6993150000000000000♠0.000015%). This small difference is negligible for most applications outside metrology. Other units of pressure include:

The bar (symbol: bar), defined as 100 kPa exactly. The atmosphere (symbol: atm), defined as 101.325 kPa exactly. The torr (symbol: Torr), defined as 1/760 atm exactly.

These four pressure units are used in different settings. For example, the bar is used in meteorology to report atmospheric pressures.[5] The torr is used in high-vacuum physics and engineering.[6][7]


v t e

Pascal Bar Technical atmosphere Standard atmosphere Torr Pounds per square inch

(Pa) (bar) (at) (atm) (Torr) (lbf/in2)

1 Pa ≡ 1 N/m2 10−5 6995101970000000000♠1.0197×10−5 6994986919999999999♠9.8692×10−6 6997750060000000000♠7.5006×10−3 6996145037700000000♠1.450377×10−4

1 bar 105 ≡ 100 kPa ≡ 106 dyn/cm2

7000101970000000000♠1.0197 6999986920000000000♠0.98692 7002750060000000000♠750.06 7001145037700000000♠14.50377

1 at 7004980665000000000♠9.80665×104 6999980665000000000♠0.980665 ≡ 1 kgf/cm2 6999967841100000000♠0.9678411 7002735559200000000♠735.5592 7001142233400000000♠14.22334

1 atm 7005101325000000000♠1.01325×105 7000101325000000000♠1.01325 7000103319999999999♠1.0332 1 ≡ 7002760000000000000♠760 7001146959500000000♠14.69595

1 Torr 7002133322399999999♠133.3224 6997133322400000000♠1.333224×10−3 6997135955100000000♠1.359551×10−3 ≡ 1/760 ≈ 6997131578900000000♠1.315789×10−3 ≡ 1 Torr ≈ 1 mmHg


1 lbf/in2 7003689480000000000♠6.8948×103 6998689480000000000♠6.8948×10−2 6998703069000000000♠7.03069×10−2 6998680460000000000♠6.8046×10−2 7001517149300000000♠51.71493 ≡ 1 lbf /in2

See also[edit]

Inch of mercury Pressure Pressure
head Atmosphere (unit) Conversion of units Centimetre of water Pascal (unit) Outline of the metric system


^ 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. ^ "Rules and style conventions". NIST. Retrieved 2012-09-29.  ^ BIPM – Resolution 4 of the 10th CGPM ^ National Physical Laboratory: Pressure
units ^ Note that a pressure of 1 bar (7005100000000000000♠100000 Pa) is slightly less than a pressure of 1 atmosphere (7005101325000000000♠101325 Pa). ^ Cohen E.R. et al. Quantities, Units and Symbols in Physical Chemistry, 3rd ed. Royal Society of Chemistry, 2007 ISBN 0-85404-433-7 (IUPAC pdf copy) ^ DeVoe H. Thermodynamics and Chemistry. Prentice-Hall, Inc. 2001 ISBN 0-02-328741-1

External links[edit]

NPL – pressure units