View source for Torr

{{short description|Traditional unit of pressure}}
{{Other uses}}
{{Use dmy dates|date=September 2022}}
{{Infobox unit
| name         = torr
| quantity     = [[pressure]]
| symbol       = Torr
| namedafter   = [[Evangelista Torricelli]]
| extralabel   = Definition
| extradata    = {{sfrac|760}} [[atmosphere (unit)|atm]]
| units1 = [[SI derived unit]]s
| inunits1     = {{convert|1|Torr|Pa|sigfig=7|disp=out|lk=on|comma=gaps}}
| units2 = [[British Gravitational System|British&nbsp;Gravitational&nbsp;System]]
| inunits2     = {{convert|1|Torr|psi|sigfig=7|disp=out|lk=on|comma=gaps}}
}}
The '''torr''' (symbol: Torr) is a [[Pressure#Units|unit of pressure]] based on an [[absolute scale]], defined as exactly {{sfrac|760}} of a standard [[atmosphere (unit)|atmosphere]] (101325 Pa).  Thus one torr is exactly {{nowrap|{{sfrac|101325|760}} [[Pascal (unit)|pascals]]}} (≈ {{convert|1|Torr|Pa|sigfig=5|disp=out}}).

Historically, one torr was intended to be the same as one "[[millimetre of mercury]]", but subsequent redefinitions of the two [[units of measurement|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&nbsp;Torr.

The unit was named after [[Evangelista Torricelli]], an Italian physicist and mathematician who discovered the principle of the [[barometer]] in 1644.<ref>Devices similar to the modern barometer, using water instead of mercury, were studied by a number of scientists in the early 1640s (see [http://www.strange-loops.com/scibarometer.html History of the Barometer]).  Torricelli's explanation of the principle of the barometer appears in a letter to Michelangelo Ricci dated 11 June 1644.</ref>

==Nomenclature and common errors==
The unit name ''torr'' is written in [[letter case|lower case]], while its symbol ("Torr") is always written with an uppercase initial; including in combinations with prefixes and other unit symbols, as in "mTorr" (millitorr) or "Torr⋅L/s" (torr-litres per second).<ref>{{cite web |title=Rules and style conventions |url=http://physics.nist.gov/cuu/Units/rules.html |publisher=NIST |access-date=29 September 2012}}</ref> The symbol (uppercase) should be used with prefix symbols (thus, mTorr and millitorr are correct, but mtorr and milliTorr are not).

The torr is sometimes incorrectly denoted by the symbol "T", which is the SI symbol for the [[tesla (unit)|tesla]], the unit measuring the strength of a magnetic field. Although frequently encountered, the alternative spelling "Tor" is incorrect.

==History==
Torricelli attracted considerable attention when he demonstrated the first [[Barometer#Mercury barometers|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 enabled.

Over time, 760 millimetres of [[Mercury (element)|mercury]] at 0&nbsp;°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&nbsp;°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]]<ref>[http://www.bipm.org/en/CGPM/db/10/4/ BIPM – Resolution 4 of the 10th CGPM].</ref> to the currently accepted definition: one atmosphere is equal to 101325 [[pascal (unit)|pascals]]. The torr was then redefined as {{sfrac|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==

{{See also|Pressure measurement#Liquid column (manometer)}}

''Manometric units'' are units such as ''millimetres of mercury'' or ''centimetres 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.<ref>[http://www.npl.co.uk/reference/faqs/pressure-units National Physical Laboratory: Pressure units].</ref>  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==
The millimetre of mercury by definition is 133.322387415 Pa<ref>{{Cite book |title=BS 350: Part 1: 1974 – Conversion factors and tables |date=1974 |publisher=[[British Standards Institution]] |page=49 }}</ref> (13.5951 g/cm<sup>3</sup> × 9.80665 m/s<sup>2</sup> × 1 mm), which is approximated with known accuracies of density of mercury and [[standard gravity]].

The torr is defined as {{sfrac|760}} of one standard atmosphere, while the atmosphere is defined as 101325 pascals.  Therefore, 1 Torr is equal to {{sfrac|101325|760}}&nbsp;Pa. The decimal form of this fraction ({{gaps|133.322|{{overline|{{gaps|368|421|052|631|578|947}}}}}}) is an infinitely long, periodically repeating decimal ([[repetend]] length: 18).

The relationship between the torr and the millimetre of mercury is:

*1 Torr = {{gaps|0.999|999|''857''|''533''|''699''|...}} mmHg
*1 mmHg = {{gaps|1.000|000|''142''|''466''|''321''|...}} Torr

The difference between one millimetre of mercury and one torr, as well as between one atmosphere (101.325&nbsp;kPa) and 760&nbsp;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<ref>[https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:31980L0181 Council directive 80/181/EEC] (20 December 1979)</ref>

:1 mmHg = 133.322 Pa

hence

*1 Torr = {{gaps|1.000|00''2''|''763''|...}} mmHg
*1 mmHg = {{gaps|0.999|99''7''|''236''|...}} Torr

Other units of pressure include:

* The bar (symbol: bar), defined as 100 kPa exactly.
* The atmosphere (symbol: atm), defined as 101.325 kPa exactly.

These four pressure units are used in different settings. For example, the bar is used in meteorology to report atmospheric pressures.<ref>Note that a pressure of 1 bar (100000 Pa) is slightly less than a pressure of 1 atmosphere (101325 Pa).</ref>  The torr is used in high-vacuum physics and engineering.<ref>Cohen E. R. ''et al.''  [https://books.google.com/books?id=TElmhULQoeIC Quantities, Units and Symbols in Physical Chemistry], 3rd ed.  Royal Society of Chemistry, 2007 {{ISBN|0-85404-433-7}} ([http://www.iupac.org/publications/books/gbook/green_book_2ed.pdf  IUPAC pdf copy]).</ref><ref>DeVoe H. Thermodynamics and Chemistry. Prentice-Hall, Inc., 2001, {{ISBN|0-02-328741-1}}.</ref>

{{pressure Units}}

==See also==
* [[Atmosphere (unit)]]
* [[Centimetre of water]]
* [[Conversion of units]]
* [[Inch of mercury]]
* [[Outline of the metric system]]
* [[Pascal (unit)]]
* [[Pressure head]]
* [[Pressure]]

==References==
{{Reflist|30em}}

==External links==
*[http://www.npl.co.uk/pressure/punits.html NPL – pressure units]

{{Portal bar|Physics}}

[[Category:Non-SI metric units]]
[[Category:Units of pressure]]
[[Category:Mercury (element)]]