Dictionary Definition
torr n : a unit of pressure equal to 0.001316
atmosphere; named after Torricelli [syn: millimeter
of mercury, mm Hg]
User Contributed Dictionary
English
Noun
torr, symbol Torr A unit of pressure that is equal to approximately 1.316·103 atmosphere or 133.3 pascals.
Old English
Etymology
Latin turrisNoun
torr mSwedish
Pronunciation
Adjective
torr dry
 boring and unexciting
Conjugation
Noun
torr torr; a unit of pressure
Extensive Definition
 For the standard botanical author abbreviation Torr., see John Torrey.
The torr (symbol: Torr) is a nonSI
unit of pressure
defined as 1/760 of an atmosphere.
It was named after Evangelista
Torricelli, an Italian physicist and mathematician who
discovered the principle of the barometer in 1644.
History
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 came to be
regarded as the “standard” atmospheric pressure. The unit of
barometric pressure (one millimeter of mercury, also written as 1
mm Hg) was
named in honor of Torricelli.
In 1954, the definition of atmosphere was revised
by the 10e Conférence Générale des Poids et Mesures (10th CGPM) to
the currently accepted definition: one atmosphere is equal to
101,325 pascals.
The torr was then redefined as 1/760 of one atmosphere. This
change in the definition of “torr” has been a source of confusion
ever since.
SI units of pressure
The SI unit of pressure is
the pascal (symbol: Pa), defined as one newton per square meter. Other
units of pressure are defined in terms of SI units. These
include:

 The bar (symbol: bar), defined as 105 Pa exactly.

 The atmosphere (symbol: atm), defined as 101,325 Pa exactly.

 The torr (symbol: Torr), defined as 1/760 atm exactly.
These four SIrelated pressure units are used in
different settings. For example, the bar is used in meteorology to
report atmospheric pressures. The torr, a more convenient unit for
low pressures, is used in highvacuum physics and
engineering.
Manometric units of pressure
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 of gravity. These units are
now regarded as obsolete, and their use is discouraged.
Nevertheless, manometric units are used routinely in medicine and
physiology, and they continue to be used in areas as diverse as
weather reporting and scuba diving.
The millimeter of mercury (symbol: mmHg) is
defined as the pressure exerted at the base of a column of fluid
exactly 1 mm high, when the density of the fluid is exactly 13.5951
gr/cm3, at a place where the acceleration of gravity is exactly
9.80665 m/s2.
There are several things to notice about this
definition:

 A fluid density of 13.5951 g/cm³ was chosen for this definition because this is the approximate density of mercury at 0 °C. The definition, therefore, assumes a particular value for the density of mercury. This assumption limits the precision of any pressure measurement (in mmHg) to six significant digits.

 The definition assumes a particular value for the acceleration of gravity: the standard acceleration gn = 9.80665 m/s2. In practice, of course, measurements are made using local values.
These assumptions limit both the validity and the
precision of the mmHg as a unit of pressure. No metrology
laboratory measures or calibrates pressure directly in these terms.
It would be extremely difficult to find a fluid with exactly this
density, and a place where g was exactly 9.80665 m/s². According to
the UK’s National Physical Laboratory (NPL):

 The need to assume fixed and exact but ultimately incorrect values of
 liquid density and acceleration due to gravity inherently limits knowledge
 of the relationship between [the millimeter of mercury] and the pascal.
 By contrast, the magnitude of pressure values expressed in the SI pressure
 unit, the pascal, can flex (albeit not by much) to take account of technological
 improvements in the underlying definitions of mass, length and time – the
 SI base quantities from which pressure is derived.
The performance of modern transducers approaches
the precision required to distinguish between the torr and the
millimeter of mercury. The NPL concludes

 Thus, in the near future, the accuracy claims being made for otherwise
 stateoftheart instruments scaled in manometric units will become
 inherently inferior. Even now, confusion and large errors abound through
 the use of differing definitions, including alternative values of ‘standard’
 gravity and varying assumptions about the density and temperature of the
 fluid. Misunderstandings about temperature assumptions alone can lead
 to errors of several tenths of a percent and there are many stories of this
 leading to major mistakes in pressure measurement.
Manometric units in medicine and physiology
In medicine, the mmHg (measured with a sphygmomanometer) is
the gold
standard for blood
pressure measurement. In physiology, manometric units are used
to measure Starling
forces. Other applications include:

 Intraocular pressure (tonometry)
 CSF pressure
 Intracranial pressure
 Intramuscular pressure (compartment syndrome)
 Central venous pressure
 Pulmonary artery catheterization
 Mechanical ventilation
 Pulmonary gas pressure
 Esophageal motility studies
 Venous ulcer compression regime
Manometric results in medicine are sometimes
given in torr. This is usually incorrect, since the Torr and the
mmHg are not the same thing. Pressures obtained with a manometer
(or its transducer equivalent) should be reported in mmHg.
Conversion factors
The mmHg is defined as 13.5951 x 9.80665 =
133.322387415 Pa. This is an exact number, although it is too long
to be of any practical use.
The torr is defined as 1/760 of one atmosphere,
while the atmosphere is defined as 101,325 Pa. Therefore, one Torr
is equal to 101325/760 of one Pa. The decimal form of this fraction
(133.322368421...) is, unfortunately, an infinitely long,
periodically repeating decimal.
The relationship between the Torr and the mmHg
is:
Torr = 0.999 999 857 533 699... mmHg 1 mmHg =
1.000 000 142 466 321... Torr
The mmHg and the Torr differ from one another by
less than 2 x 107 Torr. The difference between one atmosphere
(101325 Pa) and 760 mmHg (101325.0144354 Pa) less than 0.2 μPa/Pa
(less than 0.00002%). This small difference is negligible for most
applications outside metrology.
Notes
External links
torr in Arabic: ميلليمتر زئبق
torr in Belarusian (Tarashkevitsa): Мілімэтар
ртутнага слупа
torr in Bosnian: MmHg
torr in Catalan: Torr
torr in Czech: Torr
torr in German: Torr
torr in Spanish: Torr
torr in Basque: Torr
torr in French: Torr
torr in Croatian: MmHg
torr in Italian: Torr
torr in Hebrew: מילימטר כספית
torr in Hungarian: Torr
torr in Dutch: Mm Hg
torr in Japanese: トル
torr in Norwegian: MmHg
torr in Norwegian Nynorsk: MmHg
torr in Polish: Tor (jednostka)
torr in Portuguese: Milímetro de mercúrio
torr in Romanian: Torr
torr in Russian: Миллиметр ртутного столба
torr in Slovak: Torr
torr in Serbian: MmHg
torr in Finnish: Torri
torr in Swedish: MmHg
torr in Turkish: MmHg
torr in Ukrainian: Міліметр ртутного
стовпа
torr in Chinese: 托