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
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
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). The symbol
(uppercase) should be used with prefix symbols (thus, m
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
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) 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
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. The torr is used in high-vacuum physics and engineering.
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
Inch of mercury
^ 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:
NPL – pressure units