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The torr (symbol: Torr) is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimeter of mercury, i.e., a pressure of 1 Torr is approximately equal to one millimeter of mercury. Note that the symbol is spelled exactly the same as the unit, but the symbol is capitalized, as is customary in metric units derived from names. It was named after Evangelista Torricelli, an Italian physicist and mathematician who discovered the principle of the barometer in 1644.[1]


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. In honor of Torricelli, the torr was defined as a unit of pressure equal to one millimeter of mercury.

In 1954, the definition of the atmosphere was revised by the 10e Conférence Générale des Poids et Mesures (10th CGPM)[2] to the currently accepted definition: one atmosphere is equal to 101325 pascals. The torr was then redefined as 1760 of one atmosphere. This was necessary in place of the definition of the torr as one millimeter of mercury, because the height of mercury changes at different temperatures and gravities.

Manometric units of pressureEdit

(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 of gravity. The use of these units is discouraged.[3] 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.

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 g/cm3, at a place where the acceleration of gravity is exactly 9.80665 m/s2.[4] Under most conditions, 1 mmHg is approximately equal to 1 Torr.

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. The density can depend on temperature, exogenous pressure, and other similar variables, so those have to assume certain conventional, normal values as well.
  • The definition assumes a particular value for the acceleration of gravity, standard gravity (g0 = 9.80665 m/s2). In theory, the precise acceleration would vary, and the measurement would have to be recalibrated against the local value; in weightless conditions, this kind of measurement would not be possible.
  • The definition does not address the quality of the vacuum, including the vapor pressure of the mercury, above the column of fluid.

In practice, of course, measurements are made using local values, which vary little enough at the Earth's surface. These assumptions limit both the validity and the precision of the mmHg as a unit of pressure.

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 will inherently limit knowledge of the relationship between the millimetre 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.[5]

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 state-of-the-art 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 physiologyEdit

In medicine, the millimeter of mercury (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:

Manometric results in medicine are sometimes given in torr. This is usually incorrect, since the torr and the millimeter of mercury are not the same thing. Pressures obtained with a manometer (or its transducer equivalent) should be reported in millimeters of mercury.

Conversion factorsEdit

The millimeter of mercury by definition is 133.322387415 Pa (13.5951 g/cm3 × 9.80665 m/s2 × 1 mm ), which is approximated with known accuracies of density of mercury and gravitational acceleration.

The torr is defined as 1760 of one atmosphere, while the atmosphere is defined as 101.325 kPa. Therefore, 1 Torr is equal to 101325760 Pa. The decimal form of this fraction (133.322368421...) is an infinitely long, periodically repeating decimal, as is its reciprocal.

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 (101.3250144354 kPa), is less than one part in seven million (or less than 0.000015%). This small difference is negligible for most applications outside metrology.

The millimeter of mercury as used in medicine is in general given relative to the atmospheric pressure. This means that when a doctor tells you you have a blood pressure of 100 mmHg, this is 100 mmHg above atmospheric. So on a day when the barometric pressure is 760 your absolute pressure is actually 860 mmHg.

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.[6][7] These 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 1760 atm exactly.

These four pressure units are used in different settings. For example, the bar is used in meteorology to report atmospheric pressures.[8] The torr is used in high-vacuum physics and engineering.

Pressure units

v · d · e

pascal bar technical atmosphere standard atmosphere torr pound per square inch
Pa bar at atm Torr psi
1 Pa ≡ 1 N/m2 10−5 1.0197×10−5 9.8692×10−6 7.5006×10−3 145.04×10−6
1 bar 105 ≡ 106 dyn/cm2 1.0197 0.98692 750.06 14.5037744
1 at 0.980665 ×105 0.980665 ≡ 1 kp/cm2 0.96784 735.56 14.223
1 atm 1.01325 ×105 1.01325 1.0332 p0 760 14.696
1 Torr 133.322 1.3332×10−3 1.3595×10−3 1.3158×10−3 = 1 mmHg 19.337×10−3
1 psi 6.895×103 68.948×10−3 70.307×10−3 68.046×10−3 51.715 ≡ 1 lbF/in2

See alsoEdit


External linksEdit

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