The torr (symbol: Torr) is a non-SI unit of pressure defined as 1/760 of an atmosphere. Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface The Standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of Pressure (symbol atm It was named after Evangelista Torricelli, an Italian physicist and mathematician who discovered the principle of the barometer in 1644. Evangelista Torricelli ( ( October 15, 1608 &ndash October 25, 1647) was an Italian physicist and mathematician History The first barometer is thought to have been built unintentionally by Gasparo Berti, sometime between 1640 and 1643 [1]
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History
Torricelli attracted considerable attention when he demonstrated the first mercury barometer to the general public. Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum 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. Meteorology (from Greek grc μετέωρος metéōros, "high in the sky" and grc -λογία -logia) is the Interdisciplinary
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. Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum
In 1954, the definition of 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 101,325 pascals. The torr was then re-defined 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. The newton (symbol N) is the SI derived unit of Force, named after Isaac Newton in recognition of his work on Classical Other units of pressure are defined in terms of SI units. [3][4] These include:
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- 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 SI-related pressure units are used in different settings. For example, the bar is used in meteorology to report atmospheric pressures. [5] The torr, a more convenient unit for low pressures, is used in high-vacuum physics and engineering.
pascal (Pa) |
bar (bar) |
technical atmosphere (at) |
atmosphere (atm) |
torr (Torr) |
pound-force per square inch (psi) |
|
|---|---|---|---|---|---|---|
| 1 Pa | ≡ 1 N/m2 | 10−5 | 1. The bar (symbol bar) decibar (symbol dbar) and the millibar (symbol mbar, also mb are units of Pressure. A technical atmosphere (symbol at is a non- SI unit of Pressure equal to one Kilogram-force per square centimeter The Standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of Pressure (symbol atm The pound per square inch or more accurately pound-force per square inch (symbol psi or lbf/in² or lbf/in²) is a unit of The newton (symbol N) is the SI derived unit of Force, named after Isaac Newton in recognition of his work on Classical 0197×10−5 | 9. 8692×10−6 | 7. 5006×10−3 | 145. 04×10−6 |
| 1 bar | 100,000 | ≡ 106 dyn/cm2 | 1. 0197 | 0. 98692 | 750. 06 | 14. 504 |
| 1 at | 98,066. 5 | 0. 980665 | ≡ 1 kgf/cm2 | 0. The unit kilogram-force ( kgf, often incorrectly just kg) or kilopond ( kp) is defined as the Force exerted by Earth's gravity 96784 | 735. 56 | 14. 223 |
| 1 atm | 101,325 | 1. 01325 | 1. 0332 | ≡ 1 atm | 760 | 14. The Standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of Pressure (symbol atm 696 |
| 1 torr | 133. 322 | 1. 3332×10−3 | 1. 3595×10−3 | 1. 3158×10−3 | ≡ 1 Torr; ≈ 1 mmHg | 19. The torr (symbol Torr) is a non- SI unit of Pressure defined as 1/760 of an atmosphere. 337×10−3 |
| 1 psi | 6,894. 76 | 68. 948×10−3 | 70. 307×10−3 | 68. 046×10−3 | 51. 715 | ≡ 1 lbf/in2 |
Example reading: 1 Pa = 1 N/m2 = 10−5 bar = 10. This article deals with the unit of force For the unit of mass see Pound (mass. 197×10−6 at = 9. 8692×10−6 atm, etc.
Note: mmHg is an abbreviation for millimetres of mercury.
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. [6] 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. [7]
There are several things to notice about this definition:
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- 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. [8]
-
- The definition assumes a particular value for the acceleration of gravity: the standard acceleration gn = 9. Standard gravity, usually denoted by g 0 or g n is the nominal acceleration due to gravity at the Earth's surface at sea level 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. [9]
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 physiology
In medicine, the mmHg (measured with a sphygmomanometer) is the gold standard for blood pressure measurement. A sphygmomanometer ( or blood pressure meter is a device used to measure Blood pressure, comprising an inflatable Cuff to restrict blood flow and a In Medicine, a gold standard test or criterion standard test is a Diagnostic test or benchmark that is regarded as definitive Blood pressure is also the title of a short story by Damon Runyan in Guys and Dolls and Other Stories In physiology, manometric units are used to measure Starling forces. The Starling equation is an equation that illustrates the role of hydrostatic and oncotic forces (the so-called Starling forces) in the movement of fluid 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. In music a tonometer is an instrument used to determine the pitch or vibration rate of tones such as a Tuning fork. Intracranial pressure, ( ICP) is the pressure exerted by the Cranium on the Brain tissue Cerebrospinal fluid (CSF and the brain's circulating Compartment syndrome is an acute medical problem following injury surgery or in most cases repetitive and extensive muscle use in which increased pressure (usually caused by inflammation Central venous pressure (CVP describes the pressure of blood in the thoracic Vena cava, near the Right atrium of the heart In Medicine pulmonary artery catheterization (PAC is the insertion of a Catheter into a Pulmonary artery. In Medicine, mechanical ventilation is a method to mechanically assist or replace spontaneous breathing when patients cannot do so on their own Following is a list of average Partial pressures (in Torr) for a human at rest Factors affecting alveolar gas pressures The alveolar oxygen pressure is lower An esophageal motility study (EMS or esophageal manometry is a study performed to evaluate the pressure of the Esophagus in various stages along its length 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 10-7 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. Metrology (from Ancient Greek metron (measure and logos (study of is the Science of Measurement.
Notes
- ^ Devices similar to the modern barometer, using water instead of mercury, were studied by a number of scientists in the early 1640s (http://www.strange-loops.com/scibarometer.html). Torricelli’s explanation of the principle of the barometer appears in a letter to Michelangelo Ricci (http://web.lemoyne.edu/~guinta/torr.html) dated June 11, 1644.
- ^ BIPM - Resolution 4 of the 10th CGPM
- ^ Cohen ER et al. Quantities, Units and Symbols in Physical Chemistry, 3rd ed. Royal Society of Chemistry, 2007.
- ^ DeVoe H. Thermodynamics and Chemistry. Prentice-Hall, Inc. 2001.
- ^ Note that a pressure of 1 bar (100,000 Pa) is slightly less than a pressure of 1 atmosphere (101,325 Pa).
- ^ National Physical Laboratory: Pressure units. http://www.npl.co.uk/pressure/punits.htm.
- ^ Conventional millimeters of mercury. http://www.sizes.com/units/mmHg.htms.
- ^ The density of mercury is now known to 6 decimal places (8 significant digits). See, for example, http://www.cstl.nist.gov/div836/836.05/papers/Strouse01TM_HG_TP_cells.pdf.
- ^ National Physical Laboratory: Pressure and vacuum. http://www.npl.co.uk/pressure/faqs/unitvalidity.html
See also
External links
Inches of mercury, inHg or "Hg is a measuring unit for Pressure. Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface Pressure head is a term used in Fluid mechanics to represent the Internal energy of a Fluid due to the Pressure exerted on its container The Standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of Pressure (symbol atm Conversion of units refers to conversion factors between different Units of measurement for the same Quantity. A centimeter (centimetre of water or cm H2O is a less commonly used unit of Pressure.Dictionary
torr
-noun
- A unit of pressure that is equal to approximately 1.316·10-3 atmosphere or 133.3 pascals.
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