|Unit system||Metric system|
|1 bar in ...||... is equal to ...|
|SI units||100 kPa|
|CGS units||1.0×106 Ba|
|US customary units||14.50377 psi|
The bar is a metric unit of pressure, but not part of the International System of Units (SI). It is defined as exactly equal to 100,000 Pa (100 kPa), or slightly less than the current average atmospheric pressure on Earth at sea level (approximately 1.013 bar). By the barometric formula, 1 bar is roughly the atmospheric pressure on Earth at an altitude of 111 metres at 15 °C.
The bar and the millibar were introduced by the Norwegian meteorologist Vilhelm Bjerknes, who was a founder of the modern practice of weather forecasting.
The International Bureau of Weights and Measures (BIPM) lists the bar as one of the "non-SI units [that authors] should have the freedom to use", but has declined to include it among the "non-SI units accepted for use with the SI".The bar has been legally recognised in countries of the European Union since 2004. The US National Institute of Standards and Technology (NIST) deprecates its use except for "limited use in meteorology" and lists it as one of several units that "must not be introduced in fields where they are not presently used". The International Astronomical Union (IAU) also lists it under "Non-SI units and symbols whose continued use is deprecated".
Units derived from the bar include the megabar (symbol: Mbar), kilobar (symbol: kbar), decibar (symbol: dbar), centibar (symbol: cbar), and millibar (symbol: mbar). The notation bar(g), though deprecated by various bodies,represents gauge pressure, i.e., pressure in bars above ambient or atmospheric pressure.
The bar is defined using the SI derived unit, pascal: 1 bar ≡ 100,000 Pa ≡ 100,000 N/m2.
Thus, 1 bar is equal to:
and 1 bar is approximately equal to:
The word bar has its origin in the Greek word βάρος (baros), meaning weight. The unit's official symbol is bar; the earlier symbol b is now deprecated and conflicts with the use of b denoting the unit barn, but it is still encountered, especially as mb (rather than the proper mbar) to denote the millibar. Between 1793 and 1795, the word bar was used for a unit of weight in an early version of the metric system.
Atmospheric air pressure is often given in millibars, where standard atmospheric pressure is defined as 1013.25 mbar, 101.325 kPa, 1.01325 bar, which is about 14.7 pounds per square inch. Despite the millibar not being an SI unit, meteorologists and weather reporters worldwide have long measured air pressure in millibars as the values are convenient. After the advent of SI units, some meteorologists began using hectopascals (symbol hPa) which are numerically equivalent to millibars; for the same reason, the hectopascal is now the standard unit used to express barometric pressures in aviation in most countries. For example, the weather office of Environment Canada uses kilopascals and hectopascals on their weather maps. In contrast, Americans are familiar with the use of the millibar in US reports of hurricanes and other cyclonic storms.
In fresh water, there is an approximate numerical equivalence between the change in pressure in decibars and the change in depth from the water surface in metres. Specifically, an increase of 1 decibar occurs for every 1.019716 m increase in depth. In sea water with respect to the gravity variation, the latitude and the geopotential anomaly the pressure can be converted into metres' depth according to an empirical formula (UNESCO Tech. Paper 44, p. 25). As a result, decibars are commonly used in oceanography.
In scuba diving, bar is also the most widely used unit to express pressure, e.g. 200 bar being a full standard scuba tank, and depth increments of 10 meter of seawater being equivalent to 1 bar of pressure. (Americans alternatively measure pressure in psi.)
Many engineers worldwide use the bar as a unit of pressure because, in much of their work, using pascals would involve using very large numbers. In measurement of vacuum and in vacuum engineering, residual pressures are typically given in millibar, although torr or millimeter of mercury (mmHg) were historically common.
Engineers that specialize in technical safety for offshore petrochemical facilities would be expected to exclusively refer to explosion loads in units of bar or bars. A bar is a convenient unit of measure for pressures generated by low frequency vapor cloud explosions that are commonly considered as part of accidental loading risk studies.
In the automotive field, turbocharger boost is often described in bars outside the United States. Tire pressure is often specified in bars.
Unicode has characters for "mb" (㏔, U+33D4) and "bar" (㍴, U+3374), but they exist only for compatibility with legacy Asian encodings and are not intended to be used in new documents.
The kilobar, equivalent to 100 MPa, is commonly used in geological systems, particularly in experimental petrology.
"Bar(a)" and "bara" are sometimes used to indicate absolute pressures and "bar(g)" and "barg" for gauge pressures. This usage is deprecated and fuller descriptions such as "gauge pressure of 2 bar" or "2-bar gauge" are recommended.
The litre or liter is a metric unit of volume. It is equal to 1 cubic decimetre (dm3), 1000 cubic centimetres (cm3) or 0.001 cubic metre (m3). A cubic decimetre occupies a volume of 10 cm × 10 cm × 10 cm and is thus equal to one-thousandth of a cubic metre.
Pressure measurement is the analysis of an applied force by a fluid on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure and display pressure in an integral unit are called pressure meters or pressure gauges or vacuum gauges. A manometer is a good example, as it uses the surface area and weight of a column of liquid to both measure and indicate pressure. Likewise the widely used Bourdon gauge is a mechanical device, which both measures and indicates and is probably the best known type of gauge.
Pressure is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure is the pressure relative to the ambient pressure.
Standard temperature and pressure are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union of Pure and Applied Chemistry (IUPAC) and the National Institute of Standards and Technology (NIST), although these are not universally accepted standards. Other organizations have established a variety of alternative definitions for their standard reference conditions.
The torr is a unit of pressure based on an absolute scale, defined as exactly 1/760 of a standard atmosphere. Thus one torr is exactly 101325/760 pascals (≈ 133.32 Pa).
Atmospheric pressure, also known as barometric pressure, is the pressure within the atmosphere of Earth. The standard atmosphere is a unit of pressure defined as 101,325 Pa, which is equivalent to 760 mm Hg, 29.9212 inches Hg, or 14.696 psi. The atm unit is roughly equivalent to the mean sea-level atmospheric pressure on Earth; that is, the Earth's atmospheric pressure at sea level is approximately 1 atm.
The pascal is the SI derived unit of pressure used to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength. The unit, named after Blaise Pascal, is defined as one newton per square metre and is equivalent to 10 barye (Ba) in the CGS system. The unit of measurement called standard atmosphere (atm) is defined as 101,325 Pa.
Hecto is a decimal unit prefix in the metric system denoting a factor of one hundred. It was adopted as a multiplier in 1795, and comes from the Greek ἑκατόν hekatón, meaning "hundred". In 19th century English it was sometimes spelled hecato, in line with a puristic opinion by Thomas Young. Its unit symbol as an SI prefix in the International System of Units (SI) is the lower case letter h.
The humidex is an index number used by Canadian meteorologists to describe how hot the weather feels to the average person, by combining the effect of heat and humidity. The term humidex was first coined in 1965. The humidex is a nominally dimensionless quantity based on the dew point.
Pressure altitude is the altitude in the International Standard Atmosphere (ISA) with the same atmospheric pressure as that of the part of the atmosphere in question.
The standard atmosphere is a unit of pressure defined as 101325 Pa. It is sometimes used as a reference pressure or standard pressure. It is approximately equal to Earth's atmospheric pressure at sea level.
The North American High is an impermanent high-pressure area or anticyclone created by a formative process that occurs when cool or cold dry air settles over North America. During summer, it is replaced with an Arctic Low, or a North American Low should it move over continental land.
Inch of mercury is a unit of measurement for pressure. It is used for barometric pressure in weather reports, refrigeration and aviation in the United States.
A kilogram-force per centimetre square (kgf/cm2), often just kilogram per square centimetre (kg/cm2), or kilopond per centimetre square is a deprecated unit of pressure using metric units. It is not a part of the International System of Units (SI), the modern metric system. 1 kgf/cm2 equals 98.0665 kPa (kilopascals). It is also known as a technical atmosphere.
The ambient pressure on an object is the pressure of the surrounding medium, such as a gas or liquid, in contact with the object.
The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units. It is the pressure resulting from a force of one pound-force applied to an area of one square inch. In SI units, 1 psi is approximately equal to 6895 N/m2.
The angstrom or ångström is a metric unit of length equal to 10−10 m; that is, one ten-billionth of a metre, 0.1 nanometre, or 100 picometres. Its symbol is Å, a letter of the Swedish alphabet.
The December 2009 Midwest blizzard was a powerful extratropical cyclone which was of a category which meteorologists refer to as a cyclogenic bomb, a system which shows a drop in central pressure similar to the rapid intensification cycle of a tropical cyclone, more than 1 mbar per hour for 12 to 24 hours or more. A sustained drop averaging more than 2.5 mbar/h is termed explosive deepening/intensification, and this was the case with this rapidly deepening and intensifying storm as it traversed the Midwest and Ontario and on to Québec, Greenland and vicinity. In many locations wind, snowfall, and precipitation moisture content records dating back to the December 2, 1990 storm, the 1976-1978 period, the 1949 blizzard, or even further back were broken, with barometric pressure records falling as well. Both the central pressure (depth) and rate of change and differential over a given distance (intensity) were remarkable, and both caused hurricane-force winds in places.
Torricelli's experiment was invented in Pisa in 1643 by the Italian scientist Evangelista Torricelli (1608-1647). The purpose of his experiment is to prove that the source of vacuum comes from atmospheric pressure.
The metresea water (msw) is a metric unit of pressure used in underwater diving. It is defined as one tenth of a bar.
|Pascal||Bar||Technical atmosphere||Standard atmosphere||Torr||Pound per square inch|
|1 Pa||1 Pa ≡ 1 Pa||1 Pa = 10−5 bar||1 Pa = 1.0197×10−5 at||1 Pa = 9.8692×10−6 atm||1 Pa = 7.5006×10−3 Torr||1 Pa = 0.000 145 037 737 730 lbf/in2|
|1 bar||105||≡ 100 kPa |
≡ 106 dyn/cm2
|= 1.0197||= 0.98692||= 750.06||= 14.503 773 773 022|
|1 at||98066.5||0.980665||≡ 1 kgf/cm2||0.967 841 105 354 1||735.559 240 1||14.223 343 307 120 3|
|1 atm||≡ 101325||≡ 1.01325||1.0332||1||760||14.695 948 775 514 2|
|1 Torr||133.322 368 421||0.001 333 224||0.001 359 51||1/760 ≈ 0.001 315 789||1 Torr |
≈ 1 mmHg
|0.019 336 775|
|1 lbf/in2||6894.757 293 168||0.068 947 573||0.070 306 958||0.068 045 964||51.714 932 572||≡ 1 lbf/in2|