Bar (unit)

Bar (unit)
source
Name	Bar
Standard	Metric system
Quantity	Pressure
Units1	SI derived units
Inunits1	105 Pa
Units2	CGS units
Inunits2	106 Barye
Units3	United States customary units
Inunits3	14.5038 psi
Units4	Atmosphere
Inunits4	0.986923 atm

Bar (unit). The bar is a metric unit of pressure defined as 100,000 pascals. It is not part of the International System of Units but is widely accepted for use with it, particularly in meteorology, oceanography, and various engineering fields. The unit provides a convenient measure for pressures that are roughly equivalent to atmospheric pressure at sea level.

Definition and conversion

The bar is precisely defined as 100,000 pascals, which is equivalent to 100 kilopascals or 1,000,000 dynes per square centimetre in the Centimetre–gram–second system of units. Common conversions include approximately 14.5 pounds per square inch and 0.987 atmospheres. For finer measurements, the millibar (mbar), equal to one-thousandth of a bar or one hectopascal, is frequently employed, especially in reporting barometric pressure from weather stations. The decibar (dbar) is standard in oceanography, where an increase of one decibar corresponds closely to a one-meter increase in depth in sea water.

History and usage

The term "bar" originates from the Greek word *baros*, meaning weight, and was introduced by the British Association scientist William Napier Shaw around 1909. It was formally adopted by the International Union of Pure and Applied Chemistry in the early 20th century. Historically, the millibar became the standard unit for atmospheric pressure on weather maps issued by the United Kingdom Meteorological Office and the United States Weather Bureau, with standard sea-level pressure defined as 1013.25 mbar. Following the adoption of the pascal by the International System of Units, many national meteorological services, such as Météo-France, transitioned to using the hectopascal, which is numerically identical to the millibar.

Comparison with other units

While the bar is convenient for many practical applications, the pascal is the SI unit for pressure. One bar equals 100,000 pascals, making it a more manageable unit for pressures commonly encountered in engineering, unlike the megapascal used for high-stress materials. Compared to the standard atmosphere, which is defined as 101,325 pascals, one bar is slightly smaller, a difference of about 1.3%. In hydraulic and pneumatic system specifications within industries like automotive manufacturing, pressure is often given in bars rather than pounds per square inch, which remains common in the United States customary units.

Common applications

The bar is extensively used across numerous scientific and technical disciplines. In meteorology, atmospheric pressure is commonly reported in millibars or hectopascals on charts from agencies like the European Centre for Medium-Range Weather Forecasts. Oceanographers use decibars for measuring hydrostatic pressure in the water column. The unit is standard in scuba diving to express tank pressure, with a typical aluminum tank rated at 200 bar. In automotive engineering, tire inflation and fuel injection system pressures are often specified in bars. The food industry uses bars to specify pressures in pascalization equipment for non-thermal pasteurization.

Standardization and regulation

Although not an SI unit, the bar is recognized as a permissible non-SI unit by influential standards bodies, including the International Organization for Standardization and the National Institute of Standards and Technology. Its use is tolerated within the European Union under EU directive 80/181/EEC on units of measurement. The bar is explicitly excluded from use for health and safety purposes under regulations from the Health and Safety Executive in the United Kingdom, where the pascal or its multiples are required. Its continued prevalence in specific fields is governed by long-standing conventions within organizations like the World Meteorological Organization and professional engineering codes.

Category:Units of pressure Category:Metric units