Qu'est-ce qu'un/une Bar (bar) ?
Formal Definition
The bar is a metric unit of pressure defined as exactly 100,000 pascals (100 kPa), or equivalently 10⁵ N/m². The bar is not an official SI unit but is accepted for use with the International System of Units by the International Bureau of Weights and Measures (BIPM). Its magnitude is conveniently close to one standard atmosphere (101,325 Pa), making it a practical unit for many pressure applications.
One bar is approximately equal to 0.986923 standard atmospheres, 14.5038 psi, 750.062 mmHg, or 750.062 torr. The near-equivalence to atmospheric pressure makes the bar intuitive: 1 bar is roughly the pressure the atmosphere exerts at sea level. This close correspondence — combined with the clean decimal relationship to the pascal — explains the bar's enduring popularity in engineering and industry.
Submultiples and Multiples
The millibar (mbar), equal to 0.001 bar or 100 Pa, is widely used in meteorology and is numerically identical to the hectopascal (hPa). Standard atmospheric pressure is 1013.25 mbar (or 1013.25 hPa). The decibar (dbar), equal to 0.1 bar or 10,000 Pa, is used in oceanography because one decibar closely approximates the pressure increase per meter of seawater depth. The megabar (Mbar), equal to 10⁶ bar or 10¹¹ Pa, is used in high-energy physics and planetary science.
Etymology
Greek Origins
The word "bar" derives from the Greek word "baros" (βάρος), meaning "weight" or "heaviness." The same root appears in related English words such as "barometer" (instrument for measuring atmospheric pressure), "barograph" (recording barometer), and "isobar" (line of equal pressure on a weather map). The connection between "weight" and "pressure" is natural — atmospheric pressure was historically conceptualized as the weight of the air column above a point.
Introduction as a Unit
The bar was introduced in 1909 by the British meteorologist Napier Shaw while serving as director of the Meteorological Office. Shaw defined the bar as 10⁶ dynes per square centimeter in the CGS (centimeter-gram-second) system, which conveniently equals 10⁵ pascals in the modern SI system. Shaw's millibar quickly became the standard unit for atmospheric pressure in meteorology, replacing earlier measurements in inches or millimeters of mercury.
Precise Definition
Definition
The bar is defined as exactly 10⁵ Pa (100,000 pascals). This definition is exact and has no uncertainty. In CGS units, the bar equals 10⁶ dyn/cm² (dynes per square centimeter). The bar is listed in the SI brochure as a non-SI unit accepted for use with the SI, alongside units such as the liter, the tonne, and the electronvolt.
Relationship to the Atmosphere
The bar and the standard atmosphere are close but not identical: 1 bar = 0.986923 atm, and 1 atm = 1.01325 bar. The difference is approximately 1.325%. For most practical purposes outside of precision metrology, the bar and atmosphere can be considered interchangeable. This near-equivalence is not coincidental — the bar was deliberately defined at a round number of pascals that approximates atmospheric pressure.
IUPAC Standard
Since 1982, the International Union of Pure and Applied Chemistry (IUPAC) has recommended 1 bar (100 kPa) as the standard pressure for reporting thermodynamic properties, replacing the previous standard of 1 atm (101.325 kPa). This change affects standard enthalpies of formation, standard electrode potentials, and other tabulated thermochemical data. Chemistry textbooks published after 1982 use 1 bar as the reference pressure.
Histoire
Early Pressure Measurement
Before the bar existed, atmospheric pressure was measured using mercury barometers. Evangelista Torricelli invented the mercury barometer in 1643, measuring pressure in millimeters (or inches) of mercury. For over 250 years, mmHg and inHg were the primary pressure units. The diversity of pressure units — atmospheres, mmHg, inHg, psi, kgf/cm² — reflected the fragmented state of measurement standards.
Napier Shaw and the Millibar
In 1909, Napier Shaw introduced the bar for meteorological use. The millibar (1/1000 bar) became the standard meteorological pressure unit because its magnitude (100 Pa) was convenient for expressing atmospheric pressure values around 1000 mbar. The World Meteorological Organization (WMO) adopted the millibar as the international standard for weather reporting. By the mid-20th century, nearly all weather services worldwide used millibars.
Transition to Hectopascals
In the 1980s and 1990s, the WMO recommended transitioning from millibars to hectopascals (hPa) for weather reporting. Since 1 mbar = 1 hPa exactly, this was a change in name only — no numerical values changed. The motivation was to align meteorological practice with the SI system. Most countries completed the transition by 2000, though the term "millibar" persists in popular usage.
Industrial Adoption
In European industry, the bar became the standard pressure unit for compressed gases, hydraulic systems, and process engineering. The bar's popularity in Europe contrasts with the persistence of psi in the United States and UK. German, French, Italian, and Japanese industrial standards widely use the bar. Scuba diving internationally uses the bar for cylinder pressure and dive planning.
Utilisation actuelle
Scuba Diving
The bar is the international standard for scuba diving pressure measurement. Air cylinders are rated by their working pressure in bar: standard aluminum cylinders hold 200 bar, steel cylinders may hold 232 or 300 bar. Dive computers display depth-related ambient pressure in bar. A full 12-liter cylinder at 200 bar contains 2,400 liters of air at surface pressure.
Industrial Compressed Gas
Compressed gas cylinders worldwide are rated in bar. Medical oxygen cylinders typically hold gas at 137-200 bar. Industrial nitrogen and argon cylinders are filled to 200-300 bar. Acetylene cylinders, which are specially constructed due to the gas's instability, operate at a maximum of 18 bar. LPG (liquefied petroleum gas) cylinders hold liquid under pressure at approximately 5-15 bar.
Automotive Industry
In Europe and many other regions, tire pressure is frequently expressed in bar. A typical passenger car tire pressure of 2.2 bar equals 220 kPa or 32 psi. Turbocharger boost pressure is measured in bar: a turbocharged engine might produce 1.5-2.5 bar of boost (150-250 kPa above atmospheric). Brake system hydraulic pressure reaches 50-200 bar during hard braking.
Espresso and Brewing
Professional espresso machines operate at 9 bar (900 kPa) of pressure, forcing hot water through finely ground coffee. This specific pressure has become a defining characteristic of espresso — the 9-bar standard was established empirically in the 1960s and is considered optimal for extracting espresso crema. Home espresso machines may operate at 15 bar, with a pressure-reducing valve bringing the brew pressure down to 9 bar.
Everyday Use
Tire Pressure
In metric countries, tire pressure is commonly expressed in bar. Typical values: passenger cars 2.0-2.5 bar, SUVs 2.2-2.8 bar, bicycles (road) 6-8 bar, bicycles (mountain) 2-3 bar, motorcycles 2.0-2.9 bar. Most tire pressure gauges sold in Europe display bar alongside kPa.
Pressure Cookers
Pressure cookers operate at approximately 1 bar above atmospheric pressure (2 bar absolute). The instruction manuals for pressure cookers in metric countries specify operating pressure in bar or kPa. At 2 bar absolute, water boils at approximately 120°C instead of 100°C, reducing cooking times by 50-70%.
Water Pressure
Household water pressure in metric countries is expressed in bar. Normal domestic water pressure is 2-5 bar. Pressure below 1.5 bar is considered low and may result in poor shower performance. Pressure above 6 bar is considered high and may require a pressure reducing valve. Garden hose nozzles typically produce a spray at 3-4 bar.
Air Compressors
Home and shop air compressors are rated in bar in metric countries. A small portable compressor might produce 6-8 bar. A workshop compressor for power tools typically delivers 8-10 bar. Professional pneumatic tools require 6-7 bar for optimal operation. Spray painting requires 2-4 bar depending on the gun and paint type.
In Science & Industry
Thermochemistry
Since 1982, the IUPAC standard pressure for thermochemical data has been 1 bar (100 kPa), replacing the previous standard of 1 atm (101.325 kPa). Standard enthalpies of formation, Gibbs energies, and electrode potentials are now tabulated at 1 bar. The 1.325% change in reference pressure caused small but measurable differences in tabulated values. Chemistry databases such as NIST WebBook and CRC Handbook use the 1-bar standard.
High-Pressure Research
In high-pressure physics and chemistry, kilobars (kbar) and megabars (Mbar) are used to describe extreme conditions. Diamond anvil cells routinely achieve pressures of several megabars (hundreds of GPa). The pressure at the Earth's core is approximately 3.6 Mbar (360 GPa). Shock compression experiments in plasma physics measure pressures in megabars.
Oceanography
In oceanography, the decibar (0.1 bar = 10,000 Pa) is a standard unit because the pressure increase in seawater is approximately 1 dbar per meter of depth. This convenient correspondence allows oceanographers to use pressure as a proxy for depth. Ocean CTD (conductivity-temperature-depth) instruments record pressure in decibars. At the deepest point of the ocean (Challenger Deep, ~10,935 m), pressure is approximately 1,100 dbar (110 bar).
Vacuum Technology
Vacuum scientists use the millibar to describe low pressures. Rough vacuum is 1000-1 mbar. Medium vacuum is 1-10⁻³ mbar. High vacuum is 10⁻³-10⁻⁹ mbar. Ultra-high vacuum is below 10⁻⁹ mbar. Particle accelerators and space simulation chambers operate in ultra-high vacuum conditions.
Interesting Facts
The bar was introduced in 1909 — the same year that Robert Peary claimed to reach the North Pole and the first commercial flights by Zeppelin airships began. It outlasted both of those achievements as a relevant contribution to daily life.
Espresso's defining 9-bar brewing pressure was established by Ernesto Valente of the FAEMA company in 1961 with the E61 espresso machine. Before that, espresso was brewed at lower, inconsistent pressures using lever machines.
A scuba diver breathing at 30 meters depth (4 bar absolute pressure) consumes air four times faster than at the surface. A standard 12-liter, 200-bar tank that would last 60 minutes at the surface lasts only about 15 minutes at that depth.
The bar is almost exactly equal to one 'technical atmosphere' (1 at = 1 kgf/cm² = 98,066.5 Pa = 0.980665 bar), an older unit still found on some European pressure gauges and in Soviet-era technical documentation.
At 10,000 bar (1 GPa), water freezes even at room temperature, forming a high-pressure ice phase called Ice VI. There are at least 19 known crystalline phases of ice, most of which exist only at pressures measured in kilobars.
The pressure inside a champagne bottle is approximately 6 bar — roughly three times the pressure in a car tire. This is why champagne corks can travel at speeds of up to 50 km/h and cause serious eye injuries.
Commercial aircraft cabins are pressurized to approximately 0.75-0.80 bar (equivalent to an altitude of 1,800-2,400 meters), which is why some passengers experience ear discomfort and reduced oxygen saturation during flights.