Standard atmospheres to Bar (atm to bar)

One standard atmosphere (atm) is defined as exactly 101,325 pascals (101.325 kPa) by the 10th CGPM resolution of 1954. This value is also equal to exactly 760 millimetres of mercury (mmHg, or torr) at 0 °C under standard gravity — the equivalence is by definition, not by measurement, and was specifically constructed so that the older mercury-column convention and the pascal-based SI convention give the same numerical reference. Conversions to other commonly-encountered pressure units: 1 atm = 1.01325 bar exactly, 1 atm = 14.6959488 psi, 1 atm = 29.9213 inches of mercury, and 1 atm = 1,013.25 millibar / hectopascal. The unit symbol "atm" is recognised by the BIPM but explicitly listed as a non-SI unit whose use is discouraged in favour of the pascal — yet it persists in chemistry, hyperbaric medicine and diving because the magnitude is human-scale and the gas-law formulas are taught in terms of it. A closely related notation, "ATA" (atmospheres absolute), is used in diving and hyperbaric work to make explicit that the pressure is referenced to a perfect vacuum rather than to local atmospheric pressure — so 1 ATA at the surface, 2 ATA at 10 metres of seawater, and so on.

The standard atmosphere descends from Evangelista Torricelli's 1644 barometric experiment in Florence, in which a glass tube sealed at one end and filled with mercury was inverted into a dish of mercury; the column settled at about 760 mm above the dish, balanced by atmospheric pressure on the dish surface. Pascal's 1648 Puy de Dôme expedition extended Torricelli's work to confirm altitude dependence, and the 760 mm value became the conventional reference for "one atmosphere" of pressure across the eighteenth and nineteenth centuries. Through that period the standard atmosphere consolidated as the reference pressure for gas-law work — Boyle's law (1662), Charles's law (1787), Avogadro's hypothesis (1811), and the unified ideal-gas equation pV = nRT all referenced atmospheric pressure as the natural baseline, and chemistry developed a deep practical reliance on the atm as the working pressure unit for laboratory calculations. The modern numerical definition was fixed by the 10th General Conference on Weights and Measures (CGPM) in 1954 — the same conference that defined the kelvin against the triple point of water — at 1 atm = 101,325 Pa exactly. The CGPM framed the value as the mean atmospheric pressure at sea level, 45° latitude, 0 °C: an idealised reference rather than a directly measured quantity, closing nearly two centuries of small national variations in the assumed value. In 1982, IUPAC redefined "standard temperature and pressure" (STP) from 0 °C + 1 atm to 0 °C + 1 bar (100,000 Pa) for SI alignment. The older 1 atm definition remains in the educational literature and many engineering reference works, and the resulting "old STP" / "new STP" split is the most persistent legacy issue in introductory chemistry pedagogy.

Chemistry and gas-law calculations are the centerpiece of atm's active educational and laboratory use. Every introductory and physical-chemistry textbook in the English-speaking world (Atkins, McQuarrie-Simon, Levine, Zumdahl) presents the ideal-gas equation pV = nRT with worked examples in which the pressure variable carries units of atm, and the universal gas constant R is most commonly memorised in its convenient form 0.08206 L·atm·mol⁻¹·K⁻¹ rather than its SI form 8.314 J·mol⁻¹·K⁻¹. Dalton's law of partial pressures, Henry's law for gas solubility, Raoult's law for vapour pressure of solutions, and the equilibrium constant Kp (defined in terms of partial pressures) all conventionally use atm as the reference pressure. The molar volume of an ideal gas at the older STP convention (0 °C, 1 atm) is the famous 22.414 litres per mole, a value memorised by chemistry students for almost a century — superseded numerically but not pedagogically by the 22.711 L/mol of the post-1982 STP convention. Diving and decompression theory: recreational and technical diving teaches depth-pressure as multiples of one atmosphere absolute (ATA), with one additional atmosphere added for every ten metres of seawater depth. A diver at 10 m experiences 2 ATA, at 20 m experiences 3 ATA, at 30 m (the recreational limit on air without decompression-stop training) experiences 4 ATA, and a technical diver at 60 m experiences 7 ATA. The US Navy Diving Manual decompression tables, the PADI and SSI recreational dive tables, and the algorithm-driven dive computers (Bühlmann ZH-L16, VPM-B, RGBM) all denominate ambient pressure in ATA for the decompression-modelling calculations even when the cylinder gauge on the same dive reads in bar. Henry's law gas-loading and off-loading from body tissues — the foundation of decompression theory — is computed in terms of ambient ATA partial pressures of nitrogen and helium. Hyperbaric medicine: clinical hyperbaric oxygen therapy (HBOT) prescribes treatment pressures explicitly in ATA. The Undersea and Hyperbaric Medical Society (UHMS) approved indications for HBOT specify treatment regimes typically at 2.0–2.4 ATA for chronic wound healing, diabetic foot ulcers, radiation tissue injury, and carbon monoxide poisoning, with severe decompression sickness and arterial gas embolism treated under US Navy Treatment Table 6 at 2.8 ATA. Hospital monoplace and multiplace hyperbaric chambers display chamber pressure in ATA on the operator console as the primary clinical variable. Autoclave sterilisation: hospital and laboratory steam autoclaves operate at approximately 1 atm gauge pressure (about 2 ATA absolute) at 121 °C for 15–30 minutes per the CDC's Guideline for Disinfection and Sterilisation in Healthcare Facilities and the ANSI/AAMI ST79 comprehensive sterilisation guide; the higher 134 °C "flash" cycle uses about 2 atm gauge (3 ATA absolute) for shorter exposure. ASME Boiler and Pressure Vessel Code Section VIII pressure-vessel calculations for autoclave design carry the ratings in psi for the US market and bar for European, but clinical and microbiological literature consistently report cycle parameters in atm. Planetary science uses atm as a convenient ratio reference for comparing other planetary atmospheres to Earth's: the surface pressure of Venus is about 92 atm (a crushing nine-kilometre-deep ocean equivalent in pressure terms), Mars surface is about 0.006 atm (well below the Armstrong limit of 0.0618 atm at which water boils at body temperature), Titan's surface is about 1.45 atm, and the gas-giant atmospheres are conventionally measured against the 1 atm "surface" level of their pressure profiles since they have no solid surface.

One bar is defined as exactly 100,000 pascals (100 kPa, or 10⁵ Pa). Equivalently, the bar is one mega-dyne per square centimetre in the older CGS system in which it was originally formulated. The conversion to other commonly-encountered pressure units is: 1 bar = 14.5037738 psi exactly (rounding to five decimal places), 1 bar = 0.986923 standard atmospheres, 1 bar = 750.062 torr (mmHg), and 1 bar = 29.530 inches of mercury. The relationship to standard atmospheric pressure is the unit's defining feature: 1 atmosphere = 1.01325 bar exactly, by the 1954 BIPM definition of the standard atmosphere — so the two units are close, but not identical, and the 1.3% gap matters in precision applications such as gas-law calculations and metrology-grade barometric work. Sub-multiples in regular use are the millibar (1 mbar = 100 Pa = 1 hPa = 0.001 bar), used in meteorology for atmospheric pressure (sea-level standard 1013.25 mbar), and the kilobar (1 kbar = 100 MPa), used in geophysics for pressures inside the Earth and in materials science for high-pressure synthesis. The bar is a non-SI unit accepted by the BIPM for use with SI, alongside the tonne, the litre, and the hour.

The bar was coined in 1909 by the Norwegian physicist and meteorologist Vilhelm Bjerknes (1862–1951), founder of the Bergen School of meteorology and the figure most responsible for putting modern weather forecasting on a quantitative physical-dynamics footing. The name derives from the Greek βάρος (baros, "weight"), the same root that gives barometer and isobar. Bjerknes needed a pressure unit of convenient magnitude for synoptic meteorology, where atmospheric variations across a weather chart are fractions of an atmosphere rather than the thousands of pascals such variations would represent. He fixed the bar at exactly 100,000 pascals (100 kPa). The deliberate sizing of one bar to approximate one standard atmosphere (1 atm = 1.01325 bar) — accurate to within about 1.3% — is the unit's structural identity: a single-digit number for the pressure of the air around us and a convenient round factor of 100,000 against the SI base unit. The bar is not part of the International System of Units, but the International Bureau of Weights and Measures (BIPM) accepts it for use with SI in the same non-SI-accepted category as the tonne and the litre. The millibar (mbar, 1/1000 bar) was the working unit of synoptic meteorology for most of the twentieth century. The World Meteorological Organization recommended a transition to the hectopascal (hPa) in the 1980s for SI alignment, but because 1 mbar = 1 hPa exactly, the change was nominal rather than numeric — the same printed value, with a relabelled unit. Several national meteorological services retained "millibar" in public-facing forecasts long after the WMO recommendation, particularly in UK broadcast weather reporting.

European and Asian automotive engineering treats bar as the standard pressure unit on the consumer-facing side of the vehicle: door-jamb tyre-pressure placards on EU-market vehicles, owner's manuals printed for European, Japanese and Korean markets, and tyre-sidewall maximum-pressure markings on European tyre brands (Michelin, Continental, Pirelli) all denominate cold-inflation pressure in bar — typical passenger-car values 2.2–2.5 bar, light SUVs 2.4–2.7 bar. Continental Europe's gas-station air pumps read in bar, and the EU type-approval framework under Regulation (EC) No 661/2009 (which mandated TPMS for new passenger cars from November 2014) accepts placard values in bar as the regulatory baseline. Scuba diving is bar's globally dominant centerpiece, with no significant US-customary counterpart. PADI, SSI, BSAC and CMAS instructor materials worldwide teach depth-pressure conventions in bar (atmospheric pressure adds approximately 1 bar per 10 metres of seawater), cylinder service pressures are stamped in bar on the cylinder shoulder (200 bar for the standard aluminium S80 in metric markings, 232 bar for steel cylinders common in European technical diving, 300 bar for high-pressure steel tanks used in cave and rebreather diving), and submersible pressure gauges on every dive console — including those manufactured for the US market — read in bar. The bar is the only pressure unit a recreational diver routinely encounters in active practice. Meteorology and atmospheric science: surface-pressure analyses on synoptic weather charts have been plotted in millibars since the early twentieth century, with the standard sea-level pressure 1013.25 mbar marking the dividing line between high-pressure and low-pressure systems. The World Meteorological Organization's Manual on the Global Observing System and the technical standards published in WMO-No. 8 (Guide to Meteorological Instruments and Methods of Observation) report pressure in hectopascals, but because 1 mbar = 1 hPa, the printed values are identical. National services made the relabelling at different times: the US National Weather Service moved to hectopascals on aviation METAR and TAF reports in the 1990s, while the BBC Weather forecast retained "millibars" for UK public-facing television broadcasts well into the 2010s. European industrial process control and pressure-vessel engineering: the EU Pressure Equipment Directive 2014/68/EU regulates pressure vessels, piping and safety accessories rated above 0.5 bar gauge, with conformity-assessment categories defined by pressure-times-volume thresholds expressed in bar·litre. Industrial gauges, manifolds, valves and process control instruments installed in European chemical, petrochemical and food-processing plants are calibrated and labelled in bar; the harmonised standards EN 837 (for Bourdon-tube gauges) and EN 13136 (for refrigeration pressure-relief sizing) work in bar throughout. Hydraulic-system pressures in European mobile equipment and industrial machinery — Bosch Rexroth, Hydac, Parker (in its EU product lines) — run typically 160–350 bar, with the same product re-catalogued in psi for the North American market. Espresso and food-equipment engineering: the international convention for espresso brewing pressure is 9 bar, fixed by the Italian-machine tradition that grew up around the FAEMA, La Marzocco and Faema E61 group designs in the 1950s and 1960s. Specialty Coffee Association barista-training curricula and every major espresso-machine manufacturer document brewing pressure in bar; the 9 bar value has become specific enough that it functions as an industry shorthand for "real espresso" in coffee writing. Carbonation and CO₂ working pressures in commercial soda and beer dispense systems are similarly spec'd in bar across European equipment.