Atmospheres to Bar

The standard atmosphere was fixed in 1954 by the General Conference on Weights and Measures as exactly 101,325 pascals, the average air pressure at sea level. That equals 101.325 kPa, 14.696 psi and 760 mmHg. The bar is a rounder metric unit, exactly 100,000 Pa, introduced so pressure could be stated in clean powers of ten.

In 1982 IUPAC changed standard pressure for chemistry from 1 atm to 1 bar, so older textbooks and newer ones quote slightly different standard conditions. Gas volumes and equilibrium constants at standard pressure shift by about 1.3% between the two, which matters in precise lab work and almost nowhere else.

Under water, every 10 meters of seawater adds about 1 bar, or 0.99 atm. At 30 m a diver breathes air at roughly 4 bar absolute, the surface atmosphere plus three bars of water, which is why air is used up four times faster there than at the surface.

Going up, pressure falls. Airliner cabins are pressurized to the equivalent of 6,000–8,000 ft, roughly 0.75–0.8 atm, and the summit of Mount Everest sits near 0.33 atm. Climbers and pilots usually see these figures in hPa or inches of mercury; converting to bar or atm makes them easier to compare.

Watches use atm and bar loosely: a watch rated 10 ATM or 10 bar is tested to about 100 m of static water pressure, which does not mean it is safe for diving to that depth. For tire and compressor figures, bar to psi and kPa to psi are more practical.