Bar to Atmospheres Converter

Diving and hyperbaric medicine: dive computers use bar, but physiological calculations reference atmospheres absolute (ATA).

Pressure vessel design: European codes (PED) use bar, while thermodynamic property tables often use atm.

Gas solubility calculations (Henry's Law) where atmospheres is the standard unit.

In recreational and commercial diving per the US Navy Diving Manual, PADI, SSI, and the international IMCA/ADCI commercial diving standards, the depth gauge and submersible pressure gauge typically read in bar (or meters of seawater, MSW), but the decompression calculations and the partial pressure of nitrogen and oxygen in the breathing gas are computed in atmospheres absolute (ATA).

At a depth of 30 meters of seawater (about 4 bar absolute, or 3.94 ATA), the partial pressure of nitrogen in air (79% N₂) is 0.79 × 3.94 = 3.11 ATA, exceeding the 3.0 ATA US Navy limit for continuous exposure to compressed air (to avoid nitrogen narcosis and CNS oxygen toxicity).

The 1.3% difference between bar and ATA is small for single-dive calculations but accumulates in repetitive dives and saturation diving operations where the small systematic error can affect the decompression obligation.

Hyperbaric oxygen therapy (HBOT) chambers in hospitals and wound care clinics per UHMS (Undersea and Hyperbaric Medical Society) protocols operate at 2.0–3.0 ATA (about 2.03–3.04 bar), with the chamber gauge often reading in bar or kPa while the prescription is in ATA — the conversion is required at the prescription-to-gauge boundary.

Pharmaceutical and chemical processing gas solubility calculations use Henry's law in atm or bar, with the constant H_cp (the Henry's law constant at constant pressure, in mol/(kg·bar) or mol/(kg·atm) depending on the source) being a fundamental physical-chemistry parameter.

The conversion between bar and atm is therefore routine in the CO₂ absorption calculations for carbon capture systems, the O₂ solubility in fermentors, and the H₂ solubility in hydrogenation reactors.

Pressure vessel design per the European Pressure Equipment Directive (PED 2014/68/EU) uses bar as the design pressure unit, while the ASME BPVC Section VIII in the US uses psi (or kgf/cm² in Japanese and Korean fabrications), with the atm unit appearing in some thermodynamic property tables (e.g., NIST REFPROP, the AGA 8 equation of state for natural gas) and in some chemical process design references.

The chemical thermodynamics textbooks and most engineering reference tables (Perry's Chemical Engineers' Handbook, CRC Handbook of Chemistry and Physics) use atm or bar in the property tables depending on the edition, with the conversion required when comparing data from different sources.