Which unit is used to express the maximum oxygen fill pressure?

• A. PSIG
• B. BAR
• C. kPa
• D. psi

Answer: (B)

Understanding how cylinder pressures are labeled helps here. The maximum fill pressure of a high‑pressure gas cylinder, like oxygen used in dive operations, is typically given in bar. Bar is a metric unit that directly reflects the rating of the cylinder itself, and many gas blends and dive equipment use bar as the standard reference. A cylinder labeled with a value in bar is indicating the pressure you should not exceed when filling, which aligns with how manufacturers specify safe limits (for example, around 200 bar for many oxygen cylinders).

While psi or PSIG (pounds per square inch, gauge) are common in some regions, they are not the standard labeling for maximum fill pressure in diving oxygen cylinders, and converting between units adds a step that isn’t necessary for reading the specification. Kilopascals (kPa) are accurate but less practical for typical cylinder ratings and not the conventional display.

So, bar is the best answer because it matches the conventional labeling and practical use for maximum oxygen fill pressure in dive-related equipment.