Scuba & Nitrox Glossary: Numbers & Symbols
Nitrox and breathing-gas terminology is prioritized first. General scuba terms follow below. Definitions are educational summaries and do not replace formal training, manufacturer instructions, or an operational dive plan.
Nitrox & Gas Terms — Numbers & Symbols
- 1.4 ATA
- A commonly used working limit for oxygen partial pressure during the active or bottom portion of many Nitrox dives. Limits vary by training, conditions, workload, and plan.
- 1.6 ATA
- A higher oxygen partial-pressure limit commonly reserved for contingency or decompression exposure rather than routine working portions of a dive.
- 21% Oxygen
- The approximate oxygen fraction in ordinary air, expressed as FO₂ 0.21.
- 22–40% Nitrox
- The oxygen-percentage range commonly covered by basic recreational Nitrox training. Agency standards vary.
- 32% Nitrox (EAN32)
- A common enriched-air blend containing approximately 32% oxygen and 68% nitrogen.
- 36% Nitrox (EAN36)
- A common enriched-air blend containing approximately 36% oxygen and 64% nitrogen.
- 40% Rule
- A widely used equipment threshold: gases or equipment exposed to more than 40% oxygen may require oxygen-clean preparation under applicable standards. Requirements vary by agency and jurisdiction.
- 0.21 FO2
- The oxygen fraction of ordinary air expressed as a decimal.
- 0.32 FO2
- The oxygen fraction representing a measured 32 percent Nitrox blend.
- 0.36 FO2
- The oxygen fraction representing a measured 36 percent Nitrox blend.
- 100 Percent Oxygen
- Pure oxygen used only with compatible equipment, procedures, and training.
- 40 Percent Oxygen
- A common recreational equipment threshold referenced in oxygen-service guidance.
General Scuba Terms — Numbers & Symbols
- 3-Minute Safety Stop
- A commonly planned pause near the end of a no-decompression dive, often around 15 feet/5 metres; follow the dive computer and training plan.
- 5-Point Descent
- A structured pre-descent sequence used to confirm signal, orientation, regulator, time, and controlled descent.
- 5-Point Ascent
- A structured ascent sequence emphasizing communication, awareness, controlled ascent, and surface procedures.
- 60 Feet per Minute
- A historical maximum ascent rate used by some tables; many modern computers and agencies prescribe slower or variable ascent rates.
- 100% Oxygen
- Pure oxygen, used in some technical, decompression, first-aid, and recompression contexts by properly trained personnel.
- 1 Atmosphere
- Pressure at sea level, approximately 14.7 psi or 1.013 bar.
- 10 Metres of Seawater
- A depth increment that adds approximately one atmosphere of pressure.
- 15-Foot Stop
- A common approximate depth for a recreational safety stop.
- 200-Bar Cylinder
- A cylinder with a nominal working pressure of 200 bar.
- 500-Psi Reserve
- A traditional remaining-pressure reference, not a universal gas-planning rule.