More oxygen, less nitrogen
Air contains about 21% oxygen and 79% nitrogen. Under water, the nitrogen you breathe dissolves into your tissues, and that load limits how long you can stay at depth without decompression stops. Enriched air nitrox replaces some of the nitrogen with oxygen, usually to 32% (EAN32) or 36% (EAN36). The trade-off: oxygen becomes toxic at high pressure, so every nitrox mix has a depth limit.
Partial pressure
Each gas in a mix contributes a share of the total pressure. At depth:
ppO₂ = FO₂ × absolute pressure
In salt water the pressure rises by one atmosphere every 33 ft (34 ft in fresh water), on top of the one atmosphere at the surface. Recreational agencies set a working ppO₂ limit of 1.4 ata and a contingency limit of 1.6 ata.
The formulas
- Maximum operating depth: MOD = (ppO₂ limit ÷ FO₂ − 1) × 33 ft
- Equivalent air depth: EAD = (depth + 33) × (1 − FO₂) ÷ 0.79 − 33
- Best mix for a depth: FO₂ = ppO₂ limit ÷ (depth ÷ 33 + 1)
Worked example
A diver plans a dive to 100 ft in the ocean with EAN32 and a ppO₂ limit of 1.4:
- Pressure at 100 ft: 100 ÷ 33 + 1 = 4.03 ata.
- ppO₂ at 100 ft: 0.32 × 4.03 = 1.29 ata, within the limit.
- MOD: (1.4 ÷ 0.32 − 1) × 33 = 111.4 ft; at 1.6 it would be 132 ft.
- EAD: 133 × 0.68 ÷ 0.79 − 33 = 81.5 ft, so for nitrogen the dive counts as an 81 ft air dive.
- Best mix for 100 ft at 1.4: 34.7% oxygen.
Why the MOD is a hard limit
High oxygen pressure can trigger central nervous system oxygen toxicity. Its first sign may be a convulsion, without warning, and a convulsion under water usually means drowning. Exposure time matters too, which is why divers track an oxygen “clock” for long or repeated dives. Never descend below the MOD written on your tank.
Safety habits
- Analyze the oxygen content of every tank yourself, with your own analyzer, before you dive.
- Label the tank with the mix, the MOD and your name.
- Set your dive computer to the actual mix and ppO₂ limit.
- Nitrox reduces nitrogen loading but does not prevent decompression sickness: ascend slowly and make safety stops.
- Nitrox is not for deep diving; it is a tool for longer dives at moderate depths.
Diving nitrox requires a certification course from a recognized agency, and dive shops ask for the card before filling a tank. This calculator helps you understand the numbers and double-check a plan; it does not replace training or a dive computer.