Diving with Nitrox

In short

Nitrox is a mix of nitrogen and oxygen that contains more oxygen than air (21%). Less nitrogen in the mix means less nitrogen absorbed by your body: you get more dive time without mandatory decompression stops. In return, oxygen becomes toxic above a certain pressure: each mix has a maximum operating depth (MOD) that must never be exceeded. Nitrox requires specific training.

Why it matters

Nitrox is now offered by many dive centres. Used correctly, it increases your decompression margin. Used incorrectly, it exposes you to a risk that barely exists on air in recreational diving: oxygen toxicity, which can cause a seizure underwater, often fatal through drowning. Nitrox safety relies on three simple actions: analyse your mix, know your MOD and respect it.

What is Nitrox?

It is written EANx (enriched air nitrox), where x is the percentage of oxygen. Air is EAN21. The most common recreational mixes are:

Mix	Oxygen	Nitrogen
Air (EAN21)	21%	79%
EAN32	32%	68%
EAN36	36%	64%
EAN40	40%	60%

The app supports mixes from EAN21 to EAN40, which covers most recreational Nitrox diving.

What Nitrox gives you

• More time without mandatory stops at the same depth. For example, with the app's default settings (Bühlmann model, gradient factors 30/70), a dive at 30 m allows about 12 min without mandatory stops on EAN32, compared with about 7 min on air.
• Or an extra safety margin: if you keep the dive time planned for air, you absorb less nitrogen.

Common misconceptions

• "Nitrox lets you dive deeper": it is the opposite. The more oxygen, the shallower the maximum depth.
• "Nitrox makes you less tired": many divers feel this, but it has not been clearly demonstrated. Do not count on it.
• "Nitrox removes the risk of decompression sickness": no. It reduces it for the same dive, but the ascent and stop rules stay the same.

Oxygen partial pressure (PPO₂)

Underwater, pressure increases by about 1 bar every 10 m. At 10 m you breathe gas at 2 bar; at 30 m, at 4 bar. The oxygen partial pressure (PPO₂) is the share of this pressure due to oxygen:

PPO₂ = absolute pressure × oxygen fraction

Example on EAN32 at 30 m: 4 bar × 0.32 = 1.28 bar.

It is the PPO₂, not the oxygen percentage, that determines toxicity. The usual recreational limits are:

• 1.4 bar: recommended working limit for the dive;
• 1.6 bar: a limit never to be exceeded, in practice reserved for decompression stops and some training.

In the app, the maximum PPO₂ can be set from 1.0 to 1.4 bar (1.4 bar by default), and an additional alert triggers at 1.6 bar.

Maximum operating depth (MOD)

The MOD is the depth at which your mix reaches the chosen maximum PPO₂. The simplified formula taught in courses is:

MOD = (max PPO₂ ÷ oxygen fraction − 1) × 10

Example on EAN32 with 1.4 bar: (1.4 ÷ 0.32 − 1) × 10 = (4.375 − 1) × 10 = 33.75 m, which is always rounded down: 33 m.

The app uses standard atmospheric pressure (1.013 bar) instead of 1 bar, which gives a very slightly more conservative result. If values differ, always use the lower one, rounded down.

Mix	Simplified formula (1.4 bar)	App (1.4 bar)	MOD to use
EAN32	33.75 m	33.6 m	33 m
EAN36	28.9 m	28.7 m	28 m
EAN40	25 m	24.9 m	24 m

Never exceed the MOD of your mix, even briefly. It must be marked on your cylinder and announced at the briefing. Stay shallower with a margin, because your actual depth varies during the dive.

Oxygen toxicity

Central nervous system toxicity (Paul Bert effect)

It occurs when the PPO₂ is too high, even for a short time. It can cause a seizure, sometimes without any warning. Some divers first notice vision problems, ringing in the ears, nausea, facial twitching, irritability or dizziness. Underwater, a seizure makes you lose your regulator: this is why the MOD is strictly respected.

The risk depends on the PPO₂ and on exposure time. To track it, divers use the CNS percentage (central nervous system): it expresses how much of the maximum allowed exposure time has been used. It rises during the dive and slowly falls at the surface. The app shows it on the watch and alerts at 80% then 100%.

Pulmonary toxicity (Lorrain Smith effect)

It appears after long, repeated exposures over several days (coughing, breathing discomfort, burning in the chest). It mainly concerns technical diving or intensive trips. It is tracked with a unit called the OTU (oxygen toxicity unit). In reasonable recreational diving the risk remains low, but it justifies rest days during intensive trips.

Before the dive: analyse, label, check your equipment

1. Analyse your cylinder yourself, just before the dive, with a calibrated oxygen analyser. This is a golden rule: never dive with a mix you have not analysed.
2. Record the result on the cylinder and in the centre's log: measured oxygen percentage, MOD, date and your name.
3. Set your dive computer to the measured percentage and check the configured maximum PPO₂.
4. Check equipment compatibility: the regulator and valve must suit the mix used, following the manufacturer's instructions; oxygen-rich mixes require stricter standards (so-called "oxygen compatible" equipment). Nitrox cylinders are usually identified with specific markings.
5. Announce your mix at the briefing: the whole team must know the shallowest MOD in the group.

Nitrox training

Nitrox requires a course, usually available from the first certification level. Course names vary:

• FFESSM / CMAS: basic Nitrox, then advanced Nitrox (richer mixes, use during the ascent);
• PADI: Enriched Air Diver;
• SSI: Enriched Air Nitrox.

These equivalences are indicative; the exact rights (maximum percentage, depth) depend on the organisation and on the country's regulations. The course covers theory (PPO₂, MOD, toxicity), gas analysis and often one or two dives.

Key takeaways

• Nitrox contains more oxygen and less nitrogen than air: more time without stops, not more depth.
• Usual PPO₂ limit: 1.4 bar; 1.6 bar is an absolute maximum.
• Calculate your MOD, round it down and never exceed it.
• Analyse your cylinder yourself before every dive and set your computer to the result.
• Nitrox requires specific training.

Further reading

The app's Nitrox preparation (Dive Plus) calculates the MOD, the maximum time without mandatory stops and the forecast CNS for a mix, a depth and a bottom time. It is not a gas-supply calculation and it does not replace your training or your dive computer. During a Nitrox dive, the watch uses the Bühlmann model and shows PPO₂, MOD and CNS. The decompression guide explains what "without mandatory stops" means.