Deep diving and narcosis

In short

Beyond about thirty metres, everything changes: air runs out faster, no-stop time shrinks, it gets colder and darker, and nitrogen narcosis slows your thinking. Deep diving needs preparation: suitable level and training, precise planning, a larger air reserve, a smaller team, reliable equipment. It ends with a slow ascent and respected stops. Faced with narcosis, the reflex is simple: ascend a few metres.

Why it matters

At depth, margins shrink and mistakes cost more: running out of air at 40 m leaves much less time than at 15 m, and a narcotic diver reacts less well. Understanding what changes and preparing for it lets you enjoy deep sites without taking unnecessary risks.

What changes at depth

• Air consumption: at 40 m, the pressure is 5 bar; each breath uses five times more air than at the surface. The cylinder lasts much less time.
• No-stop time: it drops sharply; on air, it is only a few minutes beyond 40 m.
• Cold: water is often colder below the thermocline and the suit compresses.
• Light and colours: reds, then oranges and yellows disappear; everything turns blue and dark. A light brings the colours back.
• Reaction time: narcosis, cold and darkness slow decisions.
• Ascent: longer, with stops more likely.

Nitrogen narcosis

What it is

Breathed under pressure, nitrogen has an effect on the nervous system similar to alcohol. It is sometimes called rapture of the deep. The effects often appear around 30 to 40 m on air and increase with depth.

Signs

• euphoria, laughter, overconfidence, or on the contrary anxiety;
• difficulty reading or interpreting instruments, wrong calculations;
• fixation on a detail, narrowed attention;
• slow reactions, misunderstood signals, forgotten instructions.

Variable susceptibility

Narcosis varies from person to person and, for the same person, from day to day. Cold, fatigue, exertion, stress, alcohol the night before and a fast descent make it worse. Do not rely on your experience of a previous dive.

What to do

• Ascend a few metres with your buddy: the effects usually fade quickly.
• Signal it and adapt the rest of the dive.
• Watch your buddy: unusual behaviour may be a sign of narcosis that they do not notice.

Nitrox does not clearly protect against narcosis and its maximum depth is limited by oxygen; very deep dives use helium mixes, which belong to technical diving.

Depth limits

• By level: each qualification gives access to a maximum depth, supervised or independent. In France, the Sports Code sets these limits for diving on air; recreational diving on air is limited there to 60 m for the most qualified divers. Other agencies set their own limits, often 40 m for recreational diving. See the training levels guide.
• By gas: with Nitrox, depth is limited by the MOD, calculated with a usual PPO2 of 1.4 bar (1.6 bar as an absolute maximum). See the Nitrox guide.
• By the dive leader: they can always set a lower limit depending on the conditions.

Preparation

• Planning: maximum depth, bottom time, ascent profile and expected stops, defined before entering the water and respected.
• Larger air reserve: the ascent and stops use more air; plan a larger reserve and turn around earlier.
• Smaller team: fewer divers, a consistent level, clearly designated buddies.
• Suitable equipment: a regulator in good condition that performs well at depth, a twin-outlet valve and two regulators where appropriate, light, computer, surface marker buoy.
• Fitness: well rested, hydrated, no intense exertion before the dive.
• Progression: increase depth step by step, with an instructor.

Ascent and stops

The ascent is the most important part of a deep dive. Respect the rate given by your computer or table (often about 10 m/min, or slower near the surface), the mandatory stops and the safety stop. Stay together, keep air for this phase and avoid exertion. The decompression guide explains why.

Key takeaways

• At depth, air runs out faster and no-stop time drops.
• Narcosis: ascend a few metres; susceptibility changes from day to day.
• Respect the limits of your level and the MOD of your gas.
• Plan, carry a larger reserve and dive in a smaller team.
• Slow ascent and respected stops.

Further reading

The app's watch calculates stops with the MN90 model or, with Dive Plus, the Bühlmann ZHL-16C model, down to 60 m; the watch's sensor may impose a lower limit and an alert signals it. These calculations replace neither your training nor a dedicated dive computer. The guides on decompression, Nitrox and technical diving complement this one.