Understanding decompression

In short

During a dive, your body absorbs nitrogen, all the more so the deeper and longer you dive. On the way up, this nitrogen must leave slowly through your breathing. If you ascend too fast, it forms bubbles in your body: this is decompression sickness (DCS). To prevent it, divers respect an ascent rate, stops and dive times calculated by tables or a dive computer. This guide explains the principle; the actual figures always come from your planning tool and your training.

Why it matters

DCS is one of the most serious diving accidents. It can leave lasting neurological damage. It sometimes occurs even when the rules were followed, because no model offers 100% protection. Understanding what happens in your body helps you follow the procedures, keep a safety margin and recognise the signs that call for immediate help.

The principle: nitrogen dissolves, then comes out

Henry's law

The air you breathe contains about 79% nitrogen. Underwater, you breathe it at a higher pressure (about 1 bar more every 10 m). Henry's law states that a gas dissolves in a liquid in proportion to its pressure: the deeper you are, the more nitrogen dissolves in your blood and then in your tissues.

Think of a bottle of soda: closed, the gas is dissolved and invisible; opened suddenly, it forms bubbles. Opened gently, it escapes without foaming. Your ascent is the opening of the bottle.

Loading, off-gassing and compartments

• Loading (saturation) is the amount of nitrogen taken up by the tissues during the dive.
• Off-gassing (desaturation) is its elimination, during the ascent and then at the surface, over several hours.
• Not all tissues load at the same speed: blood and the brain load quickly, joints and fat slowly. Decompression models represent the body with theoretical compartments, each defined by its half-time: the time it takes to load or unload halfway.

A tissue can tolerate a certain excess of nitrogen without forming harmful bubbles. Models calculate this limit for each compartment: as long as it is not exceeded, you can ascend directly to the surface. Otherwise, you need stops.

Ascending correctly

Ascent rate

Ascending slowly gives nitrogen time to leave. Follow the rate set by your method (tables or computer) and your training. Most dive computers alert above about 10 m/min; the last metres, where pressure changes the most, are done even more slowly. Ascending too fast is one of the main DCS risk factors.

The safety stop

It is a stop of a few minutes at around 3 to 5 m at the end of the dive, even when no stop is mandatory. The tables do not make it compulsory, but it is very widely recommended: it reduces your nitrogen load and trains you to control your depth near the surface.

Mandatory decompression stops

When the dive was longer or deeper, your tables or computer require one or more stops at precise depths (for example 3 m, 6 m) for a given time. They are mandatory: never cut them short. Beginners' recreational diving is in principle no-decompression diving.

Planning tools

MN90 tables

The MN90 tables, developed by the French Navy, are the historical reference of French training. The principle:

1. You enter the table with the maximum depth reached and the dive time, both rounded up to the next value in the table.
2. The table gives any stops and a repetitive dive group, a letter that represents the nitrogen left in your body.
3. For a repetitive dive, this group and the time spent at the surface give a penalty time: a notional time added to the second dive to account for residual nitrogen.

The tables assume a "square" profile (the whole dive at maximum depth), which makes them conservative for a multilevel dive. Using them is taught in courses: this guide does not reproduce them.

Dive computers and the Bühlmann model

A dive computer measures depth continuously and calculates the loading of your compartments in real time. Many use the Bühlmann ZHL-16C model (16 compartments) with gradient factors (GF), two settings that add conservatism:

• GF low affects the depth of the first stop: the lower it is, the deeper the first stop;
• GF high affects surfacing: the lower it is, the more margin you keep when you reach the surface.

Lower values are more conservative. Your computer constantly shows the remaining no-decompression time, any stops and your ascent rate.

Tables and computers give a theoretical limit, not a guarantee. Keep a margin: do not dive up to the no-decompression limit, and always follow the instructions of the dive supervisor and your instructors.

Decompression sickness

Risk factors

• ascending too fast, shortened or missed stops;
• "yo-yo" profiles (repeated ups and downs) and many repetitive dives;
• hard exertion during or just after the dive (finning against the current, carrying heavy loads);
• cold, fatigue, dehydration, alcohol, lack of sleep;
• flying, mountains or freediving shortly after the dive;
• some individual factors, to be assessed with a doctor.

Warning signs

They usually appear within minutes or hours after leaving the water:

• unusual, intense fatigue;
• joint pain (shoulder, elbow, knee);
• itching, skin mottling;
• tingling, numbness, weakness in a limb;
• vision, speech or balance problems, dizziness;
• breathing difficulty, chest pain, feeling unwell.

Any unusual sign after a dive must be treated as a possible diving accident, even if it seems mild. Do not wait for it to "go away". Tell the dive supervisor immediately.

What to do

First-aid actions are learned in training (RIFAP in France) and applied following the dive supervisor's instructions:

1. Stop all diving for the casualty; never put them back in the water to "recompress".
2. Call for help: at sea in France, the CROSS (maritime rescue centre) on VHF channel 16 or 196; on land, 112. Emergency services direct the casualty to a hyperbaric treatment centre.
3. Give pure oxygen, by a trained person, continuously until help arrives.
4. Position, cover and monitor the casualty and note the time the signs appeared.
5. Pass on the dive details (depth, time, stops, computer) to the emergency services, and keep an eye on the other divers of the team.

Only a doctor can assess the situation and decide on any return to diving.

Good practice

• Dive deepest first: early in the day and early in the dive. Avoid reverse profiles (a second dive deeper than the first).
• Keep a margin from the no-decompression limit and do the safety stop.
• Respect a sufficient surface interval between two dives.
• Flying and altitude: DAN recommends at least 12 hours after a single no-decompression dive, 18 hours after multiple dives or multiple days of diving, and substantially longer after decompression dives. Also think about mountain passes on the way home.
• Avoid freediving and hard exertion after a dive.
• Stay hydrated and protect yourself from the cold.

Key takeaways

• Ascend slowly: it is the first way to prevent DCS.
• Do a safety stop of a few minutes at around 3 to 5 m at the end of every dive.
• Do not dive up to the limit of your tables or computer.
• Any unusual sign after a dive: call for help, oxygen, never back into the water.
• Respect the waiting times before flying and going to altitude.

Further reading

The app offers MN90 planning and, with Dive Plus, the Bühlmann ZHL-16C model with gradient factors (30/70 by default). During the dive, the watch shows the no-decompression time and stops, and alerts you if you ascend too fast; after mandatory stops, it keeps a no-fly recommendation. These calculations replace neither your training, nor a dedicated dive computer, nor your instructors' instructions.