An introduction to technical diving

In short

Technical diving covers dives that go beyond the limits of recreational diving: depths beyond 40 m, planned decompression stops, gases other than air or Nitrox, overhead environments, rebreathers. It relies on suitable gases, redundant configurations and rigorous planning. This guide is an introduction, not a method: it is in no way a substitute for dedicated training given by qualified instructors.

Why it matters

Many experienced divers are drawn to technical diving by deep wrecks, caves or longer dives. Understanding what it involves helps you measure the commitment it requires, and avoid trying it without preparation: in these dives, a direct ascent to the surface is often no longer possible, and a gas or planning mistake can be serious.

What technical diving is

It is often defined by one or more of these criteria:

• depth beyond recreational limits;
• planned mandatory decompression stops, sometimes long;
• use of several gases during the same dive;
• overhead environments (wreck, cave) where you cannot ascend vertically;
• use of a rebreather.

It requires progressive training, solid experience, specific equipment and disciplined planning and teamwork.

Gases

• Decompression Nitrox: an oxygen-rich mix (up to pure oxygen at shallow depth) breathed during stops to speed up nitrogen elimination.
• Trimix: a mix of oxygen, nitrogen and helium. Helium reduces narcosis and gas density at depth.
• Heliox: a mix of oxygen and helium, with no nitrogen, used mainly in commercial diving.

Benefits and risks

• Oxygen toxicity: each gas has a maximum operating depth; breathing an oxygen-rich gas too deep exposes you to serious oxygen toxicity. The usual PPO2 limit at the bottom is 1.4 bar, and 1.6 bar is a maximum, generally kept for decompression stops.
• Hypoxia: conversely, a very oxygen-poor bottom mix does not contain enough oxygen to be breathed at the surface or at shallow depth.
• Gas mistakes: breathing the wrong cylinder at the wrong depth is one of the major risks. Analysis, labelling and gas-switch procedures are essential.

Configurations

• Back-mounted twin set: two cylinders connected, often by a manifold with an isolation valve, worn on the back; two independent regulators.
• Sidemount: cylinders are carried along the sides of the body, which makes it easier to pass through tight spaces and to reach the valves.
• Stage and decompression cylinders: carried on the side, they hold decompression gases or extra gas for the way in and out.
• Redundancy: two gas sources, two regulators, two decompression instruments, several lights and cutters. Any critical item must be able to fail without endangering the dive.

Rebreathers

Principle

A rebreather reuses exhaled gas: carbon dioxide is absorbed by a scrubber and the oxygen used is replaced. There are semi-closed rebreathers (SCR), which release part of the gas, and closed-circuit rebreathers (CCR), which keep a set oxygen partial pressure using sensors and electronics.

Benefits and constraints

• Benefits: very low gas consumption, few or no bubbles, warm and moist gas, a mix optimised at every depth.
• Constraints: long preparation, rigorous maintenance, cost, strict checks before every dive.
• Specific risks: hypoxia, hyperoxia or carbon dioxide build-up in case of failure, sometimes without clear warning signs; this is why training on each rebreather model is mandatory.

Decompression with gas switches

In technical diving, decompression is planned before the dive: stops, gases used at each stage, switch depths, required quantities with a margin. Divers use planning software or multi-gas computers, with a backup plan in case a gas is lost. The decompression guide recalls the basic principles.

Training

National federations and international agencies offer courses in mixed-gas diving, overhead diving and rebreathers, with experience prerequisites. In France, mixed-gas diving is regulated by the Sports Code. Choose a recognised course and an instructor experienced in the discipline, and progress step by step. The useful links page lists the reference organisations.

Key takeaways

• This guide is an introduction: technical diving requires dedicated training.
• Trimix and heliox reduce narcosis; decompression Nitrox speeds up decompression.
• Each gas has its limits: oxygen toxicity too deep, hypoxia too close to the surface.
• Redundancy for everything critical.
• Rebreathers require training on each model and strict checks.

Further reading

The app's watch supports neither helium, gas switches nor rebreathers: it is not designed for technical diving. The guides on deep diving, Nitrox, decompression and specialty dives complement this one. Illustrations of the configurations will be added from the guide image library.