Tec Diving: Decompression Management

The Decompression Problem

When a diver breathes compressed gas at depth, inert gas (nitrogen in air and nitrox; helium in trimix) dissolves into blood and tissues according to Henry's Law — the amount dissolved is proportional to the partial pressure. At depth, that partial pressure is elevated, so more gas dissolves.

On ascent, as pressure decreases, the dissolved gas must come back out of solution. If the ascent is slow enough, the gas diffuses out through tissues and bloodstream to the lungs and is exhaled. If the ascent is too fast, or if the total dissolved gas loading is too high, the gas comes out of solution faster than the body can eliminate it — forming bubbles in blood and tissue. This is decompression sickness (DCS), ranging from joint pain to paralysis to death.

Compartment Models

Decompression science uses mathematical models of tissue compartments — theoretical 'tissues' with different rates of gas absorption and elimination — to predict how much gas is dissolved in different body tissues at different points in a dive.

The Buehlmann ZHL-16C model (developed by Swiss physiologist Albert Buehlmann, first published 1983) is the basis of most modern dive computer algorithms. It defines 16 tissue compartments with half-times from 4 minutes (fast tissues, like blood) to 635 minutes (slow tissues, like bone).

Decompression Stop Depths

Modern technical diving uses gradient factors — a modification of the Buehlmann model that applies a conservative factor to the M-values. A GF of 80/80 means stops are required when tissue loading reaches 80% of the theoretical maximum.

Typical tec diving decompression involves stop depths at:

• Deep stops (at 50-70% of planned maximum depth, if using a deep-stop model)
• 6 metres (20 ft): The primary deco stop for most tec dives; 50% O2 or 100% O2 accelerates off-gassing dramatically at this depth
• 3 metres (10 ft): Additional stop for conservatism

Gas Switching for Decompression Acceleration

At depth, a diver breathes bottom gas (typically trimix or nitrox) optimized for the working depth. During decompression ascent, switching to high-oxygen mixtures (typically 50% O2, then 100% O2 at appropriate depths) dramatically accelerates the rate at which nitrogen is eliminated. This reduces total decompression time significantly.

A dive that requires 90 minutes of decompression on air can be reduced to 30-45 minutes using 50%/100% deco gas switches.

Tools and Resources

V-Planner (free, desktop): Buehlmann ZHL-16C with gradient factors; widely used by recreational tec divers.

Shearwater Cloud: The dive computer app used by the Shearwater research line, which has become the de facto standard tec computer.

Ratio Deco: An empirical decompression planning method used by some technical divers as a quick-calculation alternative to full computer modeling.