Decompression Theory, Explained
At the professional level you're expected to understand not just the rules but the model behind them. Decompression theory is that model — and it's more approachable than it first looks.
The core idea: on-gassing and off-gassing
Under pressure, your body dissolves extra nitrogen into its tissues (Henry's Law). That's on-gassing, and it happens fastest at depth. When you ascend, ambient pressure falls and your tissues now hold more nitrogen than the surrounding pressure supports — they're supersaturated — so they release it. That's off-gassing. Decompression theory is the study of how to let that release happen safely, without forming problematic bubbles.
Tissue compartments and half-times
The body isn't one uniform sponge. Haldane's insight, still at the heart of modern models, is to treat the body as a set of theoretical 'compartments', each on-gassing and off-gassing at a different rate described by its half-time — the time to reach half-saturation at a given pressure. Fast compartments (well-perfused tissue like blood) load and unload quickly; slow compartments (fat, cartilage) take hours.
This is why dive planning cares about both bottom time and surface intervals: a short deep dive loads the fast compartments, while repetitive and multi-day diving keeps the slow compartments topped up long after the fast ones have cleared.
M-values: how much supersaturation is 'safe'
Each compartment can tolerate being supersaturated up to a limit before bubbles become a real risk. That ceiling — historically called the M-value, and refined in modern algorithms as an ambient-pressure limit or gradient factor — is what a no-decompression limit really protects. Your NDL for a depth is simply the longest time you can stay before any compartment would exceed its limit on a direct ascent.
Why slow ascents and stops matter
Ascending slowly keeps the pressure gradient gentle, so gas leaves solution and is carried away by the bloodstream rather than coming out as bubbles. A safety stop parks you at a shallow depth where the fast compartments off-gas rapidly while the risk of the slow ones bubbling stays low. It's the same reason a bottle of soda opened slowly doesn't foam over.
What the exam expects you to connect
Divemaster and instructor theory rewards linking these ideas together rather than memorising numbers:
- Depth drives the on-gassing gradient; time determines how far each compartment loads.
- Repetitive dives begin with residual nitrogen, mostly in the slower compartments.
- The RDP, eRDPml and dive computers are all just tools for keeping every compartment under its limit.
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Frequently asked questions
What is a tissue compartment in decompression theory?
A theoretical model of body tissue that absorbs and releases nitrogen at a particular rate, described by its half-time. Decompression models use a set of compartments — from fast to slow — to estimate your total nitrogen loading. They're a calculation tool, not literal body parts.
What is a half-time?
The time a tissue compartment takes to reach half of its saturation at a given pressure. Fast compartments have short half-times (minutes) and load and unload quickly; slow compartments have long half-times (hours) and dominate on repetitive and multi-day diving.
What is an M-value?
The maximum amount of dissolved inert-gas pressure a compartment can tolerate at a given ambient pressure before bubble formation becomes a significant risk. No-decompression limits exist to keep every compartment below its M-value on a direct ascent.
Why do you off-gas nitrogen when you ascend?
On ascent the surrounding pressure drops, so your tissues hold more dissolved nitrogen than the new pressure supports (supersaturation). The excess leaves solution and is carried off by your blood and breathed out — as long as you ascend slowly enough to keep it dissolved rather than bubbling.
Is decompression theory the same for tables and dive computers?
The underlying physiology is the same, but the tools apply it differently. Tables use a fixed model for square-profile dives, while dive computers track multiple compartments in real time and credit multilevel profiles, so they can allow more bottom time for the same safety margin.