Technical Diving

How Decompression Planning Actually Works

Tissue compartments, M-values, gradient factors, deco gas ladders and the gas density line: the full picture of modern deco theory, its limits, and how to explore it hands-on.

By Mat Mora · Updated 11 July 2026 · ~10 min read

Every dive computer, every planning app and every table you have ever used descends from a single idea, over a century old: your body takes up inert gas under pressure, and if you release that pressure faster than the gas can leave, it comes out of solution inside you. Decompression sickness is the failure mode. Decompression planning is the engineering discipline of never getting there. Here is how it actually works, from the physiology to the software.

Tissue compartments: the central trick

Your body is unmanageably complex, so in 1908 the physiologist John Scott Haldane proposed a beautiful simplification: model it as a handful of hypothetical compartments, each absorbing and releasing nitrogen exponentially at its own speed. Blood-rich tissues like brain 'fast', fat and joints 'slow'. A century of refinement later, Albert Bühlmann's ZH-L16 model from Zurich uses sixteen compartments with half-times from 4 to 635 minutes, and it remains the open, documented backbone of most modern computers and planning software.

For each compartment, the model tracks inert gas pressure through the dive, and defines a maximum supersaturation it may carry at any ambient pressure: the M-value line. Ascend until your fastest-loaded compartment approaches its M-value, and you have found your ceiling: the shallowest depth you may occupy right now. Deco stops are simply the model holding you below the ceiling while the gas drains, three metres at a time.

Gradient factors: the conservatism dial

Raw M-values proved too aggressive for comfort, so in the 1990s engineer Erik Baker introduced gradient factors: two numbers, like 30/70, that scale how close to the M-value line you are allowed to get. The first number (GF low) governs the deepest stop, the second (GF high) governs the surfacing margin, and the allowance slides between them as you ascend. GF 30/70 keeps you at most 30% of the way to the limit deep and 70% shallow. Lower numbers, softer schedule, longer hang. Every serious deco tool exposes gradient factors, and understanding them is the difference between using a plan and trusting a black box.

The deco gas ladder

Off-gassing runs on pressure difference: the more inert gas you can remove from what you breathe, the steeper the gradient pushing nitrogen and helium out. That is the whole logic of deco gases:

Each switch has a hard ceiling set by oxygen toxicity: deco gases are planned at a ppO₂ of 1.6 bar, and breathing one meaningfully deeper risks a convulsion, which underwater is usually fatal. This is why gas switches are drilled as a verified, two-person procedure, and why the accident record treats wrong-gas events with such gravity. On truly deep dives the ladder matters as much as the model: our own planner shows a 233 m schedule collapsing from an impossible 24+ hours to under 7 simply by staging the intermediate mixes real expeditions use.

The other limits: narcosis, density, the oxygen clock

A schedule that satisfies the tissue model can still be undivable. Three more gauges have to stay in the green:

• END (equivalent narcotic depth). Nitrogen at depth is an anaesthetic. Modern planning holds the narcotic load to the equivalent of air at 30 m, using helium to replace nitrogen. See how deep you can scuba dive for why air past 40 m is a bad idea long before deco enters the picture.
• Gas density. The quiet killer of the deep-air era. Dense gas raises the work of breathing until CO₂ production outruns your ability to exhale it. Research by Gavin Anthony and Simon Mitchell established the modern line: 5.2 g/L ideal, 6.2 g/L maximum. Density, more than anything else, is why the 332 m record may stand forever.
• CNS oxygen toxicity. Oxygen is the cure for deco and a poison on a clock. Planning tracks cumulative exposure against NOAA limits; long schedules on rich gas can spend the whole budget and then some.

What the models cannot promise

Honesty about limitations is part of the discipline. Bühlmann compartments are not organs; they are curve fits to survivable dives. The models are validated where the data lives, on dives shallower than about 100 m, and everything beyond is extrapolation. Individual physiology, hydration, temperature, workload and a patent foramen ovale all move your personal risk in ways no algorithm sees. Two divers can run the same profile and only one gets bent, and the model was 'right' both times, because it deals in probability, not guarantees. A gradient factor is your margin against exactly this uncertainty. Spend it knowingly.

Try all of it, hands-on and free

Everything in this article is explorable in the deco planner inside the Diving Standard app. It runs a documented Bühlmann ZH-L16C implementation with helium compartments, full gradient factor control and the standard deco ladder, and it was built to teach rather than impress:

• Plan real profiles: depth, bottom time or a touch-and-go bounce profile, trimix bottom gases, EAN50 and oxygen, switch holds, gradient factor presets.
• See every limit live: ppO₂, gas density and END for your exact mix at your exact depth, coloured against the same thresholds this article quotes, plus CNS and OTU for the whole schedule.
• Reference protocols: textbook best-mix suggestions for any depth, with the honest caveat that they are published conventions, not advice.
• Perspective mode to 300 m: push the slider past sanity and the planner keeps computing, showing you the exploding runtimes, the 40-cylinder gas bills and exactly which physiological line you crossed, because watching the schedule blow up is the fastest deco lesson there is.

The planner is a study tool, gated behind an explicit acknowledgement, and it says what most tools will not: finishing the calculation does not make a dive divable. It will happily show you a schedule and then explain why the human breathing it would not survive. That honesty is the point.