What Is Decompression, and Why Do Divers Plan for It?

Somewhere in your first dive course, an instructor gave you the rules of coming up. Ascend slowly. Hold a safety stop at five metres. Stay inside your no-decompression limit. You memorised them, you followed them, and if your course was typical, nobody explained what they were protecting you from. That is no fault of your instructor: the rules work whether or not you know the reason behind them, so a good course teaches them and moves on. But the reason behind them is one of the better stories in diving, and knowing it changes how you read every number on your computer.

This article is that reason, told from the beginning. It is not a course, and it will not qualify you to plan a decompression dive. It is the intuition, a little history, and an honest map of where to learn more. The physics stays in plain English throughout.

What depth does to your body

Air is mostly nitrogen, about four fifths of every breath, and at the surface your body ignores it. You breathe it in, you breathe it out, and the small amount dissolved in your blood and tissues stays in balance with the air around you. Nothing changes until the pressure does.

Underwater, everything you breathe is compressed. Your regulator delivers gas at the same pressure as the surrounding water, because your lungs could not inflate against the water otherwise. At ten metres that is twice the pressure at the surface; at thirty metres, four times. Each lungful now carries several times more nitrogen than the same breath on land, and your body, which was in balance at the surface, starts absorbing the excess. It has no use for it. The nitrogen dissolves into your blood and from there into your tissues, the way gas is held dissolved in a capped bottle of soft drink, and it keeps accumulating for as long as you stay down. The deeper you go and the longer you stay, the more you carry.

None of this hurts, and that is the heart of the problem. You cannot feel dissolved gas building up, so nothing warns you it is there. The trouble only starts on the way up.

As you ascend, the pressure drops, and the gas you absorbed starts to leave. Given time, it goes out the way it came in: carried by your blood to your lungs and breathed away, a gradual unloading divers call off-gassing. Rushed, it does something worse. Like the fizz when a bottle cap comes off too fast, the gas forms bubbles right where it sits, in your joints, your spine, your skin, places your lungs never get a chance to clear. Too many bubbles, or bubbles in an unlucky place, and the result is decompression sickness: joint pain, rashes, numbness, and in serious cases paralysis or death. It can be treated in a recompression chamber when it is recognised in time, but the whole point of what follows is not to need one.^{[1 ]} Divers call it the bends, a name that predates scuba entirely; the history below explains where it came from.

One more fact underlies everything else in this story. Your body is not one substance. Blood takes on nitrogen in minutes. Muscle takes tens of minutes. Fat and cartilage take hours, and they give the gas back just as slowly as they absorbed it. So "how much nitrogen am I carrying?" has no single answer; it has many answers at once, and a short deep dive loads a different part of you than a long shallow one. Every table, every dive computer, every planning app you will ever use is, underneath, a bookkeeper, tracking each of those loading speeds at once.

Why this needs planning

Most hazards in diving announce themselves. You can see the current, feel the cold, watch your pressure gauge fall. Dissolved gas is the exception. No amount of experience lets you sense it, and a diver carrying a dangerous load feels exactly like a diver carrying none, right up until symptoms start, which is usually after the dive is over.^{[1 ]} Because your senses give you nothing to go on here, the job falls to arithmetic, worked out before you reach the limit rather than felt at it. Divers lean on that arithmetic at three levels.

The first level is the one you already live by. A no-decompression limit is a precomputed answer to the question: how long can I stay at this depth and still be allowed to swim straight to the surface? Inside the limit, a slow ascent gives your body the time it needs (agencies typically teach nine to eighteen metres per minute at most, and slower near the surface), and the familiar three minutes at five metres adds a buffer on top. This is recreational diving's whole strategy. Keep the gas load light enough that the surface always remains a legal move.

Past the limit, the surface stops being a legal move. Ascend directly anyway and you arrive carrying more gas than your body can release without bubbling. So instead you pause on the way up, at set depths for set minutes, letting your tissues drain enough that the next step up is tolerable. Those pauses are decompression stops, and a dive that requires them is a decompression dive. From the moment you cross the limit there is a ceiling over your head, a depth you must not rise past yet. It is just as invisible as the gas load; only the arithmetic knows where it is.

And that is where planning stops being optional. A decompression dive is a commitment: the stops cannot be wished away, and you have to reach each one with enough gas to breathe while you wait it out. Surfacing early is no longer a safe answer to a problem, and running low on gas while you still owe stops is one of the most dangerous situations a diver can be in. So before a dive like that, divers work out its whole shape in advance: how deep, how long, what the ascent looks like, where the stops fall, how much gas each phase burns, and what the plan becomes if the dive runs longer or deeper than intended. That, in one sentence, is what a decompression planner does.

A short history

Decompression sickness was met before it was understood, and not by divers. In the 1860s and 70s, bridge builders in America sank huge inverted boxes called caissons to the riverbed and pumped them full of compressed air to hold the water out while men dug inside. The pressure kept the river back; it also, though nobody knew it, loaded the workers' bodies with nitrogen exactly as a dive does. Men who felt fine at the bottom of the Eads Bridge caisson in St. Louis, or under the East River at the Brooklyn Bridge, came up to joint pain, paralysis, and in some cases death. Hundreds fell ill between the two projects, and the Brooklyn workers gave the condition its lasting name: sufferers walked hunched over like the fashionable "Grecian bend" posture of the day, so the men called it the bends.^{[2 ,3 ]} The bridge physicians learned to manage it mostly by observation, slower transitions and shorter shifts, without knowing what was happening inside their patients.

The understanding arrived in 1908, from a Scottish physiologist named John Scott Haldane, working for the Royal Navy. By putting goats through pressure chambers, he worked out the two ideas underneath every decompression method since. First, the body tolerates a certain amount of excess dissolved gas before bubbles form, so you do not have to creep to the surface; you can ascend in steps and pause, which his team turned into the first staged decompression tables. Second, and further ahead of its time, he modelled the body as several imaginary tissues absorbing and releasing gas at different speeds, from blood in minutes to fat in hours.^{[4 ]} The tissues were never meant to be anatomy. They were a way of doing the sums, and versions of them sit inside your dive computer today.

The next half century belonged to the navies, who had the divers, the chambers, and the reasons to keep refining the tables. The most important refinement came in 1965 from Robert Workman at the US Navy Experimental Diving Unit, who gave each of Haldane's tissues its own explicit limit, the M-value: the most dissolved gas that tissue can hold at a given depth before bubbles become likely.^{[5 ]} Then Albert Bühlmann, running a hyperbaric laboratory in Zürich, spent the 1960s through the 80s extending the scheme, eventually to sixteen tissue speeds and, unusually for the field, to dives at altitude, where the thinner air changes the maths. Just as unusually, he published his work whole, every constant, every equation, in books anyone could buy: a twelve-compartment model in his 1984 book, then the sixteen-compartment ZH-L16 in 1986, documented in his 1990 book Tauchmedizin.^{[6 ]} That openness, rare in a field of proprietary navy tables, is why his name is on the default algorithm of most dive computers sold today. Manufacturers needed a documented, defensible model, and Bühlmann had put one on the shelf. The variant he tuned for dive computers is called ZH-L16C.

Hardware caught up in parallel. The first commercially viable dive computer, the Orca EDGE of 1983, carried a twelve-tissue model into the water and ran it in real time as the diver swam.^{[7 ]} That changed who does the arithmetic, and within a generation the laminated table went from essential equipment to backup.

The last piece arrived in 1998, when an engineer and cave diver named Erik Baker published a pair of articles that gave divers a dial for how close to Bühlmann's limits they are willing to go. He called it the gradient factor, and those are the two numbers, something like 40/85, in the settings of most technical-capable dive computers today.^{[8 ]} What divers then did with that dial, including a decade in which a rival school of "bubble models" (VPM and RGBM) persuaded much of technical diving to push its stops deeper and deeper, until a 2011 US Navy experiment cut against the practice,^{[9 ]} is a longer story, and we follow it in full in Unpacking Gradient Factors. For this article, the short version is enough: the model gives you a limit, the gradient factors decide your margin from it, and choosing that margin deliberately is part of modern diving.

The tools divers plan with

Everything above ends up in a diver's hands as one of three tools, and it is worth being clear about what each one is for, because they answer different questions.

Tables are the original artifact: the model run once, in advance, and printed. A table treats every dive as a simple rectangle, as if you spent the whole time at your deepest depth, which is pessimistic but safe, and in exchange it needs no batteries and no trust in electronics. Some courses still teach them, partly for redundancy and partly because working through a table teaches you what the numbers mean.

A dive computer is the same kind of model running live on your wrist, fed by a pressure sensor. It tracks the dive you are actually making rather than a pessimistic rectangle, and it continuously answers the question "what do I owe right now?": the no-deco time remaining, or once you cross the line, the ceiling and the stops. The countdown on your wrist looks like a reading, but nothing in your body is being sensed; it is the model's estimate, recalculated as you swim.

A planner answers the question the other two cannot: what would this dive cost me before I make it? Give it a depth, a time, and your gases, and it produces the whole shape of the dive: the stops, the total runtime, the gas each phase consumes, and how all of that shifts if you stay longer or go deeper than intended. For a recreational diver a planner is a way to see the machinery, to watch how the no-deco limit moves with depth, or with nitrox, air with extra oxygen. For a technical diver it is not optional, because the commitment we met earlier, required stops with the gas to breathe through them, has to be worked out before anyone gets wet. Planners began as desktop programs, run on a laptop the night before a dive, and have since moved to the phone in your pocket.

Where Dive Kit fits in

Dive Kit is our version of that third tool. The free part of the app covers the planning arithmetic every diver meets before decompression does: nitrox limits, gas consumption, cylinder capacities, buoyancy. The paid part is the Deco Planner, which runs the same Bühlmann ZH-L16C described above with gradient factors under your control, for ordinary open-circuit scuba and for rebreathers, and we publish cross-checks of its schedules against MultiDeco, the desktop planner much of the technical community grew up on, so you can verify the arithmetic rather than take our word for it. The app is on iOS and Android; the planner is a free trial, then $4.99 a month or $24.99 once, and there is a fuller comparison with the established planners if you are shopping. None of that replaces training. A planner tells you what a dive demands; a course teaches you whether you are ready to take it on.

Where to learn more

Decompression theory has an unusually good trail of public teaching material, much of it free, and the diving community is consistent about which of it is worth your time. What follows is the short list, with a note on what each item is and when to reach for it. Start anywhere; they reinforce each other.

Books

• Deco for Divers by Mark Powell (AquaPress, 2nd edition) is the community's standard recommendation: a whole book pitched exactly between "trust the computer" and the research literature, covering dissolved-gas models, bubble models, M-values, and gradient factors without assuming any maths. One caution: decompression thinking shifted after the deep-stops research of the 2010s, and Powell himself has moved with the evidence, so read the deep-stops chapter as history rather than advice.
• Deeper into Diving by John Lippmann and Simon Mitchell is the natural second book: broader diving physiology and medicine, more depth, still readable.

Lectures on YouTube

• Deco theory with Prof. Simon Mitchell (three parts, UTD) is the gentlest starting point on video: a hyperbaric physician, diver, and one of the most cited researchers in the field, teaching the fundamentals from zero.
• Decompression Controversies (Simon Mitchell, DAN Southern Africa) is the community's most recommended single talk: dissolved-gas versus bubble models, and the experiment that settled the deep-stops question. Watch it after the basics have landed; it rewards a little preparation.
• Intro to Deco Theory & Deep Stops (Mark Powell) covers the same ground as his book in an evening, in his usual plain style.

Articles

• Decompression Illness (Divers Alert Network) is the plain-language reference on what DCS actually is, its symptoms, and its treatment. Worth reading once even if you never plan a decompression dive.
• Flexible Control of Decompression Stress (Neal Pollock, on the Shearwater blog) is a clean first explanation of gradient factors from a leading researcher.
• Gradient Factors for Dummies (Kevin Watts, hosted by Andy Davis) walks through the same idea even more slowly, with worked examples.
• Understanding M-values and Clearing Up The Confusion About Deep Stops are Erik Baker's original 1998 articles, still the primary source for gradient factors, and shorter than their reputation suggests.
• Gradient Factors in a Post-Deep Stops World (David Doolette, InDepth) shows how a leading decompression physiologist sets his own computer, and why, after the deep-stops reappraisal.

Communities and the deep end

• ScubaBoard has hosted twenty years of exactly the questions you will have, with researchers occasionally answering in person; its technical reading list thread is a good rabbit hole. Reddit's r/scuba covers similar ground more casually.
• The Theoretical Diver is where to go when the plain-English versions stop satisfying you: a physicist's blog that re-derives the models from first principles, maths included.
• And when you want the full story of the two numbers on your computer, our own Unpacking Gradient Factors follows the evidence from Haldane's goats to the current research, with every claim cited.