Safe Ascents

A sponge and a soda bottle

This is about the most overlooked skill in diving: how to come up. The ascent is the part that can hurt you, because that's when dissolved gas can leave solution too fast and form bubbles, which is decompression sickness. Picture your tissues as a sponge sitting in pressurised gas. On the way down you soak it up; on the way up you wring it out. The wringing only happens when there's a difference to drive it: your tissue has to hold more gas than the water around you is pressing back in. Decompression people call that difference a gradient, and without it nothing leaves.

So you'd think the answer is simple. Drop the pressure as fast as you can, make the gradient huge, wring the sponge dry. That's where the second image comes in.

Open a bottle of soda slowly and the gas eases out clear. It stays in solution and leaves through the surface. Crack the cap off fast and the whole thing foams over the moment the pressure drops. The gas was the same; only the speed changed.

Your ascent is the cap. You need a gradient to off-gas at all (to drive the dissolved gas back out), but push it too far, too fast, and the dissolved gas stops leaving quietly and starts forming bubbles inside you. A good ascent balances those two: enough gradient to clear gas, never so much that it foams. (The soda image is a metaphor. Your tissues don't literally froth, but the same physics holds: gas kept in solution by pressure comes out as bubbles when the pressure drops too fast.)

Why the shallow part is the dangerous part

The risk isn't spread evenly up the water column. It piles up near the surface.

What matters to the foaming isn't how many metres you move. It's how much the absolute pressure changes in proportion to where you already are. And that proportion gets brutal as you get shallow.

Go from 40 m to 30 m and you drop from 5 bar to 4 bar absolute, about a 20% change. Go from 10 m to the surface and you drop from 2 bar to 1 bar. You've halved the pressure your tissues are pressing against. Same ten metres, wildly different shake of the bottle.

This is the same physics the M-values describe for each tissue, and the same reason gradient factors let you dial back how close to that foaming edge you're willing to ride.

The ascent-rate numbers, and what they really are

You'll see a rate quoted as if it were a law of nature. It isn't. It's a convention, a sensible round number that agencies agreed on, and the conventions have moved over time.

The standard you'll meet most often is 9–10 m/min (30 ft/min), used by the US Navy and NOAA. Recreational agencies historically allowed anywhere from 30 to 60 ft/min, a compromise between safety and not boring divers to tears rather than a measured cliff edge. The US Navy itself ran 60 ft/min for years before settling on the slower 30 ft/min. The number changed because the thinking did, not because the water did.

There's a second reason the rate matters, and it catches people out. On a staged dive, those numbers aren't loose guidelines. They're the rates the algorithm assumed when it built your stop schedule. Come up faster than planned and your tissues hit lower pressure than the model budgeted for, so they end up more supersaturated (holding more dissolved-gas pressure than the water around them is pressing back) than the schedule's safety ceiling allowed for. Going slower is fine for safety, but it costs gas, so plan for it if you mean to do it.

The part the research is clearest about is the end of the ascent. Faster ascents show up as higher bubble grades on Doppler (an ultrasound probe that listens for gas bubbles riding the veins back toward the lungs), meaning more gas came out of solution. So the move that actually helps is to slow the last leg right down.

Climb the ladder: 9, 6, 3

All of this turns into a simple procedure. Instead of one stop, climb the shallow zone like a ladder, pausing on fixed rungs at 9 m, 6 m, and 3 m. You spend a set time on each rung and crawl between them at that slow 3 m/min, then step up again.

For an ordinary recreational dive, a good pattern is about a minute at 9 m, two minutes at 6 m, and three minutes at 3 m. Call it six minutes total. It barely touches your gas, and each rung does a different job. The 9 m hold keeps your slow compartments under a deeper ceiling. The 6 m lets the medium tissues catch up. The 3 m is the classic stop where the fast compartments shed their last load before the surface.

Does it actually help, or does it just feel thorough? Doppler studies, which count the silent bubbles drifting back through the veins after a dive, generally find that a controlled, graduated shallow ascent leaves a lower bubble load than bolting straight up. A direct ascent tends to leave the most; easing up the shallow zone in stages leaves less.

One honest caveat. That fixed 9/6/3 ladder is the recreational pattern. On a decompression dive your stops and times come from the computer's gradient-factor schedule, not a memorised rule. The principle carries over exactly: more graduated stops mean a lower bubble load, and the algorithm just does the arithmetic for you.

Flying the stop on a tech computer

On a recreational dive you hold the stop by feel and a timer. On a technical computer you don't have to guess, because two live numbers tell you exactly how the dive is tracking against the plan. Both fall out of the gradient factors you set before you splashed.

If your computer shows gradient-factor numbers, two of them tell you exactly how the dive is tracking. GF99 is how supersaturated your leading (most-loaded) tissue is right now, written as a percentage of its limit. Surface GF is what that number would become if you surfaced this instant. As you hang and off-gas, both fall. When Surface GF has dropped to your planned GF Hi (the surfacing limit you chose before the dive), the model says you've earned the surface.

This is also where you buy cheap insurance on a dive that was harder than the model assumed. After a cold or strenuous dive your real off-gassing runs slower than the algorithm pictured, so extend a stop until Surface GF drops another 10% or so below your plan. It costs a few minutes of gas you can spare and buys a meaningfully gentler surfacing gradient.

One display tip for that final leg. Switch your ascent-rate indicator from the cartoon bar to a numeric m/min readout. The last climb wants to sit near 3 m/min, a metre every twenty seconds, and a bar graph simply can't show you that precisely.

Three things people get wrong

"Slower is always safer." It isn't. A too-slow ascent keeps you deep longer, and your slow tissues are still on-gassing while the fast ones are trying to clear. Dawdle at 15 m and you're loading bone and cartilage even as you off-load blood and brain. There's a sweet spot, not a one-way "slower wins" dial.

"The deep safety stop is the important one." It feels important, because it's deep and it feels disciplined. But the deep stop isn't where most of your gas leaves. The shallow stop does the heavy lifting, because that's where the surfacing gradient gets set. The gas you fail to clear at 5 or 6 m is the gas that follows you to the surface.

"A 5 m stop always helps." On a shallow, easy dive it can do almost nothing. Your tissues may be barely supersaturated there, with a gradient-factor reading in single digits, so there's little gradient to off-gas against. On some profiles a slightly deeper hang at 6 m clears gas better, because it sits you where the gradient is still working for you instead of where it's already spent. Stop depth is a tool, not a ritual.

What's settled and what's still argued

Settled enough to dive by:

• Slow the final ascent. That's where the bottle foams.
• Prioritise the shallow stop over the deep one for off-gassing.
• The rate numbers are conventions, not physiology. Treat them as guardrails.

Still genuinely contested:

• The exact optimal ascent rate. The research narrows it but doesn't pin it.
• Whether a brief deep stop helps at all on no-stop dives, or just delays the off-gassing that matters.

That second debate is really a question about bubbles versus dissolved gas, which is its own story in bubble trouble.

Your next ascent

1. From the bottom to your first stop, come up at about 9 to 10 m/min.
2. In the shallow zone, climb like a ladder: on an ordinary recreational dive, roughly a minute at 9 m, two at 6 m, three at 3 m.
3. Crawl the shallow legs at about 3 m/min, a metre every twenty seconds.
4. On a technical computer, surface only once Surface GF has fallen to your GF Hi, and never let GF99 climb.

This note explains the why and the habits behind a good ascent; it is not a substitute for training. No-stop and decompression dives follow different rules, and a real decompression schedule comes from your planner and your course, not a memorised ladder.

References

1. Divers Alert Network. Ascent Rates. Alert Diver, 2012.
2. Doolette DJ, Gerth WA, Gault KA. Redistribution of Decompression Stop Time from Shallow to Deep Stops. NEDU TR 11-06, US Navy Experimental Diving Unit, 2011.
3. Marroni A, Bennett PB, Cronje FJ, et al. A deep stop during decompression from 82 fsw (25 m) significantly reduces bubbles and fast tissue gas tensions. Undersea Hyperb Med. 2004;31(2). (DAN Europe DSL programme; on deep-stop bubble reduction, the effect later shown not to lower DCS.)

I drill ascent discipline, and the shallow stop especially, on every decompression course I run, from a diver's first staged ascent to deep trimix. Ask me about training.