Reading Your Dive Computer in Real Time

Part 3 of 3 · Part 1: How Dive Computers Work · Part 2: Living With Your Computer's Algorithm

From a ceiling to live numbers

A decompression ceiling answers one question: can I go shallower yet? It does not tell you how close to the edge you are, how much room you have left, or how bad it would be if you had to bolt for the surface.

Two live numbers do answer those, and on a modern technical computer they turn "hover and hope" into something you can actually read: GF99 and Surface GF. Both are standard on current Shearwater computers, and some other makers show the same idea under different names. They are the same gradient-factor framework from gradient factors, just displayed live.

GF99: how hard you're pushing right now

GF99 is your supersaturation this instant, for whichever tissue is leading, written as a percentage of that tissue's limit at your current depth.

On a stop you watch GF99 fall as the leading tissue off-gases. When it drops to the gradient factor for that stop depth, the ceiling clears and you move up. It makes the countdown concrete: instead of trusting an opaque timer, you are watching the actual tissue state.

A GF99 above 100 in mid-dive is not automatically an emergency, but it is a loud flag. It means the ascent was too fast, a stop was missed, or the dive went past the model's envelope. Stop where you are, breathe, and let GF99 come back down before you go any shallower.

Surface GF: how bad a bolt would be

Surface GF answers the other question: if you surfaced right now, skipping everything you still owe, what would your GF be up top?

That makes it the number that actually maps to your risk, because surfacing is the moment of maximum gradient. On a normal dive it counts down as you decompress, and at the last stop you simply hold until Surface GF falls to your GF Hi setting. That is the model telling you the surface is earned.

It is also your honest gauge in a pinch. If Surface GF is well under your GF Hi, you have margin in hand. If gas is short and you must trim stops, Surface GF tells you exactly how close to the line you are choosing to run. And if Surface GF is sitting above GF Hi and not budging, you are not finished, no matter what the ceiling counter says.

Ascent rate: what the numbers mean

The research here is blunt. Roughly double your ascent rate (9 m/min up to 18) and you roughly double to triple your odds of getting bent. The relationship isn't linear, because faster ascents drop the surrounding pressure faster than your tissues can keep up, and supersaturation spikes.

The last 6 m is the most dangerous stretch of any dive, recreational or technical, because that final climb to the surface is the biggest proportional pressure drop of the whole dive, hitting every loaded tissue at once. Rushing it for a boat pickup is where avoidable bends happen. And there is no upside to beating the rate: come up too fast from depth and the computer just recomputes a deeper, longer obligation from where you end up. Why this is so is the whole of safe ascents.

The staggered safety stop

Doppler studies found that swapping the old single 5 m safety stop for a staggered climb cut post-dive bubble grades by about half, for the same total time. The pattern is the familiar 9 m, 6 m, 3 m ladder, and the principle, that the shallow zone clears best in steps with a slow final metre or two, applies to the top of a technical ascent just as much. Safe ascents lays out the 9/6/3 pattern and the bubble data behind it.

Why you feel wrecked even when you felt fine

Not every bit of decompression stress shows up as symptoms. You can finish a dive your computer cleared and still be circulating silent microbubbles, with raised markers of blood-vessel stress, and no joint pain or neurological sign at all. Feeling fine does not mean your tissues were untouched.

That silent load tracks closely with post-dive fatigue. The wrung-out feeling after a heavy diving day is not just physical effort, and several big days in a row stack up blood-vessel stress even if nobody gets a diagnosable bend. The most effective thing you can do about it is cheap and dull: long surface intervals (three hours clears most of the fast and medium tissues) plus good hydration. That beats any change to your GF settings.

What your computer can't see

Your computer models an ideal, average diver. It has no way to know:

You're dehydrated. Thicker blood moves nitrogen out of tissues more slowly, and dehydrated divers show worse bubble grades on the same profile. Drink before you dive.

You're cold. Cold shuts down peripheral blood flow and slows off-gassing on ascent. A schedule that clears comfortably in warm water can be marginal at 10°C, which is why cold-water tec divers pad their stops past the computer's minimum.

You have a PFO. Around a quarter to a third of adults have a small hole between the heart's upper chambers. It lets venous bubbles skip the lung filter and reach the arterial side, raising DCS risk well above what the model assumes. Testing is available and increasingly common among tec divers.

You worked hard down there. Finning into a current at 40 m loads nitrogen faster, and drives CO₂ that further raises risk. The computer saw the same depth and time as the diver who hovered.

Your age and fitness. Older and less fit divers move nitrogen less efficiently, and no consumer computer adjusts for it.

The takeaway from the whole list is the same as the rest of this series: the model is a careful average, and you are the variable it cannot read. When the dive was cold, hard, or stacked on four others, give it more margin than the minimum.

The data your computer keeps

Current Shearwater computers download the full picture afterwards: nitrogen and helium loading for all 16 compartments at every sample of the dive. You can reconstruct which tissue was controlling your ceiling at each depth, how close GF99 came to your settings, and whether any phase loaded you in a way you didn't expect. For instructors and anyone logging complex profiles, that is genuinely useful, both for checking a plan worked and for spotting patterns across many dives.

References

• Nishi RY, Brubakk AO, Eftedal OS. Bubble detection. In: Brubakk AO, Neuman TS (eds). Bennett and Elliott's Physiology and Medicine of Diving. Saunders, 2003.
• Daubresse L, Vallée N, Druelle A, Castagna O, Guieu R, Blatteau J-E. Effects of CO₂ on the occurrence of decompression sickness: a review. Diving and Hyperbaric Medicine. 2024;54(2):110–119.
• Wienke BR, O'Leary TR. Understanding Modern Dive Computers and Operation. Springer, 2018.
• Bakovic D, et al. Acute effects of scuba diving on arterial stiffness and endothelial function in healthy divers. Diving and Hyperbaric Medicine. 2020;50(3):235–243.

Train with me

Reading your computer in the water, GF99, Surface GF, and stop management, is part of every technical and CCR course I run. Enquire about training →