Why Dive Computers Disagree

Part 4 of the Dive Computers series · Part 1: How Dive Computers Work · Part 2: Living With Your Computer's Algorithm · Part 3: Reading Your Computer in Real Time

You and your buddy roll off the boat together, drop to the same depth, swim the same reef, and come up the same line. Back on deck you compare wrists — and the numbers don't match. Your computer wants twelve more minutes of no-fly than hers. On the second dive of the day, hers called deco while yours was still happily counting down no-stop time.

Nobody made a mistake. Nothing is broken. You were simply wearing two different opinions about your own body.

That sounds like a curiosity. It isn't. It quietly decides how long your team can stay down, when you can all fly home, and — if you ever shop for a computer — what you're actually buying. This piece explains why the disagreement happens, then hands you a simulator so you can watch three real computers argue their way through two dives.

The same dive, three answers

Meet the team. Same certification, same gas, same plan — the only thing different is what's strapped to each wrist.

• Mara wears a Shearwater Peregrine: Bühlmann ZHL-16C with gradient factors, set to Medium. A dissolved-gas model that cares about depth and time, and shows you the raw maths (GF99, Surface GF).
• Tom wears a Scubapro Aladin: the same Bühlmann base, but tuned with microbubble (MB) levels and a profile-dependent intermediate stop instead of gradient factors.
• Priya wears a Suunto Zoop Novo: Suunto RGBM, a bubble model — a little more cautious on any single dive, and far more cautious once a second dive enters the picture.

Drag the slider below through both dives and watch all three screens at the same instant. Switch between a Recreational day (everyone stays inside no-stop limits) and a Decompression day (they push into mandatory stops).

Why they disagree

Three different families of decompression model are doing the thinking, and they weigh the same dive differently. (The full mechanics live in Part 2 of this series; the short version follows.)

Bühlmann with gradient factors — the Shearwater, and the Aladin's underlying model — tracks the inert gas dissolved in sixteen tissue compartments and keeps each one under a pressure limit you've scaled down with your GF setting. It's transparent and tunable. What it does not do is model bubbles directly.

RGBM — the Suunto — does. It assumes tiny gas nuclei already exist in your tissues, models them growing and persisting, and tightens your schedule to keep them in check. Crucially, those nuclei don't vanish the moment you surface. They ride into your next dive.

That single difference is the whole story of the second dive. A Bühlmann computer starts the repetitive dive carrying only your leftover dissolved gas, so it's nearly as generous as it was the first time. The RGBM computer carries bubble seeds forward too, so it begins the same dive at the same depth with a fraction of the no-stop time. Run the simulator's Recreational mode and watch it happen: on dive one the three are close; on dive two the Zoop's no-stop clock collapses while the others barely notice.

And notice what is not causing the spread: none of these recreational computers reacts to how cold you are or how hard you're breathing. Heart-rate-and-temperature adaptation exists, but it's a premium feature (Scubapro's Galileo and Aladin A2), not something a Peregrine, a Zoop, or an Aladin A1 does. The differences you see are baked into the algorithms before you ever get in the water.

Why this actually matters

This is the part that turns a trivia answer into a reason to care.

1. Team diving runs on the most conservative computer. If you and your buddy aren't on the same schedule, you aren't really diving together. The rule is simple — the team turns when the first computer says turn, and surfaces when the last one clears — but the useful skill is knowing in advance which computer that will be. Pair an RGBM diver with a relaxed-GF diver on an aggressive repetitive plan and the RGBM computer will run the entire day. Better to know that on the boat than at the six-metre stop, watching your buddy hang while your own screen says you're done.

2. Buying a computer is buying a conservatism philosophy. "Which dive computer should I get?" is really "how do I want my bottom time traded against my margin, and how much do I want to see?" A Shearwater shows you the gradient-factor maths and lets you tune it. A Suunto hides the model and makes the call for you, leaning cautious — especially across a liveaboard week. Neither is wrong. But if you buy a famously conservative computer and then dive it next to friends on Bühlmann, expect to be the one calling everyone up.

3. Your screen is only useful if you can read it — and the words differ. The same idea wears different labels on different wrists, and a few look alike but mean different things. That's its own subject; the essentials are in Part 3: Reading Your Computer in Real Time. Three traps worth carrying with you:

• GF99 and Surface GF are a Shearwater thing. They're live gradient-factor readouts — don't go hunting for them on a Zoop or an Aladin A1.
• Suunto's "L0/L1" are not tissue compartments. That left-hand bar is a single consolidated loading indicator, and the Zoop isn't even running Bühlmann. The per-tissue graph people picture lives on the higher-end Shearwaters.
• Scubapro's "L0–L9" are microbubble conservatism levels, not compartments and not gradient factors — a dial you set before the dive.

So which one is right?

None of them, and all of them. They're three defensible answers to a genuinely hard, unsolved problem: nobody can measure your actual bubble load, so every computer is running a model — an educated guess dressed up as a number. Both Bühlmann and RGBM have large, comparable real-world safety records. The conservatism that separates them is a dial, not a verdict.

What changes once you've seen the spread is how you hold the number. You stop treating the figure on your wrist as the truth about your body and start treating it as one instrument's opinion — to be read, compared with your buddy's before you splash, and backed up with the obvious things no algorithm can see: ascend slowly, do your stops, add margin after a cold or hard or repetitive day, and don't let the most generous computer on the team set the plan.

Want to pressure-test a specific profile with the actual maths? The tools page has the calculators — MOD and END, SAC rate, and the decompression-stress estimator — to put real numbers behind the intuition this simulator builds.

References

• Bühlmann AA. Decompression–Decompression Sickness. Springer-Verlag, 1984. (The ZH-L16 model; the ZHL-16C "C" coefficient set is the modern technical standard.)
• Baker EC. Understanding M-values and Clearing Up the Confusion About "Deep Stops". 1998. (The gradient-factor framework used by Shearwater and other Bühlmann computers.)
• Wienke BR, O'Leary TR. Understanding Modern Dive Computers and Operation. Springer, 2018. (RGBM, bubble-model conservatism, and the LANL comparison showing Bühlmann and RGBM produce statistically comparable DCS incidence across matched profiles.)
• Vann RD, et al. Flying After Recreational Diving Workshop. Divers Alert Network, 2004. (Origin of the 12-, 18-, and 24-hour pre-flight surface-interval guidance for single, repetitive, and decompression dives.)
• Shearwater Research. Peregrine Operating Instructions and "Understanding GF99 and Surface GF." (Live gradient-factor readouts and conservatism presets.)
• Suunto. Zoop Novo User Guide and Suunto Fused RGBM technical brochure. (Suunto RGBM behaviour, the consolidated tissue-loading bar, and personal/altitude conservatism settings.)
• SCUBAPRO. Galileo 3 (G3) User Manual, V2.0, 2024. (ZH-L16 ADT, MicroBubble levels L0–L9, and Profile-Dependent Intermediate Stops.)

The dive readings in the simulator are illustrative — hand-tuned to show how the models behave, not generated by running these algorithms. For real planning, use your own computer alongside desktop software such as Subsurface or MultiDeco.

Train with me

Choosing, setting, and reading a dive computer — and understanding why your buddy's disagrees — is part of every technical and recreational course I run. Enquire about training →