Buhlmann vs RGBM: How Dive Computer Algorithms Differ

Put two dive computers on the same dive and they can show you different no-decompression time. Neither is broken -- they are running different decompression models. The two names you will see argued on ScubaBoard are Buhlmann ZHL-16C and RGBM, and understanding how they differ explains why some brands feel more or less conservative. This is an educational explainer, not dive planning: it will not tell you how long to stay at any depth, and it is not a substitute for training.

What a decompression model actually does

Every dive computer runs a mathematical model of how your body takes on and releases inert gas as pressure changes, and it turns that into the numbers on screen -- your remaining no-stop time, your ascent guidance, your safety-stop prompt. The model is a simplification of physiology, not a measurement of your tissues. That is the first thing to internalize: two reasonable models can disagree, and both can sit within accepted practice. Dive within your training and treat the readout as an instrument, not an oracle.

Buhlmann ZHL-16C: the dissolved-gas standard

The Buhlmann model, developed by Dr Albert Buhlmann, is a dissolved-gas (Haldanean) model. It tracks inert gas across a set of theoretical tissue compartments -- sixteen in the ZHL-16C variant -- each loading and unloading at a different rate, and it keeps you inside limits meant to avoid excessive supersaturation on ascent. It is openly published and well documented, which is a big reason so many manufacturers use it: Shearwater, Garmin's Descent line, and -- as of the Suunto Ocean -- Suunto itself.

Modern Buhlmann computers add gradient factors, usually written as two numbers (a low and a high). Gradient factors let you dial the model's conservatism -- run closer to the limits or well inside them -- and they reintroduce a measure of deep-stop behavior on the way up. The practical upshot is that a Buhlmann-with-gradient-factors computer is tunable and predictable, which is exactly why technical divers favor it. A widely owned example is the Shearwater Peregrine, which runs ZHL-16C with recreational-safe presets out of the box.

RGBM: the bubble-model approach

RGBM -- the Reduced Gradient Bubble Model, developed by Dr Bruce Wienke -- starts from a different place. Instead of tracking only dissolved gas, it also accounts for free-phase gas: the microbubbles that form and grow as you ascend. In practice, bubble-model computers tend to be more conservative in the situations where bubble growth is a concern -- repetitive dives, multi-day diving, short surface intervals, and reverse profiles -- and they often push deep stops.

RGBM lives in Suunto's traditional line -- the rental-standard Suunto Zoop Novo and the older D-series -- and in a Wienke-derived form in Mares computers like the Mares Puck Pro Ultra. Implementations are proprietary and vary between brands, so RGBM on two different makers' computers is not guaranteed to behave identically. That variability is part of why the debate never fully settles.

So which is more conservative?

The honest answer is that it depends on the dive and the settings, not just the label. On a single, square, no-stop recreational dive the two families often land close together. The gaps open up on repetitive and multi-day diving, where RGBM's bubble accounting typically makes it more cautious -- while a Buhlmann computer's gradient factors let you choose to be just as conservative, or more so, if you want.

Conservatism is a setting and a modeling philosophy, not a safety score. A more conservative readout is not a guarantee, and a less conservative one is not reckless when it is inside your training. What matters more than the acronym is that you understand your own computer, dive it consistently, and stay well within its limits.

It also helps to know that "conservative" is not one single dial. A computer can be strict about no-stop time, about ascent rate, or about deep stops, and two models can be cautious in different places. This is why divers who switch brands often notice their new computer feels stingier or more generous in one specific area -- it is the model's personality, not a malfunction, and it is worth a few easy dives to learn its behavior before you read anything into the numbers.

Deep stops, and why they became contested

The two camps intersect on the question of deep stops -- brief pauses added partway up from the deepest part of a dive. Bubble models like RGBM leaned into deep stops from the outset, and early gradient-factor practice on Buhlmann computers reintroduced them as well. More recent research has made the picture less clear-cut, and computer makers have tuned their defaults accordingly, which is one reason two computers of different vintages can guide an ascent differently. The useful takeaway for a buyer is not to adjudicate the science but to recognize that ascent behavior is one of the places two models most visibly diverge -- and to follow the training and the computer you actually dive rather than improvising a profile in the water.

Why the industry is drifting toward Buhlmann

The Suunto Ocean's switch to Buhlmann ZHL-16 is a notable signal. The openness, documentation, and gradient-factor tunability of the Buhlmann model have made it the de facto shared language across brands, especially as recreational divers grow more algorithm-literate. RGBM is not going away -- it remains on many excellent, widely used computers -- but a new buyer today is more likely to end up on a Buhlmann-with-gradient-factors computer than they would have been a decade ago.

What this means when you shop

Do not buy a computer for its algorithm alone. Screen readability, interface, battery type, air-integration options, and whether you will actually wear it matter more to most divers than ZHL-16C versus RGBM. See the algorithm play out in real buying decisions in our Peregrine vs Teric and Garmin Descent vs Suunto Ocean comparisons, and let our best dive computer picks weigh the practical factors. Our how to read a dive computer guide explains the readout, and the pre-dive readiness tool helps you build a kit around it. Whatever the model on your wrist, dive within your training and certification.