I recently covered the release of the Sunnto Ocean dive computer. It’s an exciting product and looks like a return to form for Sunnto. However, I did miss one key detail in its technical specification: it doesn’t use any version of Suunto’s Reduced Gradient Bubble Model (RGBM) decompression algorithm. 

Only offering the more traditional Buhlmann ZHL-16 algorithm; the Ocean is the first Suunto dive computer not to use any version of their own RGBM algorithm in over 20 years. Suunto have long championed their RGBM, which they co-developed with the late Dr Bruce Wienke.

Known among divers for their conservative no decompression limits and the use of deep stops, Suunto computers and their algorithm have always been the subject of discussion in the diving community.

There is more evidence suggesting it’s the end of the line for RGBM in Suunto’s other recent releases. The technical diving focused Eon Steel included the option to choose between Suunto’s RGBM and the more traditional ZHL-16. Interestingly, this feature was added in a recent firmware update to the Eon Core at the end of 2024, which had previously only had Suunto’s own proprietary RGBM algorithm. 

So does this signal the end of the RGBM algorithm? 

Decompression Models

All decompression models aim to predict the intake and elimination of inert gases, particularly nitrogen, in the body. The ultimate goal is to prevent bubble formation and the risk of decompression sickness. The models originally took the form of decompression tables and latterly in the software (algorithms) of dive computers. They are both just mathematical calculations using the depth, time and surface interval to predict how long a diver can stay at a given depth.

The first recognised decompression tables were created by the Scottish scientist John Haldane in 1908. Haldane’s model assumed that different tissues absorb and release nitrogen at different rates based on a half-life (so exponentially). To simplify the calculations the body is broken down into representative tissue groups or ‘compartments’, of which Haldane’s original model had 5. 

Haldane’s model has been developed and iterated on but remains the conceptual underpinning of nearly all decompression models and algorithms used today. The Swiss physiologist Dr Albert Buhlmann, whose research began in the 1960s, developed the ZHL-16 model used in the Suunto Ocean and many current computers. Buhlmann refined the same basic principles as Haldane but broke the body down into 16 tissue groups rather than 5.

‘Using the ZHL system, both staged and continuous decompression can be calculated quite easily and computers can be easily programmed to carry out decompression calculations, which is one reason why so many dive computer programmers and decompression software designers have utilised modified versions of this system’. Deeper into Diving, Lippman & Mitchell

What is the Reduced Gradient Bubble Model?

The RGBM was developed by the American Dr Bruce Wienke in the late 1980s. It was a shift in focus from the macro view of Haldane and attempted to accurately model how bubbles are actually formed in tissue. 

The theory is based on calculating and managing the growth of bubble nuclei – the precursor to gas bubbles large enough to cause decompression sickness. One of the problems it’s always faced is it is far less intuitive than Haldane’s model – Wienke’s background was working at the Los Alamos Nuclear Laboratory so maybe it shouldn’t be a surprise. However, the simplest way to understand it is ‘is to maintain the diver at depth to both crush bubbles and squeeze out the gas via diffusion…’ 

Dives using the RGBM normally have shorter no decompression limits, slower ascent rates, deeper decompression stops if required and higher penalties for multiday or repetitive diving. This is what gives them the reputation for being conservative.

Suunto worked alongside Wienke to develop their own propriety version in the Suunto RGBM that first appeared in their Stinger and Vyper computers of the early-2000s. Versions of the RGBM also appeared in diver computers from other manufacturers like Mares and Atomic Aquatics. 

At the time it was seen as a significant advance in decompression theory. 

Why retire it?

Now we have to move into the realm of speculation. Suunto’s RGBM computers always had a certain reputation in the dive community. Often criticised for being overly conservative; they have largely been avoided by dive professionals due to their restrictive nature, especially in relation to repetitive diving.

It’s slightly ironic to criticise a dive computer, which is fundamentally a safety tool, for being too cautious. However, divers want to stay safe while getting the maximum amount of time in the water with the least amount of deco. 

My guess is it was purely a business decision from Suunto. Who likely concluded that the use of the RGBM algorithm was hurting sales rather than boosting them. Buhlmann derived algorithms are the current industry standard and used by computer manufacturers like Shearwater, Garmin, Mares and Scubapro. 

Buhlmann algorithms do have some advantages. They are easily adjusted, via gradient factors, allowing divers to manage the level of conservatism applied (a double edged sword) and with their popularity it’s easier to match profiles with divers using different computers.

Final Thoughts

There is no perfect decompression model or theory. All are simplified approximation of complex processes happening inside the body. I haven’t seen any empirical evidence proving the validity of one model or algorithm above another – just a lot of anecdotal tales (go on a diving forum if you’re brave…) 

I’ve always been happy with the Suunto RGBM computers I’ve owned (Vytec, D6 & Eon Core) and never had any objections to their restrictive or conservative nature. I’ve alway been content to err on the side of caution and do as I’m told by my Suunto. 

If the RGBM has been retired then it feels a bit like the end of an era. However, I’m sure it won’t take long for divers to find a new hot topic to discuss between dives.

References

Reduced Gradient Bubble Model: A Modern Decompression Algorithm

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