Diving Computer Algorithms Are Important

Tom Roos (London, UK) writes: "In May, in your article 'Things Are Not Always As They Seem,' you raise the safety aspect of dive computers giving significantly different no-deco times. I have been grappling with this for a while and thought I'd share my experience."

"On a dive trip to French Polynesia, my wife and I dove Suunto computers using the RGBM algorithm, while other divers had computers with more 'conventional' algorithms. As a result, we faced much longer decompression times at the end of dives. The RGBM algorithm is particularly punitive on repeat 'deep' dives after relatively short surface intervals."

"For example, on the second dive of the day to 100 feet (30m), after a surface interval a little longer than one hour, we required a deco stop 25 minutes more than other divers in the group. The diving conditions in French Polynesia mean that you can't just adjust your profile: you're often diving atoll passes in five-knot currents, and the instructions are to get down, find shelter in a gully on the bottom of the pass, hang on to the rocks and watch the show. Ending up with much longer deco times than everybody else in this situation is not safer. In fact, the opposite is true."

"To make an analogy, driving 40 mph on a highway is safer than driving 60 mph if you're the only car on the road. But if everyone else is going 60 mph, driving 40 mph is neither safer for you nor for anyone else. We ended up diving according to our-25-year-old Suunto Companions instead of our newer models."

"The diving industry needs to sort this out. If RGBM really provides a significant reduction in cases of DCS for non-high-risk individuals, it should be adopted universally (which would force a change in diving practice in many areas). If not, computers should at least be offered with dual algorithms. I would be keen to hear what experts in the field have to say."

Undercurrent Senior Editor John Bantin replies:

Computer manufacturers continue increasing the safety to their algorithms. The RGBM algorithms created by Bruce Wienke are becoming more common, with Suunto, Atomic Aquatics, Cressi and Mares using approximations of the algorithms. Modern Scubapro and Aqualung computer use similar algorithms. Other independent computer manufacturers opt for versions of the Swiss Bühlmann ZH-16 algorithm, which are freely available in the public domain.

Pelagic computers (Oceanic, Aeris, and clones thereof) originally used the older and very different DSAT algorithm. Oceanic sold a lot of these in the U.S. before it got into financial difficulty, so divers are still using them. The later models were indeed equipped with dual algorithms. (If you ask Bob Hollis, you'll discover I had a hand in that.) However, few users bothered to read the instructions and discover the option.

Later Oceanic models have both the old DSAT and the newer Pelagic+ algorithms available. The Pelagic+ is akin to a Bühlmann ZH.

Eventually, those old Pelagic DSAT computers (Oceanic, Aeris, and other clones) will be no more as they get replaced by newer models.

In the meantime, Tom, the problem you identify will continue. It should be up to dive operators and those organizing group dives to ensure that everyone in the group dives at least with a similar algorithm set.

Huish, the company that also markets Atomic Aquatics and Suunto computers (in North America), has recently brought Oceanic computers back to the market, and they look remarkably like Aqualung models, which are now using the proprietary Pelagic Z+ (PZ+) algorithm, which is based on the Bühlmann ZHL-16C algorithm.

Gradually, all modern computer algorithm writers are converging toward similar results to those with the RGBM algorithm for sport divers. The Reduced Gradient Bubble Model algorithm takes into account the possibility of subclinical DCS, and is, therefore, thought to be safer, especially for divers making repetitive daily dives.