It's All About the Algorithm

Diving computers use an algorithm -- a mathematical calculation, a process, or set of rules -- to be followed to compute real-time decompression status during a dive.

This century, no physicists have taken it upon themselves to write a new algorithm for dive computers. Manufacturers have merely tweaked the algorithms. Most use an algorithm based on the work of Swiss physicist Albert Buhlmann (died 1994), some with additional Reduced Bubble Gradient Model (RGBM) modifications by Los Alamos atomic physicist Bruce Wienke (died 2020). Others use the Pelagic DSAT algorithm, developed earlier by Rogers and Powell, a Haldanebased algorithm loosely based on data similar to the PADI Recreational Dive Planner, and more liberal (less cautious) than the Buhlmann derivatives. It allows a diver to stay down longer and come up faster, though it was intended for no-deco-stop dives to less than 100 feet deep, not the kind of profiles many sport divers use. Once a diver got into deco-stop diving, the stops tended to be punitive.

During the late 1990s and early 2000s, as Technical Editor of the UK's Diver Magazine, I regularly made side-byside comparison tests of dive computers, photographing all the displays at suitable intervals for later analysis, often on deco dives beyond 165-feet. While most computers produced similar results, it became apparent to me that the Pelagic DSAT algorithm employed in Oceanic computers and clones thereof was not suitable for the type of deeper diving Europeans and more adventurous Americans did.

As you can imagine, this made me less than popular with dive industry personnel, such as Bob Hollis, or the rest of Oceanic's management, but eventually they caved in and supplied Oceanic computers with a dual algorithm mode. Today, users can switch between the Pelagic DSAT algorithm and the Buhlmann-type algorithm, Pelagic Z+.

In many dive stores, customers are often not advised of this when they purchase their computers, and certainly not if they buy them online. And if they don't read their manuals, as many don't, they never discover this critical feature. Were you told about the choice of algorithms when you bought an Oceanic computer, or did you simply use it in DSAT default mode straight out of the box?

When I dived in Truk lagoon, many fellow divers had a second computer to back up their first, as they should. And many complained that their (Pelagic DSAT) Oceanic computers gave very different results than their Buhlmann RGBM Suunto computers. I showed them how to switch their dual algorithm Oceanic to the Pelagic Z+ (Buhlmann) algorithm, and all came into line.

It was a case, shall we say, of no RTFM or, more delicately, Not Reading the Manual.

-- John Bantin