What is the Oceanic Dual Algorithm? Why will it make a difference?
These are two very valid questions. There has been some confusing and misleading articles and discussions over the years about how it works and what it can do to improve your diving experience.
A note on Algorithms – Dive Computers work by recording depth and time measurements and then using them with an algorithm to determine decompression obligations and no decompression limits. Algorithms are created by scientists using extensive research, recording and averaging diver data. Most new algorithms are based on existing models but with modifications to suit a particular type of diving or blend different models to improve dive times and safety.
Every single current Oceanic Dive Computer has not one but two algorithms built in to them, one benefiting longer, deeper dives with longer surface intervals and the other repetitive dives with shorter intervals. You make the choice of which algorithm to use depending on what type of diving you will be doing or, if you want, you can choose an algorithm to suit your buddy's dive computer.
The two algorithms are based on different models:
- Pelagic DSAT - This algorithm is based on the studies that were used to develop the PADI Recreational Dive Planner (RDP) which is used and relied upon by thousands of divers around the world. Oceanic, along with other manufacturers, having been using this algorithm for many years.
- Pelagic Z+ - This algorithm was developed to safely maximise dive times at depth without penalising the diver for performing repetitive deeper dives. It takes into account the effects of colder conditions and more rigorous demands on the diver. The algorithm uses the Buhlmann ZHL-16C database which produces very similar dive profiles to the algorithms used in other manufacturer dive computers including Suunto, Uwatec and Mares.
Both of these algorithms allow for additional adjustments to increase the conservatism which will make the dive safer but also reduce the dive bottom time. Deep stops are also supported for no-decompression dives to help safely remove excess nitrogen at a higher rate.
How about some real world scenarios:
Liveaboard (recreational diving) – A week of repetitive diving (30m or less) can start to really load up the nitrogen, reducing your bottom time. Frequent dives and short surface intervals don't allow enough time between dives to effectively off load the build-up and rapidly starts to affect your experience. The Pelagic DSAT was built for handling this type of diving, frequent dives to 30 metres or shallower depths. A diver using this algorithm should notice longer dive times as the week goes on compared to other divers using Suunto, Mares or similar dive computers.
The graph below shows how well tuned the algorithm is for shallower dives. At 30m/100ft all the algorithms offer very similar dive times but by coming up 18m/60ft it has started to separate from the others and can give the diver longer bottom time. A shallow 12m/40ft dive will be significantly longer using the Pelagic DSAT algorithm compared to other brands or even the Oceanic Pelagic Z+ algorithm.
Day or Weekend Diving – Most of us work during the week and dive at the weekend which means we want to make the most of those two precious diving days with maximum bottom time on our favourite wreck or something similar. The Pelagic Z+ algorithm is perfect for deeper dives in more strenuous conditions like cold temperatures. In most cases we'll have longer surface intervals to have a bite to eat, warm up, chat about the dive, etc and the algorithm takes this into account to safely give us as much time in the water as possible. This algorithm will also provide similar dive profiles to those produced by Suunto and Mares.
Oceanic Dive Computers give you as the diver the choice depending on the diving you will be doing. Shallower repetitive multi-dive days work best with Pelagic DSAT whilst deeper dives with longer surface intervals benefit the most from the Pelagic Z+ algorithm.