Dive Computer Algorithms – What you need to know!
When you start diving you are taught the use of dive tables. You should also be taught the theory behind them. Today unfortunately many divers know that dive tables exist but have no idea why they exist. The same is often true when divers use dive computers. Only a few have any idea what type of algorithm the dive computer uses and how it calculates safe dive times.
Dive tables are the underlying element for all dive computers. When you get your scuba certification you most likely were exposed to the dive tables from PADI. These tables are the only ones that have been tested for recreational diving.
However, the big drawback of them is that they aren’t easy to use and that they assume you stay at a specific depth without taking the descent and ascent into consideration. They also are rather rigid when it comes to multiple dives.
Dive Computer Algorithms
Any dive computer uses a model of dive tables as the essential element of its calculations. The algorithm calculates safe dive limits constantly by taking the current values (depth, time) and matching it up or calculating the limits based on these underlying dive tables.
Different manufacturers use different algorithms for their calculations of safe dive limits. The best known algorithms are:
Modified Haldanian Algorithm
This was a model that we originally used by Mares and some updates to the original Haldane/Spencer Algorithm.
Reduced Gradient Bubble Model (RGBM) Algorithm
The RGB Model is based in part on Wienke and Hamilton’s research and work. The first dive computer manufacturer to incorporate this model was Suunto. In the meantime it’s used by nearly every manufacturer. This model considers micro bubbles in the blood stream coming from the nitrogen build-up. The thought behind it is that these micro bubbles are the starting point of larger bubbles which then lead to DCS (decompression sickness).
Some other manufacturers use adaptations or different algorithms but mostly the above mentioned are used for modern dive computers.
One thing to be aware of is that dive computers with different algorithms will produce different results! Even if they use the same underlying model, like for example a RGBM model, the results can and will vary. This is based on the fact that many manufacturers add conservative additions to the results to ensure to keep you safe.
This makes sense as a dive computer in the first place should keep you safe. This, in some cases can even lead to the situation that using a dive table might give you more time under water than using a dive computer. However, in any case, I would rather err on the side of caution than to risk being too aggressive and ending up with decompression sickness.
Oceanic specifically has two different algorithms coded into their dive computers. You can pick which of these algorithms to use. The first option is to use the DSAT model (Haldane/Spencer) which matches the PADI dive tables. Or, if you want a different conservatism or simply want to match your dive buddies results better, you can use the Pelagic Z+ algorithm which matches the more mainstream RGBM algorithms in other dive computers.
There’s no ‘best’ algorithm out there. They all keep you safe (within limits). However, all of these algorithms have some downfalls.
These specifically are that none takes your physical condition into consideration. While all dive computers measure how long your dive took so far, how many dives you took before, etc., none considers your heart rate, your weight, etc.
The only influence you have as a diver is that you can in many models set conservatism settings. This allows you to make the algorithm more conservative to reduce any chance of issues during your dive(s).
You should be very honest to yourself when looking at your current condition. Your dive computer does not know whether you worked out a few hours before diving and are somewhat dehydrated. It does not know whether you got enough sleep or not.
Many factors like this will become part of dive computers in the near future. Until then you have to assess yourself before diving.
And please do not try to trick your dive computer by using more than one at a time. Some divers decide to use more than one dive computer so that on their second dive the new dive computer has no memory of the first dive and thus does not provide the appropriate warnings for a safe dive. It’s plain stupid to do this. Dive computers are designed to keep you safe so do your part and let them do their job!