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Pursuing the Shearwater Pursuit November 27, 2008

Posted by Chris Sullivan in Equipment.
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My dive shop is making it very difficult to choose anything but this computer for my next purchase. The computer is one of 4 of 5 candidate products that would work, but the company is willing to make our little shop the first dealer in Ontario, and being one of the dive pros, I would get a great first round discount for seeding the local market by wearing one on my wrist.

I also like the unit. For the money, it does everything I need, which right now is multi-gas, including trimix, and underwater manual gas switches. It can do rebreathers, but I won’t buy that model but upgrade if that becomes necessary some day. There’s no colour, no games, no RGBM, no air integration, or anything else fancy. It allows the diver to change the battery, which is great, and has an infrared interface to a Windows computer.

Speaking of RGBM, I’ve heard about all the great advantages and how it allows shorter deco times without bending divers and all that, but right now it all seems to be a kind of secret sauce. The papers that have been published by Bruce Weinke, the developer of the RGBM algorithm, introduce the concept and then seem to spend the rest of the time selling the idea. There’s no mathematical description of what the algorithm does, presumably to protect his intellectual property rights. So while many divers advise rejection of a computer that lacks RGBM, I’m of the completely opposite opinion.

The Shearwater Pursuit is based on the 16-compartment Buhlmann model used in many a computer and desktop decompression software package. My current Apeks Quantum computer, more of a recreational model, uses 12-compartment Buhlmann, which is likely good enough for recreational and entry-level technical diving. Buhlmann’s paper clearly describes the mathematical model and parameters for his ideas, and I’m sure, given enough time, that I could even design my own decompression software using public domain information. That gives me confidence that I clearly see the path from theory to practice.

Another attractive feature (in my opinion) of the Pursuit is the way conservatism is implemented. A lot of computers seem to have a non-specific conservatism setting like 10% or 25%. What does that mean exactly? My Quantum offers conservatism by faking the altitude setting, pretending that it is in one of three higher altitudes so that it calculates a more conservative outcome. I understand that it will do that, but what does that actually do to the dive? The Quantum will tell you how the NDL is affected but you can’t determine what your decompression schedule will look like.

The Pursuit on the other hand uses Gradient Factors, which change the parameters of the Buhlmann model in a well-defined way. Some desktop software also provide Buhlmann with Gradient Factors, so you can use them to generate dive tables which should correspond with the calculations made by the computer. So if the computer fails during the dive, switching to the tables at any point should be no problem, but if you come up early, or aren’t at maximum depth for the entire dive, the computer can give you credit and shorten your decompression time.

It accommodates 5 open-circuit gases, which is plenty for anything I can anticipate. Other than that, it doesn’t do very much, which may disappoint my inner geek but will do just fine for real-world diving. I’ve never seen one in the flesh, but reports from people who have and my impressions from pictures I’ve seen seem to indicate that it’s ready for rugged service.

As an aside, I was supposed to finish off my Assistant Instructor Course last night but the pool was unavailable. It now looks like mid-December. I may have to call PADI and ask them about Divemaster renewal since I don’t want to pay the renewal fee and the AI fee in the same month, when just one will do. I also have to visit the doc to get my medical sign-off, as a self-declaration is insufficient for the PADI pro ratings.



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