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Recreational Diving with a Shearwater Predator March 7, 2011

Posted by Chris Sullivan in Equipment, Technical Diving.
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My second Apeks Quantum developed a problem last summer. The first one just died, but this one has developed the common problem (that my friends have seen) which is that the depth gauge becomes unreliable. Mine will suddenly jump to 10 feet shallower than I am, beeping mightily about the astronomical ascent rate. So while I’d normally take the Quantum on a trip where the water is warm and clear, with purely recreational dives, I elected to take my tech diving computer, a Shearwater Predator, instead.

The Predator is a great dive computer. Mine used to be a Shearwater Pursuit and was upgraded once the Predator was introduced. The main differences between the two are the OLED colour display (vs. LCD monochrome) and the Bluetooth communications to the (free!) logging program (vs. Infrared). Both features make a big difference to the operation of the computer, especially the nice bright OLED display on someone whose close-up vision isn’t quite as good as it used to be due to Presbyopia.

Predator’s have technical diving features like multiple mixed gases (5 gases for the open circuit version, 5 more for the close-circuit version), flexibility for decompression schedules, underwater gas switches and changes, etc. It lacks features that recreational computers have, especially the audible alerts and the safety stop counter. I don’t mind missing the audible alerts. I’m good at scanning my computer and other gear. Also, I like to guess what I’m going to see before I look at the computer and my air supply, so I develop a good mental picture of my situation.

I use the Predator with a conservative decompression algorithm (GF 30/85, which is actually the default).  The Quantum, at least for the NDL calculation, is less conservative and my diving buddies figure it’s around GF 88/88). The tough guys at in the dive club who use Cochran Computers have them set to about 100/100, by the looks of it. The 30/85 setting means that it will go into mandatory deco stops sooner than most of its recreational counterparts. So unlike the rules that new divers are given, if you use a computer like this you either have to abide by a very short NDL or accept the deco stops.

It’s probably no surprise that I usually do the latter. In the 18 dives I logged in Cozumel I didn’t go past 6 minutes total decompression obligation. Most of the time it was 2-3 minutes, sometimes nothing. It is my belief that a 3 minute 10′ deco stop with a conservative algorithm is more or less equivalent to a safety stop. If miss, the chances of DCS are greater but that’s also true if you miss the safety stop. So in effect, I’m just getting the additional discipline of a safety stop that my computer calls mandatory. Once I’m done the stop(s), I’ll stay a while longer to get some additional safety margin or surface – slowly of course.

Getting well into deco is an entirely different matter. 5 minutes is about all I’ll do with recreational equipment. If I have to escape to the surface I’m probably going to be OK, which is as much as you can say for missing the safety stop on a less conservative computer. Any further pushing of the limits means carrying additional redundancy to make sure that I have the means to complete the decompression under all foreseeable circumstances. That’s why we have technical diving training.

Disclaimer: This is what I do. You need to understand all the facts and risks to make your own decisions about what level of risk is acceptable to you. I certify that this level of risk is acceptable to me at the time of writing, and that’s all.


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