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Brockville 2009 & my Shearwater Pursuit October 1, 2009

Posted by Chris Sullivan in Dive Log, Shipwrecks, Technical Diving.
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The deep drift dive from the Lillie Parsons to the James B. King was a good test of my recently acquired Shearwater Pursuit SPOT. I have to say it performed admirably, calculating and tracking my deco schedule as it should. The first stop it asked for was at 80′, which is about what I’d expect, but it cleared before I got there and so my first was at 70′. I had a bit of trouble finding a good handhold in the current there, and made an unplanned excursion to 68′ for a few moments, but as these are deep stops with very low gradient factors it was of no concern. The US Navy tables show 30′ as the first stop even on a square dive profile to 5′ deeper than my maximum depth. You can see in the graph from my logging program the jump in inspired Oxygen that occurs part way through the 40′ stop, when I switched my computer after switching gases.

As the stops got shallower and longer, the gradient factors get higher and maintaining stop depth becomes critical. I’d planned to switch to my deco gas (curiously 71% 02 because someone topped off my 80% tank by mistake) at 30′, but changed my mind and used it on the 40′ stop as well, as the deco MOD of the mix was within limits.

As I mentioned before from now on if I’m only carrying one deco gas, it will be EAN50, as the penalty is low and it is a good backup at greater depths than EAN80. My next tank acquisition will likely be something smaller like an AL30 for pure O2, if I feel the need to carry something else for longer dives.

Deco stops being based on exponential models often work out close to a Fibonacci sequence, where the length of the next stop is approximately the sum of the current stop and the one before. So if your 30′ stop is 5 minutes, and your 20′ stop is 8 minutes, then the 10′ stop is likely to be around 13 minutes. This was certainly true with this dive, after adjusting for the gas switch, which shortens everything. I use this principle to memorize a deco schedule that closely approximates my plan, so if I have nothing else (which should never happen, of course) I can have something close to a reasonable decompression. All it requires is for me to remember the first two stops.

Finally, I’ve mentioned the importance of a slow ascent from the last stop, where the gradient factor is highest, and you can see from the graph that I spent a full minute ascending the final 10 feet.

Lillie-King Drift 20090907

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