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A subjective, but I hope interesting, note on first hand observations of a gas dive. At least one of the divers yesterday reads this list, and I trust will correct any errors I make (he was not one of the ones having problems). Yesterday I tagged along on one of the deep dives of a local trimix course. Our objective was the bow half of the coastal tanker Chester Poling, which lies on a mud/sand bottom in 180 fsw off Gloucester, MA. I was diving air, the other divers 23/25 trimix (END about 110 fsw). I believe this mix was chosen to cut the hang time from what would be required for a greater He fraction, since the water was in the low 40's F. What was interesting about the dive is that it illustrated some of operational constraints of gas vs. air. I felt slightly narced, but not significantly impaired, at depth. This was probably because this was my first deep dive of the season and I expected to be narced, and was thus careful to avoid swimming hard, or carrying too much gear, or getting out of breath, or becoming cold, all of which enhance narcosis. Several other divers reported significant narcosis, probably because they were expecting none, and were task-loaded and physically burdened with multiple stage bottles, gas blocks (for the two divers wearing Aga masks), argon suit inflators, etc. I should note that the other divers were all quite experienced, and all but one had made air dives on this wreck in the past. Since the wreck is upside down and hard to hook, the captain had dropped a double anchor with a long scope near the wreck. By the time we were at the second anchor, we were already ten minutes into the dive. Largely because the gas divers had a rigid deco schedule to follow, we turned the dive at this point before catching sight of the wreck. I thought of running a reel out until I reached my air turn point, but didn't feel like going solo under dark, deep, low vis conditions. The gas divers did stops starting at (I believe) 60 fsw; my first stop was at 40. As I was deco-ing on EAN50, and had cut an accelerated air table for the dive, I could have gotten out of the water in about two-thirds the time they did, but since I wasn't too cold I did the full air schedule plus about 10 extra minutes for safety. I was still out of the water before most of the gas divers. I should add that during deco, one of the Aga divers had a block problem and 1) switched to O2 instead of EAN50, but realized his error before he tested the claim that you can convulse in an Aga and still not drown, and 2) lost a lot of his O2 during a switch and barely had enough to complete the hang. Another diver burned through bottom mix quickly, and made it back to the anchor with about 500 psi left. I know it is unfair to draw conclusions from a class, but it drove home the point that gas diving adds a lot of complexity and therefore risk, in return for some narcosis reduction. For me it wouldn't be worth the trade at this depth. I am sure that when diving with gas divers with scores of gas dives, they would be much more comfortable at depth than I on air. Unfortunately, since gas is expensive and therefore impractical for shallow dives, most gas divers probably do relatively few gas dives. They therefore probably never get really experienced with gas equipment and procedures. Since I was on air, however, I was using the same equipment and procedures that I use on any wreck dive, and with which I am comfortable and reasonably experienced. The rigidity and length of trimix schedules, associated equipment complexity, and relative difficulty of becoming experienced in trimix use (as compared with air) suggest that it's not worthwhile except for very deep or very complex dives. John
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