Darren in NC,
Not sure what you're looking for, but I was surprised there wasn't a roaring response so I'll toss in my 2c and maybe it will stimulate some more discussion. I've seen a couple of posts here from folks who've done some deep commercial stuff, so maybe they'll chip in.
Read some stuff on DIR; seems to be more of a philosophy than a method. The physical side of it (stay fit, don't binge or smoke) is something I try to do anyway, so that wasn't an issue. The mental/preparation stuff seemed unnecessary for the "I'm going to swim around and look at the pretty fishes" dives but I think it would play more of a role if I was routinely pushing any comfort levels. And the equipment side of it seemed to be something that would have far more value if dive partners were also into it, and as I haven't had a regular dive partner I didn't pursue it. (Plus, as I often dive with a camera and a group rather than defined buddy I often end up effectively solo anyway.) I did adopt one thing that I didn't pick up elsewhere, and that was that an out of air diver should take the regulator from the buddy's mouth rather than take the octo. Anyone who dives airII does that ayway, but having switched to my own octo once to check on a reg problem and gotten a mouthful of grit, giving the out of air diver a clear and working air supply is a good idea. Longer hoses probably make sense if you're doing penetrations or crawling under rocks and debris, but again, probably don't make much difference for sightseeing dives.
I'm not a "deep diver" in any real sense, but I have logged one dive to 142fsw: the Blue Hole in Belize. The wall has an undercut at about 130ft and the interest--old stalagtites--are below that undercut so swimming through them requires more depth. The tour operators get around calling it a deco dive by enforcing a long--20 minute--safety stop at 15ft.
They told us on the boat we'd gather on the lip at about 40ft. Anyone who wasn't equalized in 2 minutes would be told to abort. I'd been in the water about 4 minutes when we dropped off the edge, and the rest of the profile was about 10 minutes on a diagonal descent, 10 mins wandering along the notch at 140ft, and 10 minutes back to 15 ft. Then 20 mintes there.
That dive was done with an Al-80 and no overpressure. I'd be comfortable doing that again, and I think a few problem-free 140s would probably leave me feeling that 150 or maybe 160 would be OK. I don't think I'd be comfortable pushing any further than that without some more classes and training though, and I definitely believe that if I got the bug to go deeper I'd have to train in trimix.
Stevemc's fast descent certainly helps. It's something I tend to do on deeper dives if I'm with people who are comfortable with it. I think that's something that should be progressively explored rather than saying "OK, 140 feet like a rock, here we go!." Likewise, coming up to 70 or 80 ft reasonably promptly eases the final burden but again shouldn't be done without some progressive testing. I'd guess that if someone played with the numbers or progressively challenged a computer you could get at least a 15 minute bottom time on a single 80 (and probably longer with a 20 or 30l bailout bottle to give an air cushion for that extended safety stop).
I''m very recently nitrox certified, so less experienced there and at the moment. I worry more about oxygen toxicity than narcosis or DCS so for now I feel I want to stay really conservative and keep my pO2 below 1.4. I viewed nitrox as a way to extend shallower dives and hadn't given it much thought for deeper ventures. But looking at the tables there is some utility at 140ft. Staying at my conservative 1.4, 140ft would allow me to use 26% oxygen, which in turn would make the nitrogen loading at 140ft resemble a 130ft dive which is fairly signifcant. Someone comfortable with pO2 closer to the 1.6 limit could dive 140ft on EAN30 which would buy another 10ft worth of nitrogen loading (i.e nitrogen load like the dive was to 120 ft). -- I think that's right anyway - so if you have a site that is at 140ft where you'd be making repetetive dives really looking into that could buy you a lot of minutes over the course of a season.
Not knowing what you're planning I don't know if any of that, or the following, is of any use to you. As I understand it, tech divers insist on carrying all their dive and deco gases with them as when they're swimming around or in something deep the danger of losing the anchor line is too great to chance hanging the deco air and getting separated from it. If you were working a specific recovery area you could probably remain tethered to the anchor line (or some kind of ascent line) which might make staging deco gas or even surface-supplied deco air feasible--so you'd only have one or twin bottles rather than a big cluster, which would probably be key for working. It's also getting kinda techie, but if you're planning on spending a lot of time at those kind of depths rebreathers may be woth investigating.
Of course if you have something interesting at 140+ you're gonna need some honking big mailboxes!
Bob.
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