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The tables and programs are wrong for nitrogen based deco in the first place - you have to start "deco" deeper for all gasses, and in fact could do the opposite with helium. Merely obeying a correct ascent rate would go a long way towards reducing dcs, but most divers never get taught how to do that properly. Dive shops with booster pumps would solve the problem in the US, in the UK the gas is much more expensive. What is really becomming apparent is that the training agencies who teach "deep air" and deep nitrox diving are looking more and more like the abject morons that I always insisted they are. SM wrote: > > I'm no expert but in the last year of designing a deco software package, > from what I have seen nitrogen is the big offender -- I just don't think > our tissues can handle the overpressures that occur on "standard" deco > tables as well as they can with helium. The anti-helium crowd complain that > helium is so bad, just look how deep you have to start your deco with it. > It's backwards logic, if you have to start decoing sooner (deeper) then it > means it is easier to get rid of and it will come out of solution more > readily than N2. Nitrogen deco starts shallower, therefore it's taking a > greater driving force to get rid of it. Just look at the (generally long) > shallow stops, by 6m and 3m that's nitrogen that you are trying to > decompress, most of the helium is long gone. > > I'd be reluctant to call trimix worthless. Without it we'd all either still > be doing 50m+ on air or spend our weekends fishing instead. If you can > afford heliox, which implies affording a rebreather too, then do it but the > thing to remember is that most trimix users are rec divers. Professional > diving tends to be heliox based, with us who do it for fun more or less > forced to use poor man's mix. Until rebreathers become more accessible to > the public, which means a drop in cost and increase in reliability, then > heliox really isn't an option for the majority of us. We have to find a > balance between what is desirable and what is practical, bearing in mind > that most us using trimix are using it on open circuit equipment. For an > open circuit wreck dive there is a very finite limit to the amount of gas > you can carry, even in a cave where bottles can be staged you have to ask > yourself if it is really worth the hassle of the set up dives. I watched > Rheinhard Buchaly spend a week setting up a 85m+ dive (1200m approx > distance, still a good way from the end of the sump) in the Ressel in > France last year. For two divers, 40min bottom time, there must have been > close to 40 stage bottles in place plus two deco habitats. Dives like this > were made for rebreathers. > > Trimix from a physiological viewpoint is worth less than heliox but it > beats air hands down. Open-circuit is going to be around for a few more > years which means trimix as we know it will be there with it, so it makes > sense to find the best way to use it practically. It amazes still how many > divers are still doing stuff like fooling their deco software (high ppO2 > gases used deep on Proplanner for a minute or two wipes out a shocking > number of intermediate deco stops), disregarding the benefits of deep deco > stops in favour of long shallow stops, favouring "bend'em, mend'em" style > tables, using air as a deco gas, diving heavily narcotic mixes to avoid > helium, etc. The education of a lot of new trimix divers is still done the > way it was five years ago, it's up to them to either relearn for themselves > or keep on doing the same stuff, no-one will do it for them. I don't think > we should reject trimix, just look long at how we are using it. > > ---------- > > From: Bill Mee <wwm@sa*.ne*> > > To: kirvine@sa*.ne*; techdiver@aquanaut.com > > Cc: rose@cs*.sc*.ed* > > Subject: Re: "trimix" worthless? > > Date: Tuesday, 27 July 1999 22:15 > > > > George, > > > > Frankly, I would have to completely agree. You can come up with > theoretical > > tables all day long to predict decompression from trimix dives, but you > are > > probably just fooling yourself. Any kind of serious exposure to trimix > and > > you simply cannot decompress without some type of sub clinical > physiological > > damage. You and I and the others have done our share of these kinds of > dives > > and there is no hiding the fact that no matter how good shape you're in > > there is a tangible "beating" factor. I'm sick of it. > > > > My guess is that the real problem is nitrogen and that the effect is not > so > > much partial pressure dependent as it is simply pressure dependent. It > is > > well known in chemical engineering that many reactions require external > > pressures to occur. For instance certain polymerization reactions are > > pressure dependent. > > > > Really, who knows what nitrogen is doing. We do know that it affects the > > mechanical characteristics of erythrocytes (red blood cells). The ability > of > > erythrocytes to deform is critical to their passage through some of the > > capillary beds and who would argue that gas transfer at the > microcapillary > > level is not relevant to decompression? > > > > Heliox seems to give far superior results and this should not be > surprising > > since the commercial dive industry has compiled an historically > impressive > > safety record for saturation diving. > > > > I don't know what this will mean for technical diving, but I guess it > will > > ceratinly help the market for booster pumps when everyone realizes that > > "trimix" is a bust. > > > > At least with the rebreathers you can operate at much lower drive gas > > pressures; however I am not recommending that everyone run out and buy > > rebreathers as these devices pose all sorts of other risks. As you know > I > > have been using straight heliox for the 120ft and 70ft deco gases with > the > > Halcyon with drive gas pressures of 2000-2400 and had lot's of gas left > > over, even with the 40cf bottles after long decos. I have also jacked > the > > bottom gas to 75% helium with great results. > > > > Where do we go from here? > > > > Bill > > > > > > -----Original Message----- > > From: Katherine Irvine <kirvine@sa*.ne*> > > To: techdiver@aquanaut.com <techdiver@aquanaut.com> > > Cc: rose@cs*.sc*.ed* <rose@cs*.sc*.ed*>; wwm@sa*.ne* <wwm@sa*.ne*> > > Date: Tuesday, July 27, 1999 3:14 PM > > Subject: Deco not working? Could be the gas....is "trimix" worthless? > > > > > > >I get questions all of the time as to recommendations for improving > > >deco that others are not satisfied that they are doing advantageously. > > >Many times it is obvious as to wrong shape of the deco or wrong gas, but > > >lately it is less clear. > > > > > > For my own account I have been more interseted than ever in > > >minimizing damage and maximizing my available workout time since I am > > >training for an Iron Man this fall - I can't have any down time from > > >training since I have to work three disciplines and workout twice per > > >day . My teammates have become more interested in marathons, bike > > >races, swim races, triathelons and in more conditioning, and have taken > > >the same attitide that I have taken, and that is we do not want diving > > >to get in the way of our training, even though diving was why we started > > >training in the first place. > > > > > > You have seen the progression of our deco, and may have noticed that > > >we started incorporating helium into the deco gasses, and increasing the > > >amount of helium in the mixes. As we got higher and higher on the helium > > >percentage , the truth has become clearer and clearer - nitrogen is the > > >problem, and "trimix" as we knew it is basicly worthless. > > > > > > Sure, I can walk away from most trimix dives, but I run away from > > >helium dives. We may have to revamp our entire structure on this issue. > > >Heliox may in fact be the correct gas for diving, and the answer to > > >deco, not anything more or less. > > > > > > The Navy asked us about loss of vital capacity. We can measure that > > >on the track , in the pool, and on the speedometer in one word ; > > >nitrogen. > > > > > > > > > > -- > > Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. > > Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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