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As we see with the death of the S African, three things are generally missing from deco consideration ( the way it is purveyed by the uninitiated). 1) the models don't work, so deep stops need to be incorportated as recommended by bubble mechanics. These need to be done without "padding" the shallow stops. 2) the physiology of the person needs to be considered ( see Bill Mee's information), but then some should not be diving at all ( poorly perfused, substance abusers, pfo, any other shunts, prescrition medication, dumb ass , etc). 3) The "ceiling" moves rather quickly from depth ( as one would expect in a decay curve) and then slows down. NOW GET THIS. The point at which it slows down requires a high oxygen window to achieve adequate decompression and keep the ceiling moving. You then need to get to the oxygen, ans stay there. The pulmonary and cns problems are solved by taking five minute breaks every twenty minutes, and those can be disregarded for deco time purposes. If in doubt, do more time shallow, but don't be a goober and do what is being recommended out there. When any of those who recommend 80/20 do one dive equivalent to the weeniest dive we do, then maybe they will shut up about this stupidity after they get out of the hospital. Thanks for putting this out and especially for including the stuff from Bill Mee. He has the best grip on all of these issues, especially those that involve the use of rebreathers or other complications. We see a lot of examples in WKPP diving. Jeremy Downs wrote: > > Once again, here it is (good post BTW). > (I also added Bill Mee's addendum at the end) > > At 07:40 PM 11/22/97 -0500, G. Irvine wrote: > >Could somebody replay the "Baker's Dozen" reasons why we do not use > >80/20. I do not save any of this stuff and do not know how to get it > >back up. I am being asked again about this stupdiity, > > ---------------------------------------------------------------------- > A (BAKER'S) DOZEN REASONS WHY WE DO NOT USE 80/20 (By George) > > 1) This gas was introduced in an effort to overcome the inability of > unqualified student "tech" divers to control their buoyancy in open > water, and is as such is yet one more concession to doing things in a > convoluted fashion to offset a self- inflicted set of problems brought > on by the "doing it wrong" thinking that pervades diving today. > > 2) A heavy sea is not a problem for a deco stop if it is not posing a > lung-loading problem. Look at your depth guage in a heavy sea and "see" > for yourself what the changes are - insignificant, and if they are not, > you should either not have been diving or incurring a decompression > liability of this magnitude in the first place. In the event of a change > in conditions during the dive, see below where the 80/20 becomes a > liability rather than an assett. > > 3) In the interest of using a standardized set of gases for which you > can permanently mark your bottles , it is a poor concession to inability > to sacrifice the benfits of pure O2 to accomodate a real or percived > lack of skill - learn to dive before taking up techdiving. > > 4) In this same interest you will find that when you graduate to real > diving, as in caves, you will not want to accellerate your ppo2 at > lower depths while still being faced with a long decompression at > shallower depths, and making bizarre mixes to do this is a dangerous > mistake (just like the fantasy of holding an accellerated ppo2 on a > rebreather throughout a deco). I am anticipating the thinking that the > 80/20 crowd would then go to an additional oxygen in cave without > accounting for total exposure, and subject themselves to the risk of tox > in the final deco steps. Tox you do not get out of - bends you do. > > 5) The 80/20 mix is in fact totally useless and contraindicated as a > deco gas. At thirty feet it is only a 1.52 ppo2 ( the real 1.6 ppo2 > gas would be 84/16) and as such does not either provide the right > oxygen window, nor does it does it work as well as pure oxygen without > an inert gas at any depth. The gas mixing in your lungs has already > lowerd the effective ppo2 enough to prevent spiking at 20 feet anyway > with the use of pure oxygen - in other words, we aer dealing with a > simplisitc misunderstanding here, or "old wives tale" that is typical in > diving. > > 6) If 100% oxygen is a percieved buoyancy control risk at 20 feet, then > why is the same ppo2 ( intended) not a risk at 30 feet? This shows the > total lack of reasonable logic involved in the decision to use this gas, > as well as a lack of understanding of the whole picture ( see the rest > of this discussion). > > 7) Along those lines, all we hear is howling about "oxygen cleaning" > above 40% mixtures, and dive shop proprietors on here complaining about > scuba tanks with oxygen in them being filled in their shops. With a > pure oxygen system, the tank only ever gets filled with oxygen from > oxygen tanks, not from every dive shop compressor it sees. Again , this > shows the total inconsistency of agency thinking, and reveals that the > true reason for this gas is to pretend to lower liability for teaching > incompetents to dive, which is bull, and to attempt to accrue some > inventive accomplishemts to the dive agency pundits who themseleves > prove that they do no real diving by making this recommendation > in the first place. This is like the colored regs, the stages on either > side, the quick-release buckle, and the poodle jacket: nonsense of the > most obvious nature developped through one-dimesional thinking by those > whose universe of understanding is not only severly limited, but blinded > by the hubris of not being the "inventor" of the techniques that work. > > 8) Any perceived decompression benefit of using a higher ppo2 at 30 > feet with 80/20 is then given back by the lowered ppo2 at 20 feet, not > to mention the fact that the presence of the inert gas in the breathing > mixture defeats the purpose of using oxygen in the first place ( see > the Physiology and Medicine of Diving) . The ppo2 of 80/20 at 20 feet > is 1.28, not much of an oxygen window, and at 10 feet it is 1.04 - > useless for deco. To make matters worse, you can not get out from your > 30 foot stop in an emergency ( not doing the other stops) on the 80/20 > mix without really risking a type 2 hit. > > 9) This is a dangerous method to achieve a greater total volume of gas > for the bad breathers (another obvious reason the gas is in vogue), who > should not be incurring these decos, and even that benefit of having > more gas is lost since it is breathed at 30 feet, and then has to last > for the other stops. The fact is that gas is effecively saved by using > the lower deco gas up to this point, relying on the pressure gradient > to both achieve the deco and provide a break from high the previous > gas's higher PPO2 prior to going to pure oxygen where the spike could > be a problem on an extreme exposure without an adequate low ppo2 break ( > again this shows that the 80% user is a neopyte diver with no real > experience or understanding of the true risks of these dives) . > > 10) The 20-30% longer 30 foot time on the lower ppo2 is not only > overcome on the pure oxygen at the next stops, the breaks do not come > into play until the initial good dose of pure oxygen has been absorbed, > since you are not spiking from a high pervious dose without a break > that is effectively achieved on the previous gas. These things need to > be understood and taught by the agencies, not some superficial > convolution that is designed to obfuscate the problem rather than > openly acknowledge and deal with it in a responsible fashion. > > 11) In an emergency situation, getting onto the pure O2 for 20 minutes > or so (for long dives something approximating the bottom time or a any > decent interval) would give you a real good shot at getting out of > the water having missed the rest of your deco and living through it > with pain hits only. You have to think these things all the way though, > not go for the transparent superficial thinking of those who merely are > trying to "make their mark" with some "great" idea they can call their > own. The acid test is , as always, is the caliber of the divers who > adopt these practices. > > 12) If there is some problem with your deco or you otherwise develop > symptoms and need oxygen either on the surface or back in the water, it > is silly to have not had it there all along. 80/20 is a joke for that > purpose, unless you have asthma, in which case any accellerated oxygen > mix would be a nightmare. This is again part of the "thinking it all the > way through" phiosophy which is obviously mising from the 80/20 > argument. > > 13) Only a card-carrying stroke would do somethng like this, and > showing up with 80/20 is no different than wearing a sign on your back > saying "I am a stroke, and have the papers to prove it". It announces to > all the world that you have no clue, kind of like wearing clip-on > suspenders or having dog dirt on your shoes. > > George Irvine > Director, WKPP > "Do It Right" (or don't do it at all) > ----------------------------------------------------------------------------- > Bill Mee's post: > > George, > > Thank you for exhaustively laying the reasons why we or anyone else > should not use 80/20. The only thing missing from this discussion is > the Q.E.D. at the end. > > Reason #8, reiterated here for discussion purposes is perhaps the > soundest reason, among many very cogent ones, as to why this practice > should be avoided: > > " Any perceived decompression benefit of using a higher ppo2 at 30 feet > with 80/20 is then given back by the lowered ppo2 at 20 feet, not to > mention the fact that the presence of the inert gas in the breathing > mixture defeats the purpose of using oxygen in the first place ( see > "The Physiology and Medicine of Diving") . The ppo2 of 80/20 at 20 > feet is 1.28, not much of an oxygen window, and at 10 feet it is 1.04 > useless for deco. To make matters worse, you can not get out from your > 30 foot stop in an emergency ( not doing the other stops) on the 80/20 > mix without really risking a type 2 hit. " > > The rush to embrace this practice, recommended by technical diving > diving opinion leaders, was widespread and in retrospect, irrational and > poorly thought out, like so many of the "trial balloons" in this field > of endeavor. It seemed to many, at first glance, to be a simple means of > increasing one's supply of deco gas while eliminating its bothersome > volume and mass. In fact, the perceived benefits tranform into > liabilities when subjected to a thoughtful analysis. When you view > decompression as a two pronged challenge: to progrssively widen the > oxygen window and increase the diffusion gradient to maximize passive > transport of dissolved inert gas, it becomes clear that the 80/20 > solution falls short on both requirements at a critical point in the > decompression profile. > > Section 11 emphasizes a very compelling argument for those who are > concerned with managing dive related crises. When diving in the open > ocean divers and boat operators should always be prepared to "scram" the > deco at any time. This could be for any number of reasons, not the > least of which might be a sudden change in the sea conditions or > unscheduled events such as dive accidents or impending ship collisions. > Just follow the Whitefish Point thread for an excellent example of why a > deco may require being aborted (or never started in the first place). > > Most unfortunately the "80/20 problem" bears a strikingly resemblance to > several other ad hoc technical contrivances mentioned in this same > article (section 7) i.e. dual bcs, colored regulators, bilateral stage > bottle postioning, poodle jacketed second stages and harness quick > releases. All of these ideas, while seemingly reasonable, become > tainted when subjected to thoughtful review. > > Good show Director Irvine. > > Bill Mee -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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