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Nitrox Decompression I+ve read several postings on oxygen rich decompressions, complete with calculations on how much stop time to subtract. This is all fine, but there+s more to itJthan m-values. One important approach to decompression theory is Hill+s work. In short he concluded that gases can+t really stay in solution during decompression as Haldane thought. Microbubbles form when the +oxygen window+ is broken and the physics of gases in supersaturation has a limited application. A significant supersaturation without any bubble formation requires no agitation,Jturbulence, vortices etc. - in fact it requires that one stops breathing and switches off his heart. The +oxygen window+ is the non-inert part of a breathing mix which does not contribute to bubble formation. It is, slightly simplified, the partial pressure of oxygen at a given depth. Oxygen isJconsumed and any temporary excess will be absorbed and vanish. All other gases (nitrogen, helium, neon etc.) which are used as diluent gases have one main purpose in diving, namely to avoid oxygen poisoning. (Direct and indirect effects of density is also an issue.) In an ideal world pure oxygen would be the diver+s breathing gas of choice - no tables, no DCS. One could even get bent on oxygen and just wait for it to be reabsorbed. The inert gases are in fact +inert+ because the body doesn+t deal with them. They have no active vehicles of transportation. From a decompression point of view the inert part of a breathing mix should be kept at a minimum. The only reason why air is being used is that it+s available. It+s no more natural than any other mixes under water. It+s the natural breathing mix on land, under water nothing is natural - only more or less physiologically acceptable and beneficial. In sat diving the partial pressure of oxygen should be kept down towards normoxic values to avoid long term lung burns. (The oxygen percentage of air is too high for sat dives.) For short exposures, however, partial pressures of 1.6 - 2 don+t create any problems. (To avoid the remote but disasterous possibility of a hit - 1.6 for anything short of a chamber or a Superlite 17.) Single dives are only limited by CNS tolerance. Most of this is probably well known to 90% of those who read this. Moreover, with a maximum oxygen window there is more allowance for pressure reductions without formation of microbubbles. Examples: Air at 130 feet is approximately 1 atmosphere of oxygen + 4 of nitrogen. The oxygen window allows an upward excursion of 1 atmosphere (33 feet). Nitrox 32 at 130 feet is approximately 1.6 atmospheres of oxygen + 3.4 of nitrogen. The oxygen window allows an upward excursion of 1.6 atmospheres (53 feet). Air versus Nitrox 40 at 100 feet would give oxygen windows of 26 and 53 feet respectively. Haldanian m-values may give practical limits for bubble formation while they do not assure a bubble-free ascent. In fact, the main reason why computers can+t handle sawtooth profiles and rep dives satisfactorily is that they don+t +know+ anything about microbubbles. Minute bubbles slow down gas elimination and invalidate the symmetrical equations of computer +tissues+. It would be nice to avoid all bubble formation, but that would at least require a slow sat-style decompression. In practice we can probably do well with a combination of 1) A large oxygen window 2) A slow ascent 3) Deeper stops. With the capability of mixing, analyzing and using any gas mix it+s possible to minimize inert gas loading. Further, it+s possible to unload inert gas (decompress) without moving up to traditional stop depths. I use a concentration gradient to a large extent, rather than a pressureJgradient to get rid of it. With a 60% nitrox at 50 feet the EAD is less than 10 feet. This means in effect that all stops, from 50 feet and up, can be lumped together and carried out at 50 feet! This is nothing new really, but it seems like few mixed gas divers take advantage of it. In addition to being able to unload excess inert gas well below the +fizzing zone+ one gets the second advantage of staying below cold surface water (winter), low viz surface water (summer), waves, critical bouyancy compensation and boat propellers. I use twin 70+s for diving with a pony bottle. The pony is more than sufficient for ascent and contains nitrox 40 or 50. A single 70 is placed at 50 feet on our way down. It may be clipped unto a downline, but most often it+s simply put on a ledge or in a canyon where we can+t miss it. For night dives a flashing strobe marker is practical. When the bottom time has elapsed, I switch to the pony bottle as soon as depth permits. When decompressing I can either stay put where the extra tank is or clip it to a D-ring and carry it whereever I choose to swim. For nitrox 60 my +floor+ is 55 feet and my +ceiling+ the current air stop depth. Whatever I do within that range is safe and productive decompression. Rather than hanging and getting cold and bored, the dive continues; I take photos and enjoy spending more time under water. Even though the total decompression time could be shortened considerably I never do that. I+ve got no bus to catch. One minor problem: Usually I carry an air computer - if my ascent doesn+t show any progress it won+t count down decompression time. Three solutions to that exist: 1) Don+t carry a computer 2) Move to a shallower depth 3) Finish your decompression time and let the computer finish its decompression at the end of a 10 foot line. The main point is, with nitrox decompressions one doesn+t have to move up to the ceiling depth to achieve an optimal stop. There is a wide range of depths and within that entire range decompression is considerably more efficient than air at the ceiling depth. The same goes for oxygen decompression, but since the +floor+ is shallow (P O2 of 1.6 at 20 feet) one looses flexibility. I feel that the concept of a concentration gradient rather than a pressure gradient is lost to a large extent if one has to move up to 20 feet anyway. (According to DCIEM oxygen is OK at 30 feet, but personally I find no reason to run it that high. I actually wrote them 1 year ago and suggested that 20 feet would be a better choice. Obviously, DCIEM felt that they had taken a conservative stand already, as compared to U.S. Navy standards of oxygen stops at 50 and 40 feet.) It makes sense to eliminate inert gas at a pressure where its not on the verge of going out of solution the bad way. With air that+s hard to accomplish, with nitrox it+s easy. Hans P. Roverud
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