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In reply to Richard Taylor's recent post, >Some facts about the dive & diver in question: > >1. Diver had reported shoulder pain was existing PRIOR to the dive due to >possible strain (though this can always manifest itself as a pre-existing >injury once the diver has a DCI symptom....DCI denial?). > Are you saying that it is denial, or that it isn't denial? The guys at the chamber seemed to think it was DCI. >2. Diver used a reasonable He heliar mixture (14% O2) and ONLY ONE DECO >GAS, a Nitrox 50/50. >As can be well explained by simple physiology, it is much more effective to >get off the He as soon as possible & maintain a high PO2 as often as >possible (CNS dosage taken into consideration of course). Any table using >a modified Bulhmann will only be as effective as the use of the tools >provided. For example, O2 is x2.5 as effective at eliminating N2 as air, >yet when tables are run in comparison you do not find the stops divided by >that ratio. There is a known conservatism built into any algorithm to make >it as "real" as possible, but only true emperical tests can effect a table >for each person. Decrease your O2 content & remain on the He for longer >and any conservative factor within the table is lessened. > The reason you don't see that ratio is that a simple "ratio" is a dramatic over simplification, one that is essentially incorrect. O2 does provide a greater *gradient* for off gassing, but not just some simple multiplier. The exact improvement in off gassing (in terms of table construction) depends on the maximum compartment inert gas over pressure that the table constructor has deemed appropriate. For very fast tissues this may be a very large pressure, and the elimination of the nitrogen component pressure in air may make only a very small difference in the *gradient*. For the slow tissues (the ones that usually control decompression on the shallower stops after long dives or decos) the difference in *gradient* is closer to the 2 1/2 times that you mentioned, but still not the same. Generally speaking there is no "conservatism" that will be "used up" by breathing the wrong gas. The effect you describe of the oxygen stops not being divided by a factor of two and a half is not a conservatism factor. There has been a thread on this subject before, so I won't expand on this point further. Go and read it. Table constructors pick maximum compartment inert gas overpressures and then the maths take over. Breathing a gas which reduces the off gassing rate should be taken into account in the algorithm. You may be confusing this type of practice with other activities such as multiple ascents. The difference here is that the breathing of a certain gas is accounted for in the table whereas the generation of micro bubbles in multiple ascents is not, causing a deviation from the original model. If you can complete a single cycle without breaking the minimum pressure level, you have essentially completed a dive within the model. The particular dive in question was the simplest dive possible. A pressurisation, a hold at a constant pressure and a return to initial conditions. Also, as you point out further down, "as MigPlan has built in safety in taking ascent to first stop as part of bt." So the diver wasn't off his bottom mix as early as possible, but the programme thought he was *at* the bottom. Perhaps you are saying that using a single deco gas violates the basic principles that underlie Buhlmann's approach to decompression. If that is indeed what you are saying then give me violation!! If you were to dive with his methods you would be using 36m/minute ascent rates! (120ft/minute) You would be making multiple gas switches, using Heliox on the bottom and ascent on air, argoxes and oxygen. You would need to breathe O2 for an hour prior to each dive. A dive to 250m (about 825ft) has a run time of 55 minutes. A little scary for my taste. I don't think that there is anyone diving today that follows Buhlmann's basic principles to the letter. There are valid reasons for wanting to have only one deco gas. It simplifies the equipment configuration, cuts drag, reduces diver stress, reduces chance of "wrong reg at depth" reduces the weight that has to be lifted by the diver at the end of the dive and is cheaper. (cheaper may not sound like a valid argument, but unless you have a CisLuna filled with neon, you have made some kind of cost/benefit assessment and picked a middle ground) I personally use two deco gasses for this type of dive, but that is a personal choice, and I would hesitate to say a single deco gas was wrong or violated some "simple physiology". >3. As for using MigPlan on the dive.....the diver was using a pirated >copy.....pirate beware!!! > I'm sorry, I haven't got the faintest idea how this relates to the safety or otherwise of a particular algorithm. I paid for my copy of Sheck's DRX and that didn't protect me. >4. As for the safety of MigPlan....I have used this table extensively over >the last 12 months on many gas dives to the same depth/duration. Dive plan >has off He as soon as possible & use 2 nitrox gases for deco (32 & 80). >Safety factor is only b (5%) as MigPlan has built in safety in taking >ascent to first stop as part of bt. Maybe I'm lucky for the average beer >swilling middle 30's diver, but so far so good (touch wood). The case you >mentioned may have more to do with other factors than the table. > You seem to have missed my point entirely. I wasn't saying that MiG plan was death on wheels or that you can't dive it without getting hit. What I was objecting to was that Jody implied that there had *never* been a problem with *anyone* diving on MiGplan. This is plainly absurd. He also *associated* mig plan with the *testing* of the Swiss algorithm and then with the next breath *disassociated* MiG plan from any hits sustained by users of the Swiss algorithm. >As for the diving of others...each to their own. > >Regards >Richard T >----------------------------- >Diver: do Not Bend >----------------------------- To sum up, what I was trying to point out (trust me to state the bleeding obvious) was; any diver can get bent on any dive using any table. Statements like "to our knowledge Brand X table has a perfect safety record" are misleading crap and we should all jump on anyone who says something like that without thinking first, in the same way one would correct any other silly mistake. One other thing, the statement "O2 is x2.5 as effective at eliminating N2 as air" is *wrong*. To any one else reading this, do *not* simply divide air stop times by 2.5 and use Oxygen. Did they teach you that in Technical Nitrox Instructor school Richard? (shudder) Cheers Jason
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