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On Thu, 02 Jan 1997 03:25:30 -0500, Mark Welzel wrote: >Ok, I will give you that this is a possible scenario. Right. One possible scenario. ONE of unlimited possibilities. Versatility is one of the key safety advantages of an isolation manifold. The arguments that I have seen so far in support of independent back mounted doubles have justified their use by restriction to specific conditions (valid according to the argument). Although this may be the plan, conditions change. It seems safer, therefore, to dive consistently with the system that demonstrates the greatest survival probability in the widest variety of conditions, and hence largest number of failure scenarios. >I started using my manifold on deeper dives where narcosis >and shortage of time dictate that you stay focused and >minimize distactions. I think that, in the interest of safety, the diver should at all times stay focused and minimize distractions. This is exactly why we use trimix instead of air at depth. To try and justify a lackadaisical (SP?) attitude toward this activity constitutes an admission that you have no business being in the water. >But on the majority of my dives I go to 100-130 fsw and >task loading and gas management is not a problem. I can >do two dives on one set of 120's and still have over 1200 >psi in each tank, that's using those same for deco. I >expect most people in relatively good shape can do the >same. What does this have to do with the matter in question? >I use my manifolded 120's most of the time because >people are impressed, no, because I am lazy. I think you're digging yourself into a hole here. >But I >use my independant 72's often enough and love them >for having wonderful buoyancy characteristics and >being real skinny so I can crawl in little holes. And >no I don't want to put no $300 manifold on my 72's >since I am often switching them around and I like >my $300 where it is. Do what you like with your tanks. The purpose of this discussion was to determine if there is a place for diving independent back mounted doubles, that is not equally or better suited to diving an isolation manifold. >I am not saying that independants are a better system, >I just don't feel they deserve to be condemned yet. There >are many situations where a set of independant tanks >will work fine. This is where the safety issue comes into play. Sure there are situations where independents will work fine. There are also situations where carrying 150 spare airs will work fine. The problem is that you're playing with dice. The probability of failure of any system is a compounding of the probabilities of failure of all of the components in a system. In the case of an isolation manifold, there is one more component (the isolator) than in an independent doubles system. This is (I think, and please correct me if I am mistaken) what you are basing your argument on. The major advantage of an isolation manifold is it's survival of failure probability. The manifolded system provides a degree of redundancy that, in most situations, will still provide the diver access to all of his gas. >Here's a scenario for you: >Diver A wants to dive to 70 foot on a reef. No penetration, >no collecting, no current, right off the beach, just observation. >He knows he only will use 2000 psi out of an 80, but he decides >to wear independant 80's for better balast and as a redundant >source of gas instead of a pony (can I say - pony - here?). >He's floating 20 feet off the bottom, he hears gas escaping, >he calmly checks his guages, watches the tank drain, switches >to that reg (hate to waste it all) Consciously deciding to use a malfunctioning piece of equipment when a fully functional backup is available? Charles Darwin was right. > and when all the gas is gone >he switches regs and then heads for the surface and a leisurely >swim back to the bar, I mean beach. He's calm because he is >always switching regs and monitoring his guages in relation >to individual tanks, this is no big deal to him. Takes excess gas in case of emergency - this is good thinking. Always switching regs - task loading Monitoring his gauges - task loading Monitoring his gauges - necessitating 2 pressure gauges means 1 extra hose, an extra opportunity for snagging, and yes, additional failure points. Looks like you just lost the failure point argument against the isolation manifold. >Diver B is doing the same with a manifold, he suddenly hears he >is losing gas fast, freaks, messes up his rental suit and then >strains his right arm trying to figure out which side of his >manifold is misbehaving. Shut downs the wrong side, still hears >the gas escaping shuts down the other side, sucks vacuum before >he remembers to open the first side he shut down. Loses his >Rolex in the process. Then he too surfaces while trying to >rinse his suit out. He freaks because even though he checks >that he can reach his valves, he never has had to actually do >anything, and now this! Mark, this is ridiculous. You complain about arguments that attack the competence of the diver rather than the equipment choice, and then spew this nonsense? You will note that in the scenario I submitted to you, both divers demonstrated total competence in their equipment, and reacted to the hypothetical problems as best as could possibly be expected. The problems represented were enirely resultant from the choice of equipment, and not from the divers themselves. I have yet to hear a logical argument in favour of the safety of independent doubles. >And for fun; >Diver A's kit - any old BC ($100), doubles bands ($20), >backplate ($30), twin 80's ($115 ea), watch ($20). >$ 400 US (Looks like, and is, normal, everyday dive scum) > >Diver B's kit - whatever Buoyancy device the cavers are >advocating ($750), twin 80's ($115 ea), Rolex ($2000). >$2980 US (But he did look good!) This is garbage. >I don't think independants are that bad, maybe not the best >setup that money can buy, but they are capable of being >utilized properly and safely. Maybe so, but be sure and rinse out the chlorine when you're finished. ;) >For what it's worth, >Mark not much. -Sean ---------------- >Sean T. Stevenson wrote: >> >> On Tue, 31 Dec 1996 15:44:09 -0500, Behind the Mast wrote: >> >> >I think a full face mask is a great reason for a manifold >> >system. I find, however, that most of the reasons presented >> >against independants are problems with the diver not >> >problems with independants. A hose can blow on either, >> >a guage can go on either but if you have a free flow >> >or burst hose and can't or don't shutdown your valve(s) >> >your screwed! >> > >> >I haven't heard anything that convinces me otherwise. >> >BD >> >> I think we can generally agree on the fact that, if a diver is unable >> to operate all of his valves, regardless of the system he is using, >> then he should not be in the water. With that in mind (assuming divers >> can reach all of their valves), I wish to bring to your attention the >> following hypothetical scenario: >> >> In a cave or a wreck or under ice, it really doesn't matter: >> >> Diver A, diving an independent doubles system, >> Diver B, diving an isolation manifold >> >> Both divers have same gas consumption rate, both divers following a >> rule of thirds gas management. One third for entry, one third for >> exit, one third reserve (for buddy or OAD). >> >> Diver A, being the careful and conscientious diver that he is, keeps a >> careful eye on his gauges and makes a reg switch at the midpoint of the >> entry phase, in order to consume an equal amount of gas from each tank >> (this makes sense, from a safety point of view, because either >> independent system could fail at any time). Having reached his point >> of maximum distance, he returns, effecting a similar switch on the way >> out. He now has 1/3 total gas capacity remaining in each tank, as does >> diver B. Diver A has had to perform two reg switches, but this is >> certainly no big deal to any competent diver. >> >> Now let us assume that an event occurs which prevents our two >> hypothetical divers from keeping their schedule. Entanglement with >> line or debris, added deco due to exceeding a profile, staying a little >> longer to get that perfect photograph, placing an Aunt Jemima >> sticker... you name it. This event may or may not be an example of >> multiple task loading, and as well, may or may not cause these divers >> to panic. The point is that there is some sort of event that slows our >> divers down, and distracts them from checking their gauges. >> Arbitrarily, let us assume that enough time elapses for our >> independents diver (diver A) to deplete the air supply in the tank he >> is breathing off of. In this scenario, diver B (isolation manifold) >> now has 1/6 of his original supply. >> >> Diver B still has gas, and can worry about the task at hand. Diver A >> is out of gas and should probably do something about it. For diver A >> to get gas, one of two things must now happen: >> >> 1) diver A must effect a regulator switch in order to access the gas in >> his other tank. >> 2) diver A must grab a regulator from diver B >> 3) diver B must offer a regulator to diver A >> >> We can see, from this example, that a panic or entanglement situation >> severely reduces the probability of solution (1). Solutions (2) and >> (3) require the intervention of diver B. It is good for diver A that >> diver B is equipped with an isolation manifold and has gas to donate, >> and hopefully diver B can swiftly and competently pass the long hose >> and remedy the situation. >> >> But let us assume that diver A, having complete confidence in his >> independent doubles requires no assistance from diver B, and that he >> nonchalantly switches regulators, frees himself (or completes whatever >> task was delaying him), and proceeds to surface, diver B right >> alongside. >> >> Now let us throw a monkey wrench into the works, and cause both divers >> to suffer a catastrophic HP seat failure in the regulator from which >> they are currently breathing. Both divers, being alert, aware and in >> control, notice this failure immediately. >> >> Diver B quickly reaches back and shuts his isolator. We know that he >> can do this, because otherwise he would not be in the water (see >> paragraph 1). He switches to his backup regulator, which gives him >> access to the gas remaining in his useable tank, 1/12 of his total >> starting supply. Diver B may now (depending on the profile) continue a >> normal ascent. >> >> Diver A is now up the proverbial estuary without means of propulsion, >> and is probably hoping that diver B can find it in the goodness of his >> heart to buddy breathe with his remaining 1/12th. >> >> This situation, however unlikely, is just one example of the type of >> redundancy that an isolator manifold provides, which, in an emergency >> situation, can increase the survival probability dramatically. It is >> for this reason that I choose to dive an isolation manifold, and will >> continue to support the position that they are the safest type of >> system available. >> >> Please feel free to comment (publicly). >> >> -Sean T. Stevenson >> >> -- >> Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. >> Send subscription/archive requests to `techdiver-request@aquanaut.com'. >
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