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Kevin Connell <kevin@nw*.co*> wrote: >An isolation valve is another point of failure, so why is it including in >the minimalist approach? The approach was "don't take anything you don't need", and you have to carefully analyze each piece of equipment and configuration to see what you need and how best to fit it together. You look at the different failure modes, determine what could kill you vs. what would only incovenience you, and the probability of each. For example, don't guard against something that's happened a handful of times in the history of the planet at the expense of leaving yourself open to a problem that happens to somebody somewhere every weekend. There are several plus/minus points to isolators: 1. If a tank neck o-ring or burst disk (assuming you haven't already plugged it) or valve stem bursts or breaks (cave ceiling collision) the isolator saves 45% or so of your air, depending how quickly you turn it off. Not having it would leave you completely out of air. 2. If the isolator itself fails catastrophically in a situation that a straight tube would have survived, it will lose you all your air. This situation is much less common than situation 1. 3. If the valve fails to close fully in situation 1, it's situation 2. This isn't a selection criteria, since your rig would act the same way with an isolator or a straight tube in this situation. (Likewise, situations that somehow manage to shear off your entire manifold without shearing off your head aren't criteria for deciding if an isolator would be useful or not.) 4. If you and the filling station both screw up, you could go diving with a full tank on one side and an empty tank on the other. Making sure the isolator is open before filling, after filling, and before diving resolves this. Yes, some people have made this error, and one fellow even managed to get seawater into a steel tank and rust it out. 5. If you dive with the valve closed, your SPG will show your pressure dropping either twice as fast as it should or not at all. The latter situation is more likely to be a problem, if you look at your tanks and think "yeah, still lots of air" without noting that you still have what you started with partway into a dive (a Hogarthian rig works this way, with the primary and SPG on opposite firsts). This also offsets your reserve to one side or the other instead of it being 50/50 like it's supposed to be. Again, making sure it's open before diving solves this. Since, on a probability basis, the isolator will save you more times than it will kill you, you want one. In any event, you still want a fallback plan for the remote possibility of a complete air loss, but that's why we breathe these long hoses. BTW, some people advocate having the valve just partway open, so that it can be closed more quickly in an emergency. It only has to be open a crack to keep both sides even, but because of the problems that can happen if it's a little bit closed instead of a little bit open, I'd be wary of this approach. -- Anthony DeBoer http://www.onramp.ca/~adb/ adb@he*.re*.or* (here) adb@ge*.co* (work) #include "std.disclaimer" -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send list subscription requests to `techdiver-request@aquanaut.com'.
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