I appreciate your point. In a private doubles training class, I have learned "when X happens, close the post valve," etc. Recently, in a conversation at a dive site, someone suggested something counter to what I learned, namely "when X happens, close the isolator valve." At that point, I found myself falling short on an understanding, not of the rule I learned, but of the mechanical underpinnings of the rule--ie, the precise mechanical/internal functioning of the post valve that 'effects' the "how" and "why" of the rule.
I would say that, after reading all these replies, most people understand the rules. However, it strikes me that most people understand them more in an intuitive sense, rather than a mechanically rigorous one, and this is fine. Some people understand them in terms of analogies--"think of it this way...." At least one of the diagrams offered was pretty but entirely impertinent. Suffice it to say that not everyone seems to understand thoroughly the mechanical properties of the post valve that explain the how and why--or at least most people haven't offered such an explanation (I'd have to go back and review, but Halocline's hit me as most helpful in this mechanical regard). This is what I was looking for. Maybe I played the devil's advocate a little bit in the hope that someone might hit the nail on the head, at least for me. And maybe in doing so this made me seem more ignorant than at least I hope I am. In any case, your point is still well taken. That said, I'm still dying to ask my technician to explain to me the mechanics of the post valve. Does this mean I should shelf my doubles rig?
And just for the profile, since there you went a looking, I am not a tech/cave/wreck diver. I dive short doubles because they give me more air in a manner that trims out for me better than almost any single tank (I am short), keeping in mind as well that I am partial to LP or, at most, MP tanks. I dive old SP regs and prefer not to hit them with 3500+ psi.
Looking at your profile I have come to one of two conclusions and I'm not sure at this point which one it is: Either 1) You're bored and want to see how many of us you can trick into over explaining such a simple thing or 2) you really have no business owning and diving doubles (which you perport to own in your profile). If it's option number 1, good for you you occupied our minds and hopefully made some folks think about something that many of us take for granted. If it's option number 2 I hope two things for you first that at some point someone actually communicated with you on a level that you understand and second you seek and obtain the training you need to effectively manage your life support system in an emergency. Good Luck
