That concerns me too. Actually that's the #2 thing I'm scared of, right after an asteroid hitting earth and collapsing the cave while I'm in it.
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Ok, I understand what you were saying.
I couldn't have said it better myself. This is something people on BOTH sides of the fence need to realize. I like SM. I dive SM. I don't care whether you prefer BM or SM. If you dive one way, and can do so safely, then good. If you can't do it safely, you need more training. That's true for BM and SM.
So, behind your head is the manifold and your regs. If neither of them ever fail, then what's the point of learning valve drills. Actually, what's the point of having an isolator manifold? What's the point of having more than one first stage? If the TWO things you need to learn to guard against NEVER happen, then you can safely tell people that they're wasting their time and money seeking out training and practicing those useless emergency drills......right?
2nd stages free flow, and you control those with the valves behind your head, the manifold allows you to breathe all remaining gas from the working reg rather than screwing around feathering a valve. Inflators can freeze on (in colder water, at least). People forget to turn their valves on before getting in the water from time to time. Occasionally you rub ceiling and need to make sure the valves didn't roll off.
As far as the isolator manifold, I've heard a few stories where a burst disc let's go or gets knocked loose during a dive, but that's few and far between. I have NEVER heard of an isolator or 1st stage leaking enough that it required valve manipulation.
We historically create problems or embellish where there is no historical evidence. I remember how quick releases were going to kill us. The stories that are real,do tend to snowball,and you would think Godzilla is walking the streets of New York and derailing subway trains.