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I fail to see the "full" redundancy in using a single Tank with two regulators. The source of air is still a single tank, if you run out of air in that Tank for any reason, you are really out of air. Why not have a pony bottle instead for yourself and be as true redundant as technology allows? (Assuming that you are not using double tanks).
Which reasons would those be? Redundancy is for dealing with malfunctions and a single with two valves and regulators allows for that. Even cave courses can be done with singles. Running out of air is a different matter.
The mentioned system uses tanks with dual valves. Not sure they can both fail at the same time. I guess if there is something inside the tank that blocks the passage... But when does that or an o-ring problem happen during a dive?
Not checking tank pressure is not something I'd consider. Then we'd say that divers in twinsets are not redundant as they are using gas from both tanks...
I've had a reg "blow out" on me. Early 1990s, as I recall - back when young & dumb(er?) & hadn't serviced equipment in years. Was diving by myself & had bounced to ~130-150ft & *wham* it blew out.
It just free flowed horrendously. First blast of air thought something had stuck in it (again stupid/lack of exp), saw something in mouthpiece & banged on it think something suck - but turns out that was some piece inside the reg.
Then noticed my pressure guage dropping like crazy & made a hasty retreat.
Anyhow, survived that one, hit surface & shouted to buddy nearby to turn off my tank (about 150psi left) & learned valuable lesson to service my gear ( in this case - reg rebuild ) before something goes wrong...also, of course, that regs can fail & at depth.
As far as "how often" - I'd venture to say "amost never" if you keep 'em serviced.
Anybody ever wonder why these kinds of things tend to occur with "scuba industry professionals"? And the way to avoid such incidents is to pay them more!!!!
Not that such an event can't happen, but.... So, what was the part that failed? What was the IP before the failure?