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>     Another potential failure for which the h-valve provides good
> redundancy
> for would be an o-ring extraction at the regulator/valve
> connection.  Same
> would hold true if the regulator were dislodged from a bump on
> the ceiling.
> (Obviously much less likely a scenario for a diver using DIN regs
> than one
> using Yoke/A-frame regs).  Again, the gas loss would not be
> instantaneous.
> There would be plenty of time to shut down the side with a
> problem and save a
> substantial portion of one's gas.

An H-valve doesn't really provides good redundancy for these (behind the
head) failures since you can't see what's gone wrong and the 1st stages are
only a few inches apart. If gas starts spewing out behind your head is it
the primary 1st-stage's O-ring that has failed, or that of the secondary, or
the tank neck O-ring? Which knob do you reach back and turn off? With
doubles if you're not sure then you can close off the isolator to ensure
saving half your gas supply, but not with an H-valve. If you have an
attentive buddy then he can see what's gone wrong and close the relevant
valve, but relying on that defeats the aim of self sufficiency. In the case
of a behind the head failure you're probably better off getting the hell out
of there straight away rather than wasting time guessing which knob to turn
off.

I recently converted my single tanks to H-valves, but I see it as a way of
always diving the same regulator configuration rather than as an appreciable
amount of additional safety. If I'm solo diving I won't rely on the H-valves
as my only air-supply redundancy - I either use the doubles or carry a 20 cu
ft pony slung stage-bottle style and filled with air.

Ian

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