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Rich,

There are essentially 3 tank level points of concern when discussing
redundancy - the tank-valve oring interface, the burst disk, and if the
tanks are manifolded together, then the cross bar and its points of 
attachment/seal.

Failures at any of these points occurs below the valving mechanism, and
failures at the first 2 points is such that turning off a valve will not be
of help in correcting the problem.  

If a tank neck oring or a burst disk blows while underwater, having a totally
independant-redundant configuration insures that one cylinder of gas
is saved.  How much is salvaged is dependant on the divers willingness
to make multiple regulator switches to keep both tanks essentially "balanced"
in remaining volume, thus you lose at most 50% of your gas supply present
at the time of the failure.  Obviously, if you do not alternate between
regulators (ie, plan to breathe one tank down to 1/3 of the original
pressure, and then switch to the second tank, it is a 50-50 chance that
you will have the full tank to use after a failure, and a 50-50 chance
that you will have the 2/3rds empty tank to use after the failure.

If diving twin tanks with a manifold, if the cross bar on the manifold
begins to leak (or blow an oring, or is sheared), then depending on the
precise location of the failure an isolation valve in the cross may or may
not be able to salvage the gas mixture (ie, if it begins to leak at the
attachment points to the tank-valve units, then quick action at the
isolation valve MIGHT be able to salvage meaningfull amounts of gas.

However, if the failure is at the isolation valve itself, then you are 
SOL.

As to these modes of failure, I have seen a couple of tank neck orings
let go, usually upon the first fill after a service procedure.  I have
heard of a single incidence of a tank neck oring letting go during during
a dive, but this was hearsay material.

I have personally seen multiple cases of burst disks letting go, most
while the tank was just sitting in storage (ie, in the spare bedroom)
with no changes in pressure or volume, and no significant changes in
temperature.

I have also seen burst disks rupture while in a dive on 2 occasions, and
have heard reports of quite a few other cases.  

This is the major reason that I dive european valves on my tanks, as they
do not have a burst disk system built into them.  On the valves that I dive
that do have a burst disk system built in, many cave divers would rather
risk problems during a fire, rather than risk the failure during a dive,
and they insert non-standard disks (extra heavy) or two of the regulator
disks into the burst disk assembly.

Just a couple of thoughts -

John

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