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On Tue, 31 Dec 1996 15:44:09 -0500, Behind the Mast wrote:

>I think a full face mask is a great reason for a manifold
>system. I find, however, that most of the reasons presented
>against independants are problems with the diver not
>problems with independants. A hose can blow on either,
>a guage can go on either but if you have a free flow
>or burst hose and can't or don't shutdown your valve(s)
>your screwed!
>
>I haven't heard anything that convinces me otherwise.
>BD

I think we can generally agree on the fact that, if a diver is unable
to operate all of his valves, regardless of the system he is using,
then he should not be in the water.  With that in mind (assuming divers
can reach all of their valves), I wish to bring to your attention the
following hypothetical scenario:

In a cave or a wreck or under ice, it really doesn't matter:

Diver A, diving an independent doubles system, 
Diver B, diving an isolation manifold

Both divers have same gas consumption rate, both divers following a
rule of thirds gas management.  One third for entry, one third for
exit, one third reserve (for buddy or OAD).  

Diver A, being the careful and conscientious diver that he is, keeps a
careful eye on his gauges and makes a reg switch at the midpoint of the
entry phase, in order to consume an equal amount of gas from each tank
(this makes sense, from a safety point of view, because either
independent system could fail at any time).  Having reached his point
of maximum distance, he returns, effecting a similar switch on the way
out.  He now has 1/3 total gas capacity remaining in each tank, as does
diver B.  Diver A has had to perform two reg switches, but this is
certainly no big deal to any competent diver.

Now let us assume that an event occurs which prevents our two
hypothetical divers from keeping their schedule.  Entanglement with
line or debris, added deco due to exceeding a profile, staying a little
longer to get that perfect photograph, placing an Aunt Jemima
sticker... you name it.  This event may or may not be an example of
multiple task loading, and as well, may or may not cause these divers
to panic.  The point is that there is some sort of event that slows our
divers down, and distracts them from checking their gauges. 
Arbitrarily, let us assume that enough time elapses for our
independents diver (diver A) to deplete the air supply in the tank he
is breathing off of.  In this scenario, diver B (isolation manifold)
now has 1/6 of his original supply.

Diver B still has gas, and can worry about the task at hand.  Diver A
is out of gas and should probably do something about it.  For diver A
to get gas, one of two things must now happen:

1) diver A must effect a regulator switch in order to access the gas in
his other tank.
2) diver A must grab a regulator from diver B
3) diver B must offer a regulator to diver A

We can see, from this example, that a panic or entanglement situation
severely reduces the probability of solution (1).  Solutions (2) and
(3) require the intervention of diver B.  It is good for diver A that
diver B is equipped with an isolation manifold and has gas to donate,
and hopefully diver B can swiftly and competently pass the long hose
and remedy the situation.

But let us assume that diver A, having complete confidence in his
independent doubles requires no assistance from diver B, and that he
nonchalantly switches regulators, frees himself (or completes whatever
task was delaying him), and proceeds to surface, diver B right
alongside.

Now let us throw a monkey wrench into the works, and cause both divers
to suffer a catastrophic HP seat failure in the regulator from which
they are currently breathing.  Both divers, being alert, aware and in
control, notice this failure immediately.

Diver B quickly reaches back and shuts his isolator.  We know that he
can do this, because otherwise he would not be in the water (see
paragraph 1).  He switches to his backup regulator, which gives him
access to the gas remaining in his useable tank, 1/12 of his total
starting supply.  Diver B may now (depending on the profile) continue a
normal ascent.

Diver A is now up the proverbial estuary without means of propulsion,
and is probably hoping that diver B can find it in the goodness of his
heart to buddy breathe with his remaining 1/12th.

This situation, however unlikely, is just one example of the type of
redundancy that an isolator manifold provides, which, in an emergency
situation, can increase the survival probability dramatically.  It is
for this reason that I choose to dive an isolation manifold, and will
continue to support the position that they are the safest type of
system available.

Please feel free to comment (publicly).

-Sean T. Stevenson

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