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Ok, I will give you that this is a possible scenario. I
started using my manifold on deeper dives where narcosis
and shortage of time dictate that you stay focused and
minimize distactions.

But on the majority of my dives I go to 100-130 fsw and
task loading and gas management is not a problem. I can
do two dives on one set of 120's and still have over 1200
psi in each tank, that's using those same for deco. I
expect most people in relatively good shape can do the
same. I use my manifolded 120's most of the time because
people are impressed, no, because I am lazy. But I
use my independant 72's often enough and love them
for having wonderful buoyancy characteristics and
being real skinny so I can crawl in little holes. And
no I don't want to put no $300 manifold on my 72's
since I am often switching them around and I like
my $300 where it is.

I am not saying that independants are a better system,
I just don't feel they deserve to be condemned yet. There
are many situations where a set of independant tanks
will work fine. 

Here's a scenario for you:
Diver A wants to dive to 70 foot on a reef. No penetration,
no collecting, no current, right off the beach, just observation.
He knows he only will use 2000 psi out of an 80, but he decides
to wear independant 80's for better balast and as a redundant
source of gas instead of a pony (can I say - pony - here?).
He's floating 20 feet off the bottom, he hears gas escaping,
he calmly checks his guages, watches the tank drain, switches
to that reg (hate to waste it all) and when all the gas is gone
he switches regs and then heads for the surface and a leisurely
swim back to the bar, I mean beach. He's calm because he is
always switching regs and monitoring his guages in relation
to individual tanks, this is no big deal to him.

Diver B is doing the same with a manifold, he suddenly hears he
is losing gas fast, freaks, messes up his rental suit and then
strains his right arm trying to figure out which side of his
manifold is misbehaving. Shut downs the wrong side, still hears
the gas escaping shuts down the other side, sucks vacuum before
he remembers to open the first side he shut down. Loses his
Rolex in the process. Then he too surfaces while trying to
rinse his suit out. He freaks because even though he checks
that he can reach his valves, he never has had to actually do
anything, and now this!

And for fun;
Diver A's kit - any old BC ($100), doubles bands ($20),
backplate ($30), twin 80's ($115 ea), watch ($20).
$ 400 US   (Looks like, and is, normal, everyday dive scum)

Diver B's kit - whatever Buoyancy device the cavers are 
advocating ($750), twin 80's ($115 ea), Rolex ($2000).
$2980 US  (But he did look good!)

I don't think independants are that bad, maybe not the best
setup that money can buy, but they are capable of being
utilized properly and safely.

For what it's worth,
Mark


Sean T. Stevenson wrote:
> 
> 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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