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I want to comment on the breathing supply redundancy from the 
point of view of a cave diver.  I believe Dan Nafe's comment 
stressing the primacy of the dual outlet manifold is simplistic 
and rather, I agree with Richard Pyle that one must design a rig 
that is appropriate to the type of diving.  No diving equipment 
of any sort fills every need.  Here in Australia the standard 
cave diving breathing setup is a minimim of two independent 
cylinders either back or side mounted.  I believe that this is 
the optimum system for overhead enviroment despite the increase 
in task loading.  You should reduce your task loading in other 
ways (diving within your comfort zone etc.) rather than by 
reducing your equipment redundancy.  Here we do alot of solo, 
long penetration diving, and only completely independent gas 
supplies are appropriate.  I do not believe that a solo diver 
would have much luck operating an isolating manifold.  On the 
other hand, for deep diving in a situation where you can dive 
with a buddy and are not too far from stage bottles, I think 
that manifold tanks are ideal.  Where there is considerable 
narcosis, the task loading of monitoring independent supplies 
can be a problem, the speed with which gas is used at depths is 
a problem in a single cylinder, and additionally you should 
continuously have the regulator in your mouth (or full face mask).

By the way I know of someone who had both blowout disks rupture 
(tanks overpressured) wat the beginning of the dive.

In regard to Richard's mention of using two different mixes 
in independent tanks, I realize this may be a necessity in 
some deep diving in open water, but this was the cause of 
death in an Australian wreck diver recently (see Greg Ryan's 
post on the subject), who inadvertantly used his 50/50 nitrox
at 47 metres depth.

David Doolette
ddoolett@me*.ad*.ed*.au*

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