On Tue, 2 Nov 1999 20:20:50 -0500, Dave Sutton wrote:

>Sorry, but unable to reply in more detail to all
>of your well thought out mesage, but
>there's an 0-dark-30 flight tomorrow AM
>that I'm preparing for and thus little time.
>
>But to highlights:
>
>>Using a single tank for any
>>dive requiring decompression stops is an unreasonable assumption of
>>risk, if you are giving any consideration to a gas sharing scenario.
>
>Explain to me, please, what the difference is between a
>single 120 with a moderate overfill and a set off twin
>72's, with which loads of deco diving was done forever
>and a day, and with which many divers still perform
>these dives?

Certainly.  My comment, more specifically, was referring to the
difference between a system in which a single component failure
(freeze, o-ring extrusion, first stage knock-off, LP hose rupture, IP
failure) causes a catastophic system failure, such as the case with a
single tank and reg, versus the isolatable dual outlet manifold which
provides many more failure recovery options.  In the case of
non-decompression (no-stop) diving, the single tank is acceptable
because in the event of a catastrophic failure the diver can make a
controlled ascent, or even a buoyant ascent without a high probability
of debilitating DCS.  When the diver has a decompression obligation,
such a failure would be life threatening, without the redundancy and
gas supply access provided by the manifold.  Also, you need to consider
the possibility that you could be gas sharing, at the time of failure. 
Even a single with a pony bottle does not solve this problem.  Assuming
that the probability of a regulator failure at any time is equal for
each reg, the two tanks must necessarily be the same size.  I'm sure
you can see why the manifold is preferable to independent singles. 
Even with rule of thirds gas management, incorporating regulator
switches during the dive to ensure equal gas consumption across both
tanks, if the independents diver loses a reg (tank) at the turn point
of the dive, he has just enough gas for egress with none for sharing. 
The manifolded diver can shut down the offending post and still have
access to the entire gas supply.


>> Independent
>>singles are a bad idea - do the failure analysis, and consider an out
>>of gas buddy at the point of maximum penetration/turn point of the
>>dive, add a first stage failure during egress and you're SOL.
>
>
>Explain to me why this system is any different than
>sidemounts, considering that I do not consider
>changing regulators underwater a procedure that
>I would ever apply in a non-overhead environment,
>and similarly that I would not be considering giving
>up a bottle to a buddy (again assuming non overhead
>environment). I mean, if buddy teams use the rule 0f
>thirds, and I lose one of my bottles, we are still getting
>out. I -do not- wear, nor do I advise the use of dual singles
>as doubles, but it baffles me that sidemounts (which are
>the -exact- same thing worn differently) are considered
>the holy grail, while the other system is considered unsafe.
>Forgive me for observing that if you cannot ID one reg
>over one shoulder and another over another and decide
>that L=L and R=R using dual singles, how can that be
>considered different than managing side mounts?

The logistical difference is essentially that component failure is
catastrophic failure when the cylinders are on the back.  When on the
side, regardless of regulator malfunctions, you still have access to
all of your gas, since in an absolute worst case scenario you can
remove the regulator entirely and breathe from the orifice by manual
control of the valve.  Sidemount diving is inferior to back mounting
with the isolation manifold because of the survival options provided by
the manifold, and is only necessitated in environments where back
mounted cylinders are contraindicated, such as in tight cave passages
or sumps.  


>I do not presently use either technique as a normal one,
>(nor do I use a pony), preferring isolation valve ideal
>manifolds when not diving rebreather. But the logic
>still escapes me how 2 singles worn on your side are
>any different than 2 worn on your back if U/W regulator
>changes are not contemplated.

Quite simply, if you can put tanks on your back at all, you use an
isolation manifold because it is the safer system.  If you can not fit
tanks on your back due to the restrictions you are traversing, you
sidemount, but accept the additional risk.

-Sean



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