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?




> 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?

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.




>Open circuit is not "old" technology, but rather the
>correct equipment to use  <snip... see below>

As is a pony bottle when used with a single for environments
where minimal deco is required and no real overhead environment
is anticipated. A single 104 and a 30 Ft/3 pony provides adequate
margin for much of the 80-100 foot-ish diving done here on Nitrox,
and is safely used by many divers of widely varying experience levels.



>when the additional complexity and failure
>modes introduced by a closed circuit system are not warranted, such as
>is the case with any dive that could be accomplished on open circuit,
>without respiratory heat loss or dehydration becoming so significant an
>issue that to use the rebreather would be safer.


The mixed gas rebreather has more redundancy modes than any
OC system and frankly I find that the system is far less complex,
and far less bulky than the tech rigs worn to do the same or similar
dives. Having a realistic 6-8 hour life support duration, several modes
of operation (to quickly run through them: Normal control, sensor monitored
manual add, constant bag volume CC bailout, SCC bailout by nose
exhale method, OC mode through auto add-valve, OC bailout with
carried OC gas, access to offboard bailout gas through rebreather
for CC or OC use, etc...)  And the ability to dive trimix, switch to nitrox
at 130 on ascent, and then to pure 02 for deco, all in a package weighing
in at about 80 pounds (with adequate on-board gas for all OC bailout
deco) and with milspec design characteristics (read that several
million man-hours of open water use by the Navy) and I feel that the
equipment that would need to be carried to perform the same dive
profiles OC is actually more complex to manage (not to mention about
twice the weight and at least twice the drag). The added goodies of
not needing to carry argon for inflation (you don't get cold breathing
100 F gas) as well as the hydration issue are just free plus factors.
The bad side, of course, is that it requires additional training and
discipline, and proper predive inspection. The Mark-15 gets serviced
at home the day or two before the dive, is sealed and carried to the boat,
and is then usable for about  8 hours of diving by just filling
the 02 bottle and checking the diluent bottle (and maybe topping it off).
After returning home the rig has the sofnolime dumped, the sensors
removed and stored, and bottles filled. The electronics are simple
and robust, are not exposed to the environment, and are reliable.
I mean, it's less complex than a video camera, and when was the last
time yours just fried itself for no reason at all? Plus, if you understand
the system, the raw sensor voltage is displayed and you can manually
add 02 based on just that data for bailiut and deco, no problem.
The raw sensor data just requires that the wires from the sensors
to the secondary display are intact, and even if the electronics module
floods the raw data is still there. The system described is a US Navy
Mark-15 with modifications for tech use, but also describe Mark-16,
CCR-1000, and other rigs including one that we are building up using
a Russian IDA-71y as the chassis and adapting to US made electronics
and subsystems for additional capability.

Bottom line: If you showed your Hogarthian cave rig to a PADI diver,
he would saythat it is complex and unmanagable, but to you it is
streamlined and intuitive. As you describe the system to a new diver, he
will beginto understand it and feel the same way. It's the same with
rebreathers.
You might look at it and think it way too complex and difficult, but to
an exerienced rebreather diver it's as clear as day. The key to
using it intuitively is to use it all of the time, for dives where it has
no clear advantage other than the fact that you are building the
habit patterns needed to safely dive it on dives where it _is_ needed.
thus the seemingly disadvantagous use of such devices in 100
feet of water.  These devices make 300 foot dives really very
little different than 150 foot ones, save for deco. Much deeper dives
are possible, but now the limit really is deco, not the system of
life support. The only answer to that issue is the application of
transfer under pressure dry deco systems (IE: Closed bell) and
that is just off the shelf commercial diving technology. It's only
a matter of time before this is done to push deeper and longer.

Eventually, the difference between available tech techniques and
those used by industrial divers will be little other than if you are
paying or being paid....  In this vein, the 300 foot surface oriented
doves (as opposed to saturation diving) that we were doing some
20 years ago in the gulf were routine, hardly worth mentioning.
That was using the correct techniques (surface supplied semi
closed circuit rebreathers, hot water suits, open bottom bells,
and deck Sur-D deco). This stuff is not difficult to do, nor is it
-really- all that expensive. f you have access to a 60 foot boat
you can do it with relative ease and virtually absolute safety.

So why are we patting ourselves on the back and calling
ourselves special when what we do (wreck diving anyhow)
is really just using the -least safe- technique (SCUBA) when
a 300 foot wreck dive would be just another days work for
a diver using the correct technique? What we -are- doing is
deliberately making it hard to do, in a way, not unlike a rock
climber forgoing assisted climbing (polts and pitons) for
free climbing. We use a relatively inefficient technology as
as a result we need to be pretty good to stay alive. The
reward is a personal one. But the same dives can be done
much more safely and much more certainly using non-OC
scuba techniques, and CCR's are one of the steps towards
making it routine. Surface supplied techniques will not, of
course, work in caves, nor would it provide the sense of
accomplishment desired. But working in a nuclear reactor
inlet system, we were doing 1500 foot penetrations into
24 inch diameter pipes to get samples from the pump
impeller tips and it was routine. Sense of sport? No.
Easy? Yep. 1500 is not a lot, by cave standards,
but it's a start. The whole point of this being, of course,
that OC is not the end-all of techniques and selecting it
at the expense of even considering alternate methods
is a handicap from the start.


As for rebreathers, as prices fall and acceptance increases the days
of OC will be waning, promise you. Might be 10 years off, but
it's coming.


Dave Sutton







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