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Again, thanks for the response.

> Apparently we are becoming bogged down in semantics. When I refer to
> electrical devices I am including galvanic oxygen sensors. In Bill Stone's
> otherwise excellent recently published article about failure probabilities
> and  nonlinear redundancy, the sensors weren't mentioned. The reason that
> electronically controlled rebreathers have three sensors and sensor
> averaging circuits in the first place is because of the historical
> unreliability of oxygen sensors in marine environments. I can't speak from
> detailed knowledge of the Cis-Lunar, but on the CCR-1000's and decendants,
> there are five places on the secondary or passive display that water
> intrusion can occur and short out all three sensor outputs (only
> millivolts) simultaneously: two seals on the wire penetrator entering the
> display case, two seals on the rotary selector switch shaft penetrator
> entering the case and the analog meter seal. The analog meter that is
> directly driven by the sensor outputs selected by the switch is also a
> single point failure source without water intrusion. If it fails (keep in
> mind that it's on the end of a multiconductor wire outside the rebreather
> case and subject to many shocks), NO sensor outputs can be read. In
> addition the elastomeric cover on the wire bundle is a source of water
> intrusion shorts to all three sensor circuits if abraded, cut or punctured.

I've forwarded this to the Cis-Lunar engineers.  All good points, about 
which I am neither qualified, nor authorized to respond to on Cis-Lunar's 
behalf. I'll forward on any response from them.  Off the cuff, most of 
the failure scenarios you suggested have either been eliminated or 
reduced to extremely low probability in their rebreather.

> If any of these things occur, a life supporting diluent (which is extremely
> limited) is your only onboard bailout, and cannot be monitored with the
> secondary display. Even if you are still getting SOME reading from one or
> more sensors after a partial flood, that reading cannot be trusted.

It can only be trusted if it is verified.  The verification process 
assumes three things: 1) Boyle's law of partial pressures is reasonably 
reliable - at least to a level of precision that is physiologically 
meaningful; 2) you really do know, within physiologically meaningful 
precision, what the O2 percentage is inside the diluent; and 3) you know 
your depth (I always carry at least one backup depth gauge).  With these 
three things, and a well-designed rebreather, you can verify which, if 
any, of the O2 sensors are giving accurate readings.

I don't think we have a semantics problem, because when I say no 
electronics I also mean no O2 sensors. It is possible (and not too 
difficult) for a well-trained diver to maintain a life-sustaining breathing
mixture within the breathing loop of most 
fully-closed rebreathers without ANY electronics and without any O2 
sensors.

> The reason for using a rebreather instead of open circuit at depth is
> because it reduces the bulk of the primary breathing supply by a
> substantial margin. The bailout supplies would be required regardless of
> the primary breathing source. There are other advantages that you should
> ask George Irvine about.

Knowing how George does his dives, I suggest that with a rebreather, he 
will NOT be carrying enough gas on his person to make a full abort back 
to the surface with full decompression.  For his present penetrations on 
open circuit dives, he incorporates the use of stage bottles for 
decompression (correct me if I'm wrong, G).  I suspect that, if he 
followed your suggested standard of carrying all the OC bailout he needs 
on his person, he wouldn't get any farther into the cave with a 
rebreather than he does now with OC and stage bottles (Again, correct me 
if I'm wrong, G).

I only questioned the usefulness of a rebreather in the context of your 
suggestion that all OC bailout gas should be carried by the diver.  On 
"no-decompression" dives and dives with only a small amount of 
decompression, the rebreather can provide significant advantages of 
course.  But for the sorts of dives that I do, and I *think* the sorts of 
dives George does, carrying all the OC bailout gas throughout the dive 
deminishes the value & purpose of a rebreather.

Thanks for your continued input.

Aloha,
Rich

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