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On 26 Jan 1996, Dan Volker wrote:
>This year at DEMA...
snip
>The Realities
>
>They are quiet, no bubbles. Huge bottom times are "possible", and even
>at "extreme depths" on a few of the units, such as the Cis Lunar and
>the RBC Odyssey.
snip
>...more so than scuba...
snip
Dan, been very busy, so couldn't respond any sooner.
First, let's look at "no bubbles". Semi-closed rebreathers *ALWAYS* vent
some amount of gas (except sometimes on descent). There is no such thing
as "no bubbles" with a semi-closed system. There are a number of semi-
closed rebreather folk who make such a claim, but IT SIMPLY AIN'T TRUE!
In fact, systems that work on a similar method as the Odyssey tend to
vent MORE gas than some other semi-closed systems because it is always
venting a constant percentage of each breath, albeit depth compensated.
A closed circuit rebreather, on the other hand, is "bubble-free" except
on ascent, when the expanding gas has gotta go somewhere.
Secondly, "Huge bottom times" is an accident waiting to happen, or at
best, a disappointed customer. While semi-closed systems are more gas-
efficient than open circuit, "huge bottom times" are not possible without
"huge cylinders" and/or helium compatibility. Closed circuit systems, in
general, are more gas efficient than semi-closed.
Thirdly, saying that one can take a rebreather to "extreme depths"
without qualifying the statement is, IMO irresponsible or misinformed.
One of the common scary statements regarding Nitrox is "Let's get Nitrox
certified so we can go really deep." Most semi-closed systems use
Nitrox, which, when everything works, and you appropriately limit your
depth, is great. However, they are certainly not capable of "extreme
depths" per se. The only exception to this that I know of is non-
nitrogen based rebreathers. Is the Odyssey helium compatible? As an
aside, in rebreathers, the amount of O2 being breathed is less than what
is in the cylinder because some O2 is metabolically consumed. In
mechanical-only rebreathers a higher than required amount of O2 must be
in the breathing loop because the system does not "know" how much O2
that particular diver in his current situation is metabolizing. So there
are really two problems here, one is the well-known nitrogen problem,
and the other is a too high PPO2.
Fourthly, what do you mean "more then scuba"? A rebreather, even the
Odyssey is SCUBA. Surely you meant "open-circuit SCUBA".
>With closed circuit, you rebreathe *ALL* the gas,...(emphasis added)
Technically, Dan this is not correct. May be a nit-pick, but there are
some folks reading this who may not know much about rebreathers. For
their benefit; the diver does not rebreathe either CO2 (hopefully) or
the metabolized O2. The CO2 is removed in the scrubber (to be replaced
by heat and moisture in an exothermic reaction) and the metabolized O2
is replaced from the cylinder. And if you really want to nit-pick, a
very small amount of nitrogen (assuming a nitrogen based gas) is also
absorbed by the body. In semi-closed systems some amount of gas is
always vented, and therefore, not rebreathed.
>The real issue here would be the probability of an electronic failure,
>and if planned redundancy will work if electronically based. Some day
>this will be the answer---we don't know if it is today or not.
Dan, we think it is in the PRISM II which works in any of these modes:
-----------------------------------------------------------------------
Electronic | Electronic monitored, | Manual
control | manual control | control
--------------------------+----------------------------+---------------
Closed circuit | Closed circuit | Closed circuit
Semi-closed, constant % | Semi-closed, constant % | Semi-closed
Semi-closed, constant PPO2| Semi-closed, constant PPO2 | bail-out
| | Open circuit
-----------------------------------------------------------------------
The PRISM II comes with an independent PPO2 monitoring system that, in
the event of a main computer and/or mechanical controller crash, allows
the system to be flown manually. It also comes with a completely
independent dive computer for depth, bottom time, and air-based deco
computations. Available as an option is a completely independent backup
PPO2 monitor/dive computer with deco computations based on the actual gas
you are breathing and redundant data logging capability. Of course, you
can always fly the thing by the seat of your pants.
Dan also wrote some info about how the Odyssey works by sensing reducing
counterlung volume, and adding gas. My question is, how does the Odyssey
know about this if, during ascent, the counterlung volume is constant (or
expanding) in spite of the fact that the diver is consuming O2?
>And some units like the Cochran model, have elaborate CO2 sensors which
>further enhance the diver's ability to recognize a CO2 scrubbing
>failure.
Thanks for the plug, Dan, but I'd prefer to not exaggerate. FYI, it's a
single sensor, and non-elaborate, but functions quite well.
>On the positive side, a photographer can use a rebreather like the RBC
>Odyssey for 134 minutes on a 99 foot dive, all the while staying bubble
>-free and silent...
Dan, while one must admit that relative to open circuit there are fewer
bubbles, semi-closed systems, whether the Odyssey type or not, are *NOT*
"bubble-free", but we've been over this already.
Also, you refer to a "rebreather like the Odyssey for 134 minutes on a 99
foot dive". The 134 minutes could be a good number, but it just doesn't
seem right based on our limited knowledge of how the Odyssey works. What
cylinder size, cylinder pressure, gas percentages, and worker breathing
rate does this assume?
>...the rebreather can be used for "Massive bottom times" even at 300
>feet."
Dan, if you *REALLY* believe this statement, you shouldn't be discussing
rebreathers here. Not just ANY rebreather will allow the diver to do
this. You certainly are NOT referring to the current crop of Nitrox-
based, semi-closed systems. IMO, to use as described requires helium
compatibility and closed-circuit capability (like the PRISM II). <G>
But, it all depends on your definitions of "huge" and "massive".
Although not discussed much here, it's not always the "Massive bottom
times even at 300 feet" that gets you, it's the trip back up. Some
semi-closed rebreathers can be worse than open circuit in this regard.
There sure seems to be a lot of mis/dis/information and outright BS
being flung around about this rebreather subject.
Kind Regards, Mike C.
On Fri, 26 Jan 1996, Dan Volker wrote:
> >Dan, you still didn't answer my question about what is not safe about
> >the "other" rebreathers, including our PRISM II.
> >
> >Dan Volker wrote:
> >
> >>Wrong Dennis!
> >> The BMD had a hypoxic event, not the RBC, and the BMD had it because
> >> the instructor wanted to let a student...
> >
> >Dan, we heard two divers went hypoxic in the Miami area on a BMD and
> >that's why it was pulled from the market. You know any details?
>
> Mike,
> First, let me say once more, RBC is not BMD---they are VERY different, like
> Cochran and Uwatek. I have not been following BMD training in Miami, though
> I will check in to this for you. The way a BMD works, a hypoxic event would
> almost have to be diver error in using air instead of nitrox as gas supply.
> But without checking I can't contribute much on this.
>
>
> >
> >Re the RBC not having working units at Tek/DEMA, I understand and
> >sympathize about the shortage of working units when you're supposed
> >to be in production. As is all too frequently said, "Been there,
> >done that, won't do it again."
> >
> >>The handouts which you must have seen clearly showed the enormous
> >>difference in safety margins.
> >
> >Nope. Didn't get any. All I have is what was downloaded. Why don't
> >you publish the handouts here? Downloaded stuff didn't show "the
> >enormous difference".
>
> Once I catch up in my work (put off through Tek and DEMA), I'll visit the
> RBC guys and get more specifics coded and placed on the web site. And since
> this is a freebie, it will have to wait a week or so until I can free up the
> time. This thread alone is eating up my schedule, and I'm sure yours as well!
>
> >
> >>Safety is superior in the Odyssey because it does not rely on
> >>electronics which will potentially fail, and in so doing allow a
> >>diver to become hypoxic should there be a gas addition failure.
> >
> >So does this mean that if electronics is added to the Odyssey it
> >suddenly becomes unsafe? What if there existed a rebreather with
> >primary electronics control, but if the electronics crashed was
> >"safer" than the Odyssey?
>
> If you created a mechanical system with the electronics as extra warnings,
> that would interest me a lot. My favorite anology right now is the BC
anology.
> The RBC is more like a BC than the highly electronic units are. I have an
> old Seaquest Explorer BC with over 10,000 dives on it. Its not even what you
> would call a technical quality BC, but it's mechanical system is so simple,
> it is very reliable. If I were to acquire an electronic BC, that added or
> removed air bladder volume electronically, now I would have batteries to
> worry about , shorts to cause failure, and electric motor failure.
> Lets say it had a electronics in it to automatically detect if I was
> neutral or not, and it would automatically inflate or deflate just enough to
> regain neutral bouyancy. While this would be very cool if it was "failsafe",
> few of us would be comfortable at the potential risk of the unit going
> haywire and blowing us up to the surface, or any one of a half a dozen other
> problems this new electronic BC could cause. Of course, we could add a huge
> emergency dump valve (mechanical) and a secondary mechanical inflation
> bladder, but the convolutions are becoming extreme. Someday, someone
> probably will make one of these, and with Cochran's edge in elctronic
> technology, it will probably be you guys. But today, right now, ALL
> electronics used in diving is still prone to more failure than mechanical
> systems.
>
> And if we take a vote right now for the traditional mechanical BC,
> or the conceptual "Electronic BC", I believe most divers on the tech list
> will opt for the superior reliability of the mechanical one. But all of us
> would be thrilled if some techno-breakthrough in the next few years changes
> the reliability equation.
>
> Of course I use a dive computer. But I don't rely on it. Divers need to be
> computer assisted, not computer dependent. Your company may very well be
> leading the way towards the perfect electronics of tomorrow, but the
> electronic equipment "used" today needs to be for assistance. If dependance
> is on an electronic sensor or other electrical system, I would be nervous.
> Where you suggest having both mechanical control, and electronic backup, I
> think you will have the ideal system.
>
> >>Apparently you have never simulated hypercapnea. It is VERY apparent
> >>that your breathing rate is sharply elevated. CO2 is not really all
> >>that sneaky---as a human your breathing is controlled by CO2 levels-
> >>--you are the most dependable CO2 sensor you can ever find.
> >
> >Dan, can't believe you said that. A task-loaded diver could easily
> >overlook those symptoms. Furthermore, the huffing and puffing method
> >only works in shallower water.
>
> OK Mike, you do have a good point here about tolerance to CO2/O2 at depth.
> This is an area where as a diver, I know I have to swim at an even, steady
> pace at 280 feet, and not exert. The times I have had to exert, I have felt
> the sudden CO2 buildup, and felt the difficulty in ridding my system of it.
> In the old days, when I would have been doing this on air, the threat of O2
> tox was increased enormously by this large buildup of CO2, but still, I
> would always feel the increased CO2 levels, even with the diminished mental
> acuity that 280 ft on air brings. Now I am willing to agree with you an
> extra warning system would be desirable here, particularly when running as
> high a PO2 as I was when I did these 280 ft dives on air. Now that I am
> "enlightened" by the tri-mix god, my PO2's on the same 280 ft profile are
> low enough so that the "Real Danger" would probably not be instant blackout
> without warning. With the lower PO2's of trimix for this depth, the diver
> who is "forced" to exert will still build a high CO2 level, but will begin
> breathing very heavily, and know it...he/she will also be aware that either
> they must slow down, or if on a rebreather, and not exerting, assume a
> scrubber failure. The main problem with large CO2 buildup at 280 feet is a
> decreased ability by the body to remove CO2 at this depth. This is one of
> the reasons that all tech divers at deeper depths should be close to elite
> level aerobic athletes, to have the best internal gas exchange system
> possible.{ This is ONE of MANY reasons why the WKPP divers are among the
> best deep exploration divers --they make use of an optimal cardiovascular
> system.} And awareness of high PO2 levels will mean the diver should pop up
> about 50-80 feet (at least) higher in the water collumn if the exertion gets
> to be too severe to remove a dramatically high level of CO2. Of course cave
> divers will not have this option, so they need better cardiovascular
> systems, and better awareness of their exertion and CO2 levels.
> >
> >
> >That's enough for now. Got to go do some real work.
> >Mike C.
>
> ditto.
>
> Regards,
> Dan
> >
> Dan Volker
> SOUTH FLORIDA DIVE JOURNAL
> "The Internet magazine for Underwater Photography and mpeg Video"
> http://www.florida.net/scuba/dive
> 407-683-3592
>
> --
> Send mail for the `techdiver' mailing list to `techdiver@terra.net'.
> Send subscription/archive requests to `techdiver-request@terra.net'.
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