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Dennis,
Sounds like you had a lot of fun. Stuart does have a nice operation there
and the wall dives are certainly excellent. Did you go on his shark dive as
well?
Thought you might like to hear about an alternative to the Draeger that is
now on the market and addresses some of the concerns many have about
rebreathers being in recreational/tech diver hands. The unit is the RBC
Odyssey a passive addition rebreather with a compound bellows and breathing
bag which is depth compensating so the bottom time will not be so variable
like with the Draeger Aquarius. It also addresses the problem of mass flow
precipitating hypoxic and/or hyperoxic "incidents".
I've enclosed two documents.
Dave.
>>Precedence: bulk
>>Errors-To: owner-techdiver@terra.net
>>Date: Mon, 17 Jul 1995 12:37:59 -1000 (HST)
>>From: Dennis Pierce <epic@so*.ha*.ed*>
>>X-Sender: epic@ak*
>>To: cWalter <hg334wa@rs*.un*.de*>, techdiver@terra.net
>>Subject: Re: Draeger Atlantis
>>
>>
>>Howzit all,
>>
>>got back from the Rebreather course and will finally post some info for you
>>all.. i tried to get online from nassau and couldn't, even tried to access
>>cis but couldn't get thru.. after about $20 in phone charges i gave up.. so
>>here goes, a little late...
>>
>>the diving was the best part... at stuart coves dive center nassau.. what an
>>opperation he has.. five boats 15 minutes from an incredible wall.. lots of
>>fish and wild life... wish we had those perfect training grounds. the first
>>ledge was at 45 feet then at 130fsw, 190fsw, 240fsw, 355fsw, and could almost
>>make out one past that but it could have a temp change..
>>
>>the unit: perfect for a rec level unit, worked best at 60 feet due to the
>>way it functions (constant mass flow). i averaged from 5 to 9 cf of gas use
>>at 60fsw for 50 minutes... not bad.. the best part is the silence of course,
>>you could hear the rubber one way valves opening and closing in the mouth
>>piece, and a very slight hiss of the constant flow reg.. but other than that
>>no noise. there is a diffuser on the top that covers a dry suit type over
>>inflation valve.. it's adjustable to control the breathing bag volume..
>>it was difficult at first to get this adjusted and we used alot of gas the
>>first day, but after we figured it out it was easy to control... a good
>>functioning unit would have a very small lline of bubbles flowing from the
>>valve once in a while. only upon assecent would you hear any bubbles at
>>all. the units were equipped with a four liter bottle but will come out
>>with a five or six liter. the canister is the same one used in the lar 5,
>>in fact a lot of the parts were the same. my unit was leaking on the second
>>day and we decided to put it thru the test and got back in the water to see
>>how long it would work with water inside. other than a little gurgling and
>>a slight increase in the taste of the dragersorb it worked fine.. in fact
>>i was suprised to see about two cups of salt water pour from the canister
>>when i took it apart. (i got 40 minutes at 80fsw)
>>
>>price: they (uwatec) started out at $6500 retail plus the cost ofthe
>>computer (which btw is the best part of the system.. if you havent seen
>>one of these units find a way, they are hot). we seem to have made an
>>impresssion on them that the price was too high, they are considering
>>a better offer to make the market. the training course is 40 hours with
>>six dives (it takes about three to get used to the unit, there are many
>>things you do automatically that arent necessary any more).. cost there:
>>from 600 to $1000 depending on the location and dives.
>>
>>when: september is the schedule time for 200 units in the us (more world
>>wide). but will be suprised if they make that date with that number.. we
>>gave them some ideas to take back to the drawing board (minor ones but).
>>
>>opinion: this is a great entry level rec unit.. super for photographers
>>(i spent most of my dive time floating head down looking at all the wild
>>life up close... you won't believe how much life runs from the bubbles
>>of open circuit). the unit is not controled by a sensor in fact there is
>>no electroics at all, but will be completely upgradable to the atlantis II,
>>and the a III... so any money spent now will not be lost on upgrades or
>>better
>>units. weighs about 44lbs without bcd (not needed, completely neutral, but
>>will be included in price) and bail out bottle. there is a sending unit on
>>the nitrox computer if you take the upgrade ($1200) that clocks consumption
>>and deco... but no o2 sensors (there is a place tho for the atlantis II
>>upgrage.. next year).
>>
>>if i left out any info that yo want please just ask, ill look it up..
>>my eyes are still crossed from the return flight (i sat by tia carrea
>>on the way out.. no such luck coming home)..
>>
>>there were instructors from chicago (really cool guy ricky thomas, black
>>magic dive shop), new york, canada (another super fellow named chester),
>>two from australia, john jordan from guam, bret, mitch, a super guy from
>>egypt, and others..
>>
>>gotta go..
>>
>>sea ya,
>>
>>dennis
>>epic dives hawaii
>>--
>>Send mail for the `techdiver' mailing list to `techdiver@terra.net'.
>>Send subscription/archive requests to `techdiver-request@terra.net'.
>>
>>
>Dan Volker
>407-683-3592
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"Safety Considerations In The Use of Rebreathers By The Technical Diving
Community"
J.Kellon
I've had numerous discussions lately with long time military and scientific
users of rebreathers that are now involved with the technical diving
community. The consensus of opinion is that although there are some very
qualified and capable divers out there, the community as a whole lacks the
level of ongoing training, discipline and level of support (including
diving operations officers to conduct rebreather operations that any of us
would consider acceptably structured and within the bounds of reasonable
risk.
My personal view is that hyperoxia can be reduced through avoidance of any
system that has a separate oxygen supply and hypoxia can be reduced through
avoidance of any system that has a separate oxygen supply and hypoxia can
be reduced through avoidance of active addition systems. Intensive high
quality training with some type of periodic recertification or review
requirement is also high on my list. Even with these constraints, risk
management still needs a considerable boost in this community. Hopefully,
we can minimize the number and severity of inevitable accidents that will
occur as this technology reaches a less disciplined group of users.
There are three basic fatality-inducing conditions that are peculiar to
rebreathers:
1. Hyperoxia (excess oxygen reactions) due to system malfunctions.
2. Hypoxia (insufficient oxygen reactions) due to system malfunctions or
rapid ascents.
3. Scrubber related hypercapnia (excess carbon dioxide reactions).
The degree to which these problems are present in a recirculating breathing
system depends on whether the system is:
A. An active addition system, in which the addition mechanism operates
independently of the counterlung gas volume available to be rebreathed.
OR
B. A passive addition system, in which addition is effected by a demand
regulator that replaces the shortfall in a counterlung caused by control
of the previous exhalation.
NOTE: A third consideration that affects the diver's operation of both
the above categories of systems is complexity. The more complex the system,
the more likely a failure will occur. In addition, some of the more complex
closed circuit systems (electronic oxygen controllers) have caused
accidents because of the diver's ability to activate an oxygen manual
addition valve while having less than full usage of his reasoning ability.
1. Rebreather divers are subject to the same time and depth restrictions
for oxygen partial pressures that open circuit divers must adhere to, but
the rebreather diver is subject also to equipment induced hyperoxia. This
is a particular danger in electronic, active addition, closed circuit mixed
gas units. Aside from basic electronic malfunctions, hyperoxia with these
units becomes more and more possible as the trend toward using higher
oxygen partial pressure set points (the point at which oxygen addition
ceases) escalates. Oxygen sensors should be calibrated before every dive
because they are constantly degrading with use (similar to a battery), but
can only be calibrated to 1 ATA without hyperbaric equipment (such as a
chamber). When set points such as 1.4 or 1.6 are used, the danger exists
that the sensor(s) are not capable of producing the electrical potential
necessary to satisfy the set point value. The result is that the unit will
inject oxygen into the system at regular intervals (generally every five
seconds) with resulting hyperoxia. Central nervous system oxygen poisoning
usually leads to convulsions that take several minutes to abate even when
the cause is removed. Plenty of time to drown in.
SOLUTION:
The only sure solution is not to use mixed gas units that have a separate
oxygen supply.
2. Hypoxia is a condition that occurs all too often in active addition
systems. Active addition systems are those that, both closed and
semi-closed, rely on the control mechanism to add the oxygen necessary to
meet the metabolic demands of the diver. Unfortunately, at a steady depth or
on ascent, the diver will be able to breathe normally even though an oxygen
addition malfunction has occurred. This is possible because carbon dioxide
is still being removed and the diver still has a full counterlung to
breathe in and out of. The result is a degrading oxygen partial pressure in
the counterlung(s) that usually leads to spontaneous unconsciousness. In
addition, oxygen partial pressures that are safe at depth can cause hypoxia
on ascent if the addition method has failed.
Fixed orifice, variable orifice and mass flow injection systems, and all
electronic mixed gas closed circuit systems are active addition systems and
subject to the problems described in the preceding paragraph.
Hypoxia is the condition, because of spontaneous or near spontaneous
blackouts, that has traditionally given recirculating breathing systems a
bad name. While this image is well deserved for active addition units,
passive addition systems should not be painted with the same brush.
SOLUTION:
The only sure solution is to use passive addition rebreathers. These units
rely on the lack of a full breath when the diver inhales to passively add
oxygen, nitrox, heliox or trimix to the breathing loop with a standard
demand regulator. If gas addition fails to occur, each successive breath
after the failure will be shorted, thus giving the diver the same warning
that he would get with open circuit SCUBA, but less abruptly.
In fully closed circuit oxygen rebreathers, addition occurs when metabolic
activity has depleted the counterlung contents. These units are very
reliable when proper counterlung purging and depth limitation requirements
are observed. If automatic addition does not occur, the diver is again
warned of the failure by the inability to get a full breath from the
counterlung.
In semi-closed circuit rebreathers passive addition is achieved by
controlling the diver's exhalation. There is a French unit that was
designed for military use that expels 20% of every breath into the water
with a double bellows arrangement. There is a Canadian unit that was
designed for pipe penetration bailouts that expels 25% of every breath with
a spring-loaded proportional discharge valve. The U.S. built RBC Odyssey
was designed for the civilian
tech/scientific/photographic/cave/wreck/advanced deep diving community and
expels 20% of every breath at the surface and less with depth because the
unit is depth compensated to optimize gas efficiency. None of these units
will cause hypoxia as a result of ascents. All of these units are keyed to
the diver's respiratory minute volume to tighten oxygen partial pressure
control. All of these units will warn the diver through successive
shortness of breath if an addition failure has occurred.
All the units in the preceding paragraph are absolutely silent in the water
unless counterlung overpressures created by very rapid ascent are vented.
The amount of gas discharged by the control mechanisms is too small to be
audible. All passive addition units vent less overpressure gas on ascent
that active addition systems if the diver is breathing normally.
3. All rebreathers share the problem of hypercapnia. Sloppy scrubber packs,
improperly stored absorbents, breathing loop water leaks at loose fittings
and divers trying to exceed the scrubber's rated time limits are all
contributing factors. Semi-closed passive addition systems (which discharge
upstream of the scrubber) have an advantage here because the discharged
gasses don't have to be scrubbed, thereby extending absorbent life.
The onset of hypercapnia is relatively easy for a well-trained rebreather
diver to identify, primarily as a result of the increased breathing rate.
A side issue here is "caustic cocktails", the ingestion of water that has
entered the breathing loop and been passed through a highly alkaline
scrubber bed, some materials being more alkaline than others. At this point
the loss of scrubber efficiency due to flooding becomes moot.
One of the attractions of some semi-closed units is that they allow outside
sources such travel and decompression gases to be run through the
recirculation loop, thus requiring smaller cylinders. The same sort of
organization on the divers part is required here that would be required on
open circuit equipment to keep from inadvertently breathing a gas that
would cause either hyperoxia or hypoxia at the depth the change is made.
As a rebreather designer with over thirty years of experience in the field,
I'd like to make some basic points that should be considered by every
prospective rebreather user:
1. No single unit is a panacea. Rebreathers should be chosen just as
carefully as technical divers choose their mixes, equipment and procedures
for any given dive.
2. By virtue of varying degrees of additional complexity and the
possibility of breathing a mixture from the counterlung that can cause
unconsciousness, rebreathers should be treated as tools that allow the user
to accomplish a desired goal that would not be practical or even possible
with open circuit equipment. In other words, REBREATHERS ARE NOT TOYS. They
can allow scientists, photographers and overhead environment divers to
accomplish remarkable thins, but they should not be purchased or used to
enhance one's self-image. The trivial gain is not adequately offset by the
additional risks.
3. The most rigorous training possible should be sought by the prospective
rebreather user, both from the manufacturer and nationally recognized
technical diver training groups. Knowledge, attitude and attention to
detail are more important to the diver's well-being in rebreather
operations than in any other type of diving.
Please dive safely.
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RBC Odyssey Passive Addition Semi-Closed Circuit Rebreather
General specifications
Method of Operation:
The "Odyssey" passive addition semi-closed circuit rebreathers were
designed to be used with a single gas source of nitrox or trimix containing
not less than 32% oxygen when being breathed at the surface. Using this gas
composition, the units are approximately five times more efficient in gas
utilization than open circuit equipment AT THE SURFACE. The relative
efficiency ratio increases with depth. The addition mechanism is keyed to
respiratory minute volume.
The units utilize an all new operating system that, unlike all other
semi-closed systems, is based on an automatic depth adjusting compound
bellows. The depth adjustment not only keeps both the primary and secondary
counterlungs equalized during depth increases, but also changes the ratio
between them. Thus, at the surface, approximately 1/5 of every breath is
discharged to ambient, to be replaced by a demand regulator on the
subsequent inhalation (passive addition). At 2 ATA this becomes 1/10, at 3
ATA 1/15, etcetera. This feature makes the Odyssey more efficient than any
other semi-closed rebreather and approaches the efficiency of mixed gas
closed units while eliminating the possibility of hyperoxia due to
electronic, solenoid, regulator or bypass valve failure. The possibility of
spontaneous unconsciousness due to undetected hypoxia is greatly reduced
because failure of the passive addition will result in the diver getting
less gas with each successive breath, thus providing the same type of
warning the diver would experience when open circuit equipment fails to
deliver gas.
The units maintain a counterlung oxygen percentage of 22% to 24%, depending
on the diver, regardless of depth, with a NOAA Nitrox I (32% oxygen)
supply. It is recommended that oxygen exposures by calculated based on 25%
O2 and nitrogen decompression exposures be calculated based on air. This
will change with other supply mixes.
FEATURES:
* 4 lb. to 8 lb. "Convertible" scrubber delivered standard. Other sizes
available on request. This is a side full scrubber. It is exceptionally
easy to fill or clean.
* 28 cu. ft. or 56 cu. ft. gas supply delivered standard. Other sizes
available on request.
* Standard "Wings-type" BCD to be mounted on unit. Other BCD's are optional.
* Gas fill port can be used to mount bailout regulator.
* Breathing loop adjustable over-pressure valve and adjustable regulator
can be reached without removing case cover.
* Hinged cover for easy scrubber access.
* Counterlung is weighted to minimize breathing effort variation during
diver attitude changes.
* Counterlung plus all gas discharge and addition components are located
within and protected by the case.
* Unit is neutrally buoyant and evenly trimmed.
* 1 year parts and labor warranty, exclusive of transportation costs.
* Fast service turnaround.
* Separate PPO2 analyzer included in base price.
TRAINING:
Unit-specific training is provided by the dealer and included in the base
price of $8,500.
Buyers are required to satisfactorily complete a Rebreather physics and
physiology course from a recognized technical diver training organization
such as the International Association of Nitrox and Technical Divers
(IANTD) or the Professional Scuba Association (PSA) prior to delivery of
the unit.
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