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Devon,
I think you have a valid concern when discussing the tendency of the
Draeger (and most other current rebreathers with or without computers) to
add gas (or shut off) independently of the diver's needs or control. At the
recent UHMS meeting in Palm Beach, FL. one of the discussions I overheard
concerned the gas addition mechanisms of rebreathers. The terminology used
by Dr. Morgan Wells and others was "active" vs. "Passive". The issues that
were discussed as potential rebreather problems were: hyperoxia, hypoxia,
and scrubber-related hypercapnia.
What happens with malfunctions? What happens if O2 sensors are inaccurate
(highly likely since they can't be calibrated above 1.0 ATA without a
chamber and are in a constant state of degradation). Are we supposed to
bring chambers along on every dive so we know the electronic units are
doing appropriate things to our gas supply?
To quote Jack Kellon, the developer of the RBC Odyssey and one of the
persons attending at UHMS:
"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 casued by
control of the previous exhalation.
Kellon and Wells indicated that a passive system would be safer in that
hypoia and hyperoxia of the type you discussed could not occur. As far as I
know no papers have come out concerning this but the logic is unassailable.
Consider the "insidious death" navy divers used to toast drinks to before
missions. It's now loosely (and more politically correct) defined as
"spontaneous unconsciousness" . Whatever we call it the cause was likely
either hypoxia or hyperoxia and the result was death. The fact that highly
trained navy divers could succumb makes one pause since they have always
used active addition rebreather systems similar to the Draeger, Prism, and
Oceanic (since no alternative existed at that time).
I've attached portions of an article Mr. Kellon says will be published
soon. In it he discusses the issues you raise and which absolutely must be
brought out before the recreational/technical birth of rebreathers turns
into a stillborn child. I've also attached a document showing the specs of
a passive system rebreather, the RBC Odyssey.
Dave
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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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