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Dan, what's the story on Odessey Rebreathers? Just heard they went 
belly-up and won't be selling the unit? Rod

On Fri, 2 Feb 1996, Dan Volker wrote:

>        With the last post to me by Mike Cochran, I felt the rebreather  
> arguements had gone far enough without bringing in some people that know 
> rebreathers as well as some of us know open circuit....I asked Jack Kellon 
> to put something down on paper that would explain the systems we are dealing 
> with in more detail, and end the misinformation.
> Below is the first of three articles he will be contributing to this list.
> Dan
> 
> 
> 
> 
>  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 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 hypoxia is likely 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.         Some systems, once turned on, are constantly adding a 
> predetermined   amount of supply gas to the breathing loop, regardless of 
> the diver's activity level/respiratory minute volume. In fact, the same 
> addition is made if the unit is sitting on the dock with no diver at all. 
>           
> 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. These units are keyed to RMV and make a 
> full   correction with every breath. No diver, no addition.
> 
>      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 unless mission requirements dictate it, such as the 
> longer duration cave penetrations that Dr. Bill Stone and his associates 
> conduct.  Effective dive risk assessment  and response for extreme exposures 
> in overhead environments usually requires weeks of study and equipment 
> preparation.
> 
> 2.   Hypoxia is a condition that occurs all too often in active addition 
> systems. Active addition systems are those that 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 that usually leads to 
> unconsciousness with little warning. In addition, oxygen fractions that are 
> safe at depth can cause hypoxia on ascent if the addition method has failed 
> or the counterlung is not purged.
> 
>      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  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 shorter, thus giving the diver 
> the same warning that he would get with open circuit SCUBA, but less abruptly.
> 
>      In fully closed circuit pure 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 abruptly though the overpressure relief valve on ascent than 
> 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 or through the diver interface 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.  Additionally, if the 
> addition regulator is located downstream from the scrubber, resistance 
> caused by any degree of flooding will 
> automatically by compensated for with whatever amount of fresh gas is 
> required to allow the diver to take a normal breath.
> 
>      The onset of hypercapnia is relatively easy for a well-trained 
> rebreather diver to identify in temperate water, primarily as a result of 
> the increased breathing rate. There are accidents on record where divers in 
> cold water attributed this effect to hypothermia. This
> is particularly unfortunate since scrubber efficiency is reduced 
> considerably by the cold.
> 
>      A side issue here is "caustic cocktails", the inhalation of water that 
> has entered the breathing loop and been passed through a caustic scrubber 
> bed, some materials being more caustic 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 as travel and decompression gases to be run through the 
> recirculation loop, thus requiring smaller cylinders.  The same sort of 
> organization on the diver's 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 things, but they should not be purchased or 
> used to enhance one's self-image. The additional risks are not adequately 
> offset by the trivial gain.
> 
>      3.   The most rigorous training possible should be so ught 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.
> 
> 
> Dan Volker
> SOUTH FLORIDA DIVE JOURNAL
> "The Internet magazine for Underwater Photography and mpeg Video"
> http://www.florida.net/scuba/dive
> 407-683-3592
> 
> --
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