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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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