A learner's guide to
Closed-Circuit Rebreather Operations
by Richard L. Pyle
I have been using a Cis-Lunar Mk-4P mixed-gas, closed-circuit rebreather
since 1994 for exploration of coral reefs at depths of 200 to over 400 ft
/61-122 meters (the "Twilight
Zone"). On a recent expedition to Papua New Guinea, my diving partner
and I discovered nearly 30 new species of reef fishes as well as several new
invertebrates. Among the most important lessons I have learned about
decompression diving using rebreathers are: 1) the importance of knowing the
oxygen partial pressure in the breathing loop at all times; 2) vast amounts
of open-circuit diving experience does not help one learn how to dive with a
rebreather as much as a solid understanding of gas physics and diving
physiology does; 3) rebreather training regimes should place emphasis on
manual operation and bailout procedures; and 4) divers should always have an
alternate safe route to the surface, even in the event of a catastrophic,
unrecoverable breathing loop failure. I have developed an assortment of
protocols for conducting decompression diving using multiple diluent
mixtures with closed-circuit rebreathers, as well as procedures for various
emergency bailout scenarios. I believe that it is vitally important that
past, current and future rebreather divers maintain an open line of
communication in order to share experiences and techniques, in an effort to
minimize the potential for fatal or otherwise harmful accidents.
My interest in advanced mixed-gas diving technology, including
rebreathers, stems from my ongoing endeavor to document marine life
inhabiting deep coral reefs. Biologists using conventional air scuba have
been limited to maximum depths of about 130-190 ft /40-57 m for productive
exploratory work. Scientific research utilizing deep-sea submersibles has
primarily focused on habitats at depths well in excess of 500 ft /150 m. The
region in between, which I have referred to as the undersea "Twilight
Zone" (Fig. 1), remains largely unexplored (Pyle,
Montres Rolex S.A., 1996).
an effort to safely investigate this region, I designed an open-circuit
mixed-gas diving rig that incorporated two large-capacity cylinders, two
pony cylinders, five regulators, and a surface-supplied oxygen system for
Sharkey & Pyle, 1993). Using this open-circuit rig, Charles "Chip" Boyle
and I discovered more than a dozen new species of reef fishes on the deep
coral reefs of Rarotonga in the Cook Islands (e.g.,
Pyle & Randall, 1992). The extent of these discoveries was remarkable
not only because of the extremely limited amount of time spent at depth
(12-15 minutes per dive), but also because Rarotonga lies far from the
center of coral reef species diversity (Fig. 2).
Given the unexpected wealth of diversity in the "Twilight Zone", it was
clear that I would need to conduct dives with longer bottom times in order
to adequately explore this region, especially if I was to examine the deep
reefs of the more species-rich western Pacific. Unfortunately, transporting
large quantities of oxygen and helium to remote tropical islands can be
extremely expensive and logistically difficult, if not impossible. The
obvious solution was to use closed-circuit mixed-gas rebreather technology.
1994, Cis-Lunar Development Laboratories provided me with two of their MK-4P
closed-circuit, mixed-gas rebreathers, so that my diving partner John Earle
and I could continue exploration of deep coral reefs. After nearly a year of
training in Hawaii, we shipped the rebreathers to Papua New Guinea for a
series of exploratory dives on the deep reef drop-offs. Diving from the M/V
Telita (live-aboard vessel), we logged a total of 96 hours on the
rebreathers, including 28 trimix dives to depths of 200-420 ft/61-122
meters. Although we only intended to conduct preliminary observations during
this expedition, we nevertheless discovered nearly thirty new species of
fishes and several new invertebrate species (e.g.,
Gill et al., in press;
Earle & Pyle, in press;
Allen & Randall, in press;
Randall & Fourmanoir, in press).
Staying alive on a closed circuit rebreather
Having spent the past two years developing my own procedures and
protocols for decompression diving using closed-circuit rebreathers, I have
learned some important lessons (Comper
& Remley, 1996;
Pyle, 1996d). After my first 10 hours on a rebreather, I was a real
expert. Another 40 hours of dive time later, I considered myself a novice.
When I had completed about 100 hours of rebreather diving, I realized I was
only just a beginner.
Now that I have spend more than 200 hours diving with a closed-circuit
system, it is clear that I am still a rebreather weenie. In my experience,
the underlying quality that divers must have to consistently survive
rebreather dives is discipline. The first step in exercising this
discipline is to realize that it takes a fair amount of rebreather
experience just to comprehend what your true limitations are. You should
leave a wide margin for error between what you think your limitations are,
and what sort of diving activity you actually do. To help new rebreather
divers survive the early overconfidence period, I offer these suggestions:
Know your pO2 at all times
Without doubt, the single most hazardous aspect of closed-circuit
rebreathers is the fact that the oxygen content of the breathing mixture is
dynamic. With open-circuit scuba, inspired gas fractions are constant. Thus,
as long as gas mixtures are not breathed outside their respective
pre-defined depth limits (assuming proper filling and mixture verification
procedures have been followed) an open circuit diver can be confident that
the inspired gas is life-sustaining.
One of the fundamental advantages of closed-circuit rebreathers is their
ability to maintain an optimal gas composition at all depths. However, the
disadvantage of this dynamic gas mixture system is the potential for oxygen
content to drop below or exceed safe levels without any change of depth. The
real danger is the insidious nature of hypoxia and hyperoxia. Neither malady
has any reliable warning symptoms (although see
Pyle, 1995), and both can be deadly in the underwater environment. It is
therefore of utmost importance that rebreather divers always know the oxygen
partial pressure inside the breathing loop.
Simply checking the primary electronic instrumentation on a regular basis
is not sufficient. Most electronically-controlled rebreather designs
incorporate at least three oxygen sensors, and most will provide divers with
at least two different displays of the oxygen sensor values. Many people
refer to these as the "primary" and "backup" displays; however, I prefer the
term "secondary" to "backup" because most backup equipment is used only
after the primary component has failed. Instead, the secondary oxygen
display of a closed-circuit rebreather should be monitored almost as
regularly as the primary display, to verify that both displays are giving
the same value.
Ironically, the most reliable rebreathers can potentially be the most
dangerous to an undisciplined diver. If the primary oxygen control system
virtually never fails, then a diver may become complacent about checking the
secondary display. Due to the oxymoronic nature of the phrase "fail-safe
electronics" (especially in underwater applications), complacency of this
sort can have disastrous consequences.
2. Open-circuit scuba
experience is not as useful for rebreather diving as a good grasp of diving
physics and physiology.
Many experienced open-circuit divers who are new to rebreathers may fall
into the "trap" of overconfidence. While vast amounts of open-circuit diving
experience can increase a person’s over-all comfort level in the water and
enhance one’s respect for the hazards of sub aquatic forays, these qualities
alone are insufficient for consistent rebreather survival. Diving with
closed-circuit rebreathers differs considerably from open-circuit diving in
many respects, ranging from methods of buoyancy control, to gas monitoring
habits, to emergency procedures. Development of the proper knowledge,
skills, and experience takes time and practice, regardless of how many
open-circuit dives (mixed-gas or otherwise) one has successfully completed.
What is probably the most dangerous period in any rebreather diver’s
learning curve occurs relatively early on; after enough time to be
comfortable with the basic operation of the unit, but before there has been
enough practice and experience to adequately recognize problems and correct
them before they become serious (the period when one’s confidence exceeds
one’s abilities). In some ways, experienced scuba divers may be at greater
risk than non-divers when learning how to dive with a rebreather for the
first time, because the initial discrepancy between confidence and abilities
will be larger.
On the other hand, a good working knowledge of gas physics and diving
physiology is probably more important for rebreather diving than for
open-circuit mixed gas diving. Well-designed closed-circuit rebreathers will
provide users with many ways to control the gas mixture in the breathing
loop, and divers must have an intuitive understanding of the effects their
actions (gas additions, loop-purges, depth changes, etc.) will have on their
breathing gas and decompression status. With the additional control a diver
has over the inspired breathing mixture in a closed-circuit rebreather,
comes the need for greater discipline and understanding of the dynamics
3. Training should emphasize
failure detection, manual control and bailout procedures.
Diving with closed-circuit rebreathers is relatively easy when the system
is functioning correctly. Recognizing component failures before they lead to
serious problems and knowing how best to respond to various failures is a
bit more tricky. The solution to problem response is fairly
straight-forward: training regimes should include a great deal of time
simulating failure situations and practice of appropriate response actions.
Manual control of the rebreather is probably the most important skill to
learn; in fact, I recommend that new rebreather divers
first learn to
control the unit manually, and only be allowed to activate the automatic
control system after manual control has been mastered. Unfortunately, even
the most well-practiced skills, and all the best backup systems in the
world, are completely useless to an unconscious diver. Thus, perhaps even
more important than knowing how to respond to a problem is knowing how to
recognize a problem before it is too late.
The most critical failure conditions a rebreather diver may encounter are
hypoxia, hyperoxia (due to failure of the oxygen control system), and
hypercapnia (due to failure of the absorbent canister). Although the former
two do not provide any reliable physiological warning, some people in some
circumstances may detect symptoms of hypoxia or hypercapnia prior to
blackout or convulsion.
Text descriptions of possible "pre-cursor" symptoms might help but, as
any teacher knows, first-hand experience is much more useful. The question
is: should a rebreather diver be exposed to hypoxia and hyperoxia under
controlled conditions during training? (Obviously, "controlled conditions"
would not include a diver experiencing these things underwater, or without
trained supervision.) Hypoxic symptoms probably occur with more consistency
than hyperoxic symptoms. Furthermore, hypoxia can easily be experienced on
dry land using a rebreather with a disabled oxygen injection system, whereas
hyperoxia (to the point of convulsion) would require a hyperbaric chamber.
Therefore, it seems that experience with hypoxia would be both more
useful and logistically more feasible during a training regime than
experience with hyperoxia would be. Nevertheless, even for hypoxia the
answer to the question is not obvious. While having first-hand experience
with symptoms might save a diver’s life in some situations, it might also
falsely boost a diver’s confidence in his or her ability to detect the onset
of such conditions (i.e., induce complacency). Another consideration is that
any exposure to hypoxia likely results in the death of brain (and other)
cells. Thus, even with the discipline to avoid the complacency problem, it
is not clear whether the benefits of first-hand experience of possible
warning symptoms outweigh the cost of lost brain cells during a "hypoxia
experience" session. In my case, I believe the experience was well-worth the
Less ambiguous is the issue of hypercapnia. Although testing by the U.S.
Navy indicates that symptoms of hypercapnia cannot be considered as reliable
pre-cursors to blackout, the experience of several civilian rebreather
divers (myself included) indicate that they can be considered reliable. One
possible explanation for this discrepancy of experience may be individual
variation. Perhaps some individuals (e.g., so-called CO2-retainers")
cannot reliably detect the onset of hypercapnia, while others (perhaps
including the aforementioned civilian rebreather divers) can. If this is the
case, it makes a great deal of sense to include deliberate exposure to
hypercapnia (again, under controlled conditions) as part of a rebreather
training regime. This can easily be accomplished on dry land by breathing
off a rebreather without a carbon dioxide absorbent canister installed.
4. Cover your ass.
This is probably the most important piece of advice that my rebreather
instructor, Bill Stone, gave to me. This point doesn’t need much
elaboration, but is nevertheless vital to rebreather survival. It is
fundamentally the same principle that all cave divers and mixed-gas divers
should already understand: always have an safe alternate pathway back to the
surface. For open-circuit divers, this usually means a second regulator and
following "rule of thirds" for gas consumption.
On rebreather dives, especially those requiring extensive decompression,
the logistics of providing for an alternate means to safely return to the
surface, even in the event of catastrophic, unrecoverable breathing loop
failure, can be difficult. See the section on bailout procedures below for a
description of some of the solutions I have developed for my rebreather
Protocols for Closed-Circuit Rebreather Diving
Procedures and protocols for closed-circuit rebreather diving will vary
according to specific rebreather models and specific diving conditions and
objectives. In this section, I will outline the procedures and protocols
that I have developed for rebreather model I use, in the environments that I
I. System Configuration & Equipment
A. Diluent Supplies
1. Dives Without Required
Most closed-circuit rebreather dives that do not involve ‘required’
decompression stops will be conducted using a single diluent gas (usually
nitrogen or helium). If only one non-oxygen cylinder is carried by the diver
on such a dive, that cylinder must be accessible via an open-circuit
regulator, and the mixture in that cylinder must contain a fraction of
oxygen that will sustain the diver at all depths during the dive (air is
usually the easiest choice). Furthermore, that cylinder must be of
sufficient capacity that all buoyancy control gas, drysuit gas (if
applicable), and rebreather gas needs are met with enough remaining that a
safe, controlled ascent to the surface in open-circuit mode can be
accomplished with sufficient margin for error at any point during the dive.
2. Dives With Required Decompression
Rebreather dives that require substantial decompression times often
(although not always) involve more than one diluent gas type (usually
nitrogen, helium, and/or a combination of both). More often than not, it
would be entirely impractical for a diver to carry a large enough gas supply
to complete full decompression in open-circuit mode. This leaves two
options: 1) the diver carries a completely independent rebreather system
(including independent breathing loop, counterlung, and absorbent canister);
or 2) the diver carries enough gas supply to safely reach a staged
life-support system (e.g. another rebreather, more open-circuit gas supply,
an underwater habitat, etc.) while breathing the carried gas in open-circuit
The difficulty with option number 1 includes not only the problem of
physical placement of the secondary rebreather, but also the need to monitor
and control the gas content within the secondary breathing loop during depth
changes. More frequently, one form of option number 2 will be used, in which
case much thought must be given to the question of how much of each type of
gas will be carried by the diver, and how much will be staged. There are
many variables that affect this ratio, including whether or not buddies can
be relied upon for auxiliary open-circuit gas supplies, whether or not full
face masks are used, whether there is a guideline physically connecting the
diver with the staged gas supply, maximum depth and duration of the dive,
strength of current, among many others.
The oxygen content of the diluent gas mixture(s) should be such that the
diver has access to at least one life-sustaining mixture in open-circuit
mode at any point (depth) during the course of the dive. Choosing a diluent
configuration to optimally meet the needs of the dive is among the most
difficult aspects of decompression diving with rebreathers.
I have experimented with a wide variety of con-figurations, and have
settled upon one basic configuration that I use for almost all dives to
depths in excess of about 220 ft (66 m), with total ‘required’ decompression
times exceeding about 15 minutes. This configuration includes a total of 80
cubic feet (cf) of gas in three cylinders: one 20 cf "on-board" cylinder,
and two 30cf "off-board" cylinders. One of the 30 cf cylinders will contain
a trimix that is safe to breathe at the maximum possible depth of the dive.
The other two cylinders will include one with air, and one with heliox-10
(10% oxygen, 90% helium); which of these two gases that is in the 20 cf
"on-board" cylinder and which is in the 30 cf "off-board" cylinder will
depend on the planned decompression profile of the dive. The placement of
the staged gas cylinders will depend on a variety of factors (discussed
below under the "Bailout" section).
B. Oxygen Supplies
Most dives without ‘required’ decompression stops can be safely
accomplished using only one oxygen cylinder. If the single oxygen cylinder
is accessible via open circuit mode, then dives with limited ‘required’
decompression can also be conducted safely with a single oxygen supply
(limited by whether or not the oxygen supply can sustain the diver in
open-circuit mode for the duration of the shallowest decompression stops,
with appropriate margin for error).
Although dives requiring extensive decompression can be conducted with a
single oxygen supply (provided a large supply of open-circuit decompression
gases can be reliably accessed in an emergency bailout situation), it is
usually better to carry a backup oxygen supply on such dives. If any part of
a single oxygen delivery system fails on a closed-circuit rebreather, then
the diver will essentially be forced to conduct an open-circuit bailout (or
perhaps some form of semi-closed circuit bailout), at least for as long as
it takes to access a staged rebreather oxygen supply. For dives requiring
extensive decompression, I carry two independent oxygen supplies, both
contained in 13.5 cf cylinders. Either cylinder contains enough oxygen to
complete the entire dive in closed-circuit mode, and both can be accessed in
open-circuit mode should the need arise.
C. Full Face Mask Considerations
The question of whether or not a full face mask should be used on a
rebreather dive depends on several factors; primarily whether or not
electronic through-water communications systems are to be used, whether or
not the dive is conducted solo or with other divers, and to what extent a
diver must "go blind" in order to access additional gas supplies (either
closed-circuit or open-circuit). In most cases, a full face mask is
preferable, but there are some costs to using them.
Obviously, if the dive requires electronic through-water communications,
a full face mask is probably needed. A full face masks can mean the
difference between life and death if the diver blacks out due to hypoxia or
hyperoxia, but this advantage is diminished if the dive is to be conducted
solo (especially with regard to hypoxia) or with an inattentive buddy.
Conversely, a full face mask can increase the risk of drowning if the diver
has to "go blind" by removing the mask in order to access additional gas
supplies (if the need to access an open-circuit bailout gas supply arises,
it is likely to be the least convenient moment to lose one’s ability to
This hazard can be minimized to some extent by masks and mouthpieces that
allow access to additional gas supplies without the need to remove the mask
(or the part of the mask that allows the diver to see). In any case, divers
should carry a spare conventional mask if a full face mask is to be used.
Once the decision to use a full face mask has been made, an additional
consideration is what sort of mask to use. Some full face masks have a
single airspace that includes the eyes, nose and mouth. Others divide the
airspace into two isolated compartments; one for the mouth, and one for the
eyes and nose. This latter type of mask (often referred to as a "half-mask")
is preferable for rebreather diving for three main reasons.
First, a single-compartment full face mask increases the amount of "dead
space" in the breathing loop (especially if an oral-nasal cup is not sealing
properly), which increases the risk of carbon-dioxide build-up in the mask.
Second (as is detailed below), a convenient way to vent excess gas from the
breathing loop is by exhaling through the nose; if the compartment that
seals the diver’s nose is part of the breathing loop, then the excess loop
gas must be vented by some other means. Third, the entire mask can serve as
a diaphragm, contracting and expanding on inhalation and exhalation,
increasing the overall work of breathing (Rod Farb, personal communication).
The relative costs and benefits of full face masks must be taken into
account for each different set of dive parameters.
D. Emergency Line and Float
Each diver carries a reel with line, and an emergency float of some sort.
The length of line on the reel depends primarily on the depth of the dive,
and the depth of the first "required" decompression stop, but is usually a
minimum of 200 ft (60 m) in length. The ideal emergency float for the sorts
of dives I do is inflatable, cylindrical in shape, about 3-6 ft /1-2 m in
length and 2-6 inches /5-15 cm in diameter, is bright orange in color, and
has an overpressure relief valve. It is often useful to have a small slate
with its own pencil attached to the emergency float. This float is used
mainly to alert the surface-support personnel that a diver has commenced a
bailout from a dive (see discussion below).
For all dives involving substantial decompression, additional equipment
associated with the surface-support vessel is usually needed.
1. Decompression Line
A basic decompression line includes a relatively large float, a
relatively thick line, and a weight. The length of the line depends on the
decompression profile expected, but is usually at least as long as the depth
of the first anticipated "required" decompression stop. A float is attached
at one end of the line, and a weight, not exceeding 10 lb. (2 kg) is tied to
other end. The end with the weight also has a large clip of some sort
(ideally a stainless steel, slip-locking carabiner). Sometimes markers or
loops are placed at 10-ft/3 m intervals along the line. This line serves as
the decompression "station" (to which additional equipment or gas supplies
may be connected), and may or may not be deployed prior to the start of the
2. Open Circuit Gas Supplies
a. Self-Contained Gas
It is always a good idea to keep extra supplies of breathing gas aboard
the surface-support vessel in case of an open-circuit bailout situation. In
most cases, supplies of both oxygen and oxygen-nitrogen mixtures (air or EAN)
should be on hand, and mixtures incorporating helium may be needed for more
extreme dive profiles. In some cases, some or all of this gas will be staged
underwater prior to the dive, but in other cases, it will remain in the
surface support vessel until (and if) it is needed. Of critical importance
is that the diver can reliably reach additional gas supplies, with at least
a 30% margin for error, should the need arrive. If only one diver is
conducting a decompression rebreather dive (i.e., a solo dive), the volume
of total gas supply should be twice that required by the diver for a
complete decompression on open circuit. If two divers are conducting the
dive simultaneously, then the total supply should be three times the amount
that any one diver would need to complete decompression in open-circuit
mode. Teams of three or more divers might require even larger gas supplies.
b. Surface-Supplied Oxygen
The emergency open-circuit oxygen supply could include a surface-supplied
oxygen system. Such a system reduces the bulk of equipment in the water,
which can be beneficial for extended shallow-water decompression stops
(especially for in-water recompression treatment of Decompression Sickness [DCS]).
A full discussion of these systems is beyond the scope of this article, but
it should be noted here that if two or more divers are conducting
decompression dives simultaneously, there needs to be at least one
self-contained oxygen supply per diver to guard against the unlikely event
that two or more separated divers simultaneously need additional supplies of
3. Other Equipment
Most other equipment for decompression dives using closed-circuit
rebreathers will depend on the particular objectives and environmental
conditions of the dive. Two items that most divers should carry are a sharp
cutting tool, and one or more sets of decompression tables. The knife should
be small and easily accessible by either hand, and the decompression tables
should include a variety of depth and bottom-time contingencies, as well as
schedules for both closed-circuit (constant oxygen partial pressure) and
open-circuit (constant oxygen fraction) decompression with available gas
In addition to general gas mixing, equipment testing, rig preparation,
team briefing, and other obvious pre-dive activities, rebreather divers
should perform several additional pre-dive routines.
A. Loop Leak Test
An essential pre-dive test for any rebreather is a loop leak (or
"positive pressure") test. This step involves adding gas to the rebreather
loop until the over-pressure relief valve vents, and observing for a
subsequent drop in remaining loop volume or pressure that might indicate a
poorly sealed connection or leak somewhere in the breathing loop.
B. Oxygen Control System Test
Another test prior to commencing the dive is a verification of the oxygen
control system function. Minimally, this test involves flushing the loop
with diluent, activating the oxygen control system, and verifying that the
solenoid fires correctly. If the unit allows the user to easily adjust the
PO2 set-point, the test could be conducted with a low set-point
(such as 0.3 atm) to verify that the solenoid stops firing after set-point
has been achieved. If this latter test is conducted, it is imperative that
the PO2 set-point be returned to the correct value prior to the
C. Final Checklist
Beyond the standard checklists frequently used by open-circuit mixed-gas
decompression divers, a separate checklist should be developed specifically
for the particular rebreather unit that is to be used. Minimally, this
checklist should include verification of absorbent type and remaining
canister life, accurate oxygen sensor calibration, correct PO2
set-point, oxygen and diluent cylinder pressures, diluent gas composition(s),
and correct position (open or closed) of all valves in the system.
Additional model-specific verifications may also be required for certain
If the descent is abrupt (i.e., a straight, fast descent to depth), the
breathing loop should be flushed with diluent prior to commencement of the
dive. If the oxygen partial pressure is allowed to increase at the surface
prior to the dive (for example, by the action of the oxygen injection
solenoid), there is a risk that the oxygen partial pressure in the breathing
loop will exceed safe levels during a rapid descent. Correction for this
would involve flushing the loop with diluent at depth, which results in an
unnecessary loss of potential open-circuit breathing gas supply.
If the dive is to be conducted with only helium and oxygen in the loop
during the deep portion of the dive, the loop should be flushed with heliox
before beginning the descent. Some people (myself included) have experienced
impaired concentration when breathing heliox at depths in excess of about
250 ft /75 m following rapid descents. This impairment seems to be
alleviated when the nitrogen partial pressure in the breathing loop is
maintained at about 2.5-3.0 atm (less than the level at which significant
narcosis is usually experienced).
There are two basic methods of introducing trimix into the breathing
loop. The most obvious is to use a blend of trimix as the diluent supply.
The advantage of this method is that the helium-to-nitrogen ratio remains
relatively constant; the disadvantage is that nitrogen partial pressure in
the breathing loop increases with increasing depth (hence, the trimix must
be blended for the maximum depth of the dive, and will be ideal only at that
maximum depth). A less obvious method is to blend trimix from separate air
and heliox diluent supplies. With this method, the descent begins with a
loop full of air, and air as the diluent supply. Upon reaching a depth of
about 100 ft /30 m, and allowing the oxygen partial pressure to achieve
set-point, the diluent supply is changed to heliox and the descent
continues. This results in a relatively constant partial pressure of
nitrogen in the breathing loop (calculated as [ambient pressure at time of
diluent change] minus [oxygen partial pressure at time of diluent change]).
The advantage of this method is that the nitrogen partial pressure does
not increase with increasing depth. The disadvantage is that there may be
deviations from the predicted nitrogen partial pressure in the event of loop
volume fluctuations and loop gas venting (as from mask clearings, etc.).
Combinations of these two methods are also possible, but it is vitally
important that, whichever method is followed, the software used to generate
the decompression profiles (both for real-time decompression and backup
decompression tables) take into account the predicted fluctuations of the
IV. System Monitoring &
The most critical variable to monitor on a closed-circuit rebreather is
the oxygen partial pressure in the breathing loop. The PO2
set-point of the oxygen control system should be no less than 0.5 atm, and
no greater than 1.4 atm. The lower limit maintains a margin for error above
hypoxic levels, and the upper limit maintains a margin for error below
dangerously hyperoxic levels.
Although some standards allow for inspired oxygen partial pressures as
great as 1.6 atm, such partial pressures would be unsafe set-points on a
closed-circuit rebreather for two reasons. First, oxygen partial pressures
in the breathing loop can "spike" above set-point during short, rapid
descents; and second, rebreather divers should incorporate a more
conservative upper oxygen partial pressure limit than open-circuit divers
due to the fact that the diver is exposed to that partial pressure
throughout the entire dive (as opposed to open-circuit dives, where the PO2
limit is experienced only at the deepest depth of each breathing mixture).
Each rebreather diver should become intimately familiar with the rates at
which their metabolism affects the oxygen partial pressure within the
breathing loop at different levels of exertion, on the specific rebreather
that diver intends to use. For example, with the oxygen control system
disabled on the rebreather model that I use, the oxygen partial pressure
will drop from 1.4 atm to 0.2 atm over the course of about 30-40 minutes at
low to moderate exertion levels. My diving partner consumes oxygen at about
twice the rate I do at a given workload, and thus causes the same PO2
drop to occur in about 15-20 minutes at the same exertion level.
Once a diver knows the oxygen consumption rates, the PO2
levels in the loop should be checked with a frequency no more than one-half
the amount of time it would take for the PO2 to drop to dangerous
levels. For the example above, if the PO2 setpoint was 1.4 atm, I
would check the PO2 in the breathing loop at least every 15
minutes, and my diving partner would check his at least every 7 or 8
minutes. The PO2 should also be monitored during and after every
substantial depth change.
Divers should also be in the habit of frequently comparing the primary PO2
display with the secondary PO2 display, should note whether or
not all oxygen sensor readings are in synchrony, and should note whether the
readings are dynamic or static (static readings are often indicative of some
sort of oxygen sensor failure). Some rebreather designs allow divers to
verify that sensors are providing correct readings; such tests should be
performed periodically throughout the dive, and whenever some reason to
doubt about the accuracy of the readings presents itself.
B. Gas Supplies
Although cylinder pressures are of critical importance to open-circuit
divers, they are somewhat less critical to closed-circuit rebreather divers.
Diluent supply pressure(s) should be monitored to ensure a safe open-circuit
bailout can be performed at any point during the dive. Oxygen supply
pressure(s) should be monitored to ensure there is a sufficient quantity of
oxygen remaining in each oxygen cylinder to complete the remainder of
the dive in closed-circuit mode (with a comfortable margin for error).
C. Remaining Absorbent
The amount of time that a given canister of carbon dioxide absorbent will
sustain a diver should be clearly and confidently known prior to the
commencement of any dive. For dives requiring substantial decompression,
there should be at least a 50% margin for error and preferably a 100% margin
for error (i.e., an absorbent canister should be able to last one and a half
to two times the predicted total dive time).
In the absence of reliable carbon dioxide sensors, the ability to
reliably predict the remaining life of an absorbent canister can be
difficult. The most frequently-used method is a simple "clock" of how much
dive time is spent using a particular canister of absorbent. Unfortunately,
the rate of this clock can vary among different divers and different
workloads by as much as a factor of ten. In the same amount of time that one
diver may have completely exhausted the canister, another diver may have
used up only 10% of the active life of the absorbent (considering the
maximum possible extreme cases).
An alternative method of monitoring canister life is to monitor the
amount of oxygen consumed. This includes the total volume of oxygen entering
the loop, both from oxygen and from diluent supplies. Calibration of this
value should be done empirically under controlled conditions (i.e., minimal
venting of gas from the breathing loop), with each particular canister
design of each particular rebreather (values cannot necessarily be
extrapolated based only on volume of absorbent material). A sample size of
empirically-derived values should be large enough such that scale of
variation can be inferred. Venting of loop gas during dives (e.g., ascents,
mask clearings, etc.) will result in a more conservative estimation of
remaining canister life. If done correctly, this method of canister life
prediction is probably among the most accurate (assuming consistent and
proper canister packing techniques and absorbent quality).
Divers should be on the alert for potential symptoms of hypercapnia
(e.g., shortness of breath, headache, dizziness, nausea, a feeling of
"warmth", etc.) during all phases of the dive. If such symptoms are
suspected, the dive should be immediately terminated and the ascent should
commence. Short-term relief of symptoms following an ascent should not be
interpreted as evidence that the canister is functioning properly, because
ascents will inherently lead to a short-term drop in the carbon dioxide
partial pressure in the breathing loop, and often involve a concurrent
reduction of workload (i.e., CO2 production rate).
Hypercapnia symptoms might also be a result of improper breathing
techniques (i.e., the "skip-breathing" pattern that many scuba divers do,
which, of course, confers absolutely no advantage to a rebreather diver).
Canister failure can be tested with short-duration periods of high exertion
(in shallow water). If a diver feels unusually "starved for breath" after
such short bursts of exertion, the canister is probably near the end of its
effective life (note, these periods of high exertion should be kept brief,
so as not to unnecessarily waste remaining absorbent life). As discussed
earlier, it is probably beneficial for rebreather students to undergo
first-hand experience with hypercapnia symptoms as part of their basic
D. Loop Volume
The volume of gas contained in a rebreather loop (the hoses, canister,
and counterlung(s) of the rebreather plus the diver’s lungs) is seldom
fixed. I define "minimum" loop volume as that volume of gas occupying the
rebreather loop when the counterlung(s) are completely "bottomed-out", and
the diver has completely exhaled the gas from his or her lungs. Conversely,
"maximum" loop volume is the volume of gas in the breathing loop when the
counterlung(s) are maximally inflated, and the diver has maximally inhaled
gas into his or her lungs. Although the magnitude of the difference between
these two volumes, ([Vmax] [Vmin]), will vary from one rebreather design to
another, it will always be non-zero.
Rebreather divers must learn to maintain the loop volume close to its
optimal level for their particular model of rebreather. If the volume is
maintained too close to Vmin, the counterlungs will tend to "bottom-out" on
a diver’s full inhalation. If the loop volume is maintained too close to
Vmax, the overpressure relief valve will tend to vent excess gas at the peak
of a diver’s full exhalation. Furthermore, total loop volume will influence
work of breathing due to hydrostatic effects.
On rebreather models with a relatively large value of ([Vmax] [Vmin]),
the optimal volume should ideally be closer to Vmin; for models with a
relatively small value of ([Vmax] [Vmin]), the optimal loop volume should be
ideally close to the mid-point. In either case, the diver should maintain
the loop volume at whatever level results in the minimum total work of
breathing and gas loss.
Scuba divers have two main components of "compressible buoyancy"; namely,
the buoyancy compensator, and the thermal protection suit. Rebreather divers
add to this a third component of "compressible buoyancy"; the breathing
loop. Many rebreather divers utilize fluctuations in breathing loop volume
as fine-tune control of buoyancy. To maintain a constant PO2 in
the breathing loop and a constant loop volume while changing depths, a diver
must be skilled in minor gas addition and venting techniques.
On descents, most rebreathers will automatically compensate for a
dropping loop volume by the addition of diluent. Depending on the fraction
of oxygen in the diluent, this may also lead to a concurrent drop in loop PO2
(it should never lead to a rise in loop PO2, because the PO2
of the active diluent at ambient pressure should not exceed the PO2
set-point of the breathing loop). This then leads to subsequent injection of
oxygen into the loop by the solenoid, which increases the loop volume.
Practiced rebreather divers should be able to indirectly detect changes
in loop volume based on changes in buoyancy and work of breathing. Increases
to loop volume can be made by the addition of diluent or oxygen (depending
on whether the current PO2 is greater than, or less than
[respectively] the PO2 set-point). Decreases to loop volume can
be accomplished by manually venting gas from the loop, either by exhaling
through the nose (except for certain kinds of full face masks), allowing gas
to escape from the seal of the lips to the mouthpiece, or dumping gas from a
valve somewhere on the rebreather loop. Ideally, a fully-dressed rebreather
diver should be neutrally buoyant (or very slightly negative) at the
surface, with optimal loop volume, and empty buoyancy compensator. Under
such conditions, gas needs to be added to the buoyancy compensator only to
compensate for compression of the thermal protection suit, if any. In any
case, a diver should be weighted such that he or she is close to neutral
when the breathing loop volume is at or near optimal.
During an ascent from a rebreather dive, especially a deep dive, the
oxygen partial pressure in the loop will begin to drop (due to the dropping
ambient pressure). The oxygen control system will likely begin to compensate
for this by injecting oxygen; however, except for the slowest of ascents,
the solenoid valve will not likely be able to keep up the with drop in loop
PO2 due to drop in ambient pressure. Although it may be tempting
for a diver to "help" the solenoid achieve PO2 set-point by
manually adding oxygen to the loop, this is probably not a good idea in most
During the ascent, loop gas will be vented from the breathing loop due to
expansion. The diluent component of this lost gas is unrecoverable (it
cannot be put back in the cylinders, and it is not used by the body), and
assuming a continuous ascent, no more diluent will need to be added to the
loop for the remainder of the dive.
The oxygen component of the vented gas, however, is wasted especially if
the system continuously injects more into the loop to bring the PO2
back up to set-point. This waste of oxygen can be minimized by allowing the
PO2 to drop relatively low during the ascent. Obviously, the PO2
level in the loop should be continuously monitored to ensure that it does
not drop dangerously low (i.e., below about 0.5 atm). There is seldom any
real advantage to adding additional oxygen into the loop manual in a futile
attempt to maintain PO2 set-point.
My procedure is to allow the PO2 in the loop to drop during
the ascent. I manually add oxygen to the loop only if the PO2
drops below 0.5 atm, or when I reach the first decompression stop. At the
first decompression stop, I will usually manually add oxygen to the loop to
bring the PO2 back up to set-point. Proper manual oxygen addition
requires a great deal of practice and training; it’s easy to accidentally
over-compensate by adding too much oxygen, escalating the loop PO2
to dangerously high levels. If oxygen is manually injected in large bursts
(rather than several short bursts), a "pocket" of high-PO2 gas
will move around the breathing loop for several breaths.
On most decompression dives involving helium during the deep phase of the
dive, the diver will want to flush the helium out of the loop and replace it
with nitrogen. I usually do this during an ascent at a depth of about
130-150 ft/40-45 m, and start the flush by venting gas from the loop until
the loop volume is at Vmin. I then inflate the loop to Vmax with air, and
repeat this cycle at least three times. The partial pressure of any
remaining helium in the loop is negligible, and will continue to drop as
more gas is vented from the loop during the remainder of the ascent. When I
reach the 20-ft /6-m decompression stop, I shut the diluent input supply,
and flush the loop with oxygen until the loop PO2 reaches
set-point. I will generally remain at this depth until the decompression
ceiling has been cleared. If I ascend shallower, I reduce the PO2
set-point to 1.0 atm.
VI. System Recovery and
The most valuable skills a rebreather diver must learn are the skills
which enable recovery and/or bailout from various failure modes. These
skills should be practiced routinely, because a diver should only rarely
have to use them in a real emergency situation.
A. Oxygen Control System
1. Solenoid Failure
One potential failure mode of most closed-circuit rebreathers is that the
solenoid valve can potentially get stuck in the open position. In such a
case, oxygen would be continuously injected into the breathing loop, and the
PO2 of the breathing loop would reach dangerously-high levels
relatively quickly. The first response to this situation (which is usually
immediately evident to the diver via audible cues and an increase in loop
volume) is to temporarily switch to open-circuit mode. After the oxygen
supply to the solenoid has been manually shut, the diver can flush the loop
with diluent until the gas is safe to breathe, return to closed circuit
mode, and abort the dive while manually maintaining the PO2 in
the breathing loop.
The obvious response to a solenoid valve that is stuck shut is to abort
the dive and maintain PO2 set-point manually.
2. Partial Electronics
If either the primary or the secondary PO2 display systems
fail at any time during the dive, the dive should be aborted. If the
automatic oxygen control system has concurrently failed, the diver should
manually maintain the PO2 in the breathing loop following the
functional PO2 display.
3. Total Electronics Failure
A total electronics failure generally means both the primary and
secondary PO2 display systems have failed simultaneously.
Although an open-circuit bailout will often be the most appropriate response
to this situation (especially if there is no "required" decompression stop
and the dive is relatively shallow), there are at least two alternative
a. Semi-Closed Operation
Any closed-circuit rebreather can be manually operated as a semi-closed
rebreather by the diver. To accomplish this, the diver simply vents every
third, fourth, or fifth exhaled breath out of the loop, replenishing it with
more diluent. The optimal rate at which exhaled breaths should be vented
from the loop depends on the depth, the fraction of the oxygen in the
diluent, and the metabolic rate (workload) of the diver. This system is not
perfect, but a well-trained rebreather diver should be able to maintain a
life-sustaining breathing mixture in the loop until reaching staged bailout
cylinders, or a depth where it is safe to use the "Oxygen Rebreather" method
(see below), while consuming substantially less gas than a bailout in full
open-circuit mode would. This method requires a great deal of practice while
the PO2 displays are fully functional to master. Obviously,
appropriately conservative decompression schedules should be followed
following this bailout method.
b. Manual Gas Mixing
A more difficult, but more gas-frugal method of maintaining a
life-sustaining gas mixture in the breathing loop is to manually mix oxygen
and diluent within the breathing loop. During the initial bailout ascent,
the diver occasionally adds just enough oxygen to the loop manually to
prevent hypoxia from occurring (the proper rate of gas injection can only be
learned after much practice and experience). Upon reaching the first
decompression stop, the diver blends the first pre-calculated gas mixture.
Available to the diver are at least two known gas mixtures (oxygen and at
least one diluent with some known fraction of oxygen in it), and two known
breathing loop volumes (Vmin and Vmax). Presumably, the difference between
the two, ([Vmax] [Vmin]), will not be identical to the absolute value of
Vmin. With these known variables, the diver can create (within reasonable
limits of accuracy) at least four different gas mixtures. The first gas
mixture is achieved by flushing the loop completely with diluent. Once doing
this, the diver can manually add oxygen to compensate for the drop in volume
of the breathing loop (as oxygen is metabolized and carbon dioxide is
absorbed by the absorbent, the loop volume will drop).
If a diver is sufficiently sensitive to changes in loop volume, the PO2
in the loop can be maintained relatively constant. The diver continues using
this method until reaching a depth shallow enough where the next mixture can
be blended. To create the second mixture, the diver flushes the loop with
diluent and then achieve Vmin, then manually adds oxygen until Vmax is
reached. After allowing the gases to mix for a few breaths, the loop is
vented back to optimal volume (if the gas mixture is sufficiently mixed, the
FO2 should remain constant). The diver then maintains optimal
loop volume with the addition of oxygen.
The third mixture involves flushing the loop first with pure oxygen
followed by venting until Vmin is reached. The loop is then "topped-off"
with diluent until Vmax is achieved, and the loop is vented back to optimal
volume after mixing has occurred. This is the most difficult mixture to
create, because the diver must breathe in open-circuit mode to avoid
hyperoxia during the gas mixing process.
The fourth gas mixture is pure oxygen, which can be maintained by using
the "Oxygen Rebreather" method outlined below.
With two diluent supplies with different oxygen fractions, the number of
gas mixtures that can be created increases to 9. With three diluent
supplies, there are 16 possible gas mixtures that can be blended. This
method is most difficult in deep water, because with a given PO2,
the FO2 is relatively small. This means that relatively small
changes in loop volumes equate to relatively large changes in PO2.
This makes the task of trying to replenish metabolized oxygen considerably
more difficult. It cannot be over-emphasized that these methods require a
great deal of practice to master. Practice sessions should be conducted
while the rebreather electronics are fully functional, so the diver can
monitor the various gas flushes and how they affect actual PO2.
c. Oxygen Rebreather
The simplest and most reliable method of manual oxygen control is to
maintain only oxygen in the breathing loop. Unfortunately, this method can
only be used at depths of about 15-20 ft /3.5-6 m or less (depending on the
maximum PO2 the diver wants to be exposed to). The diver simply
flushes the loop with pure oxygen, and replaces and drop in loop volume with
more oxygen. Regardless of how precise the diver is at maintaining a
constant loop volume, the PO2 in the loop stays constant at any
constant depth, and life-sustaining at any depth shallower than about 20 ft
B. Partial Absorbent
A partial failure of the absorbent canister usually means that the
absorbent in the canister can no longer remove carbon dioxide from the loop
as fast as the diver is producing it, leading to a rise in loop PCO2.
If this occurs during a high-workload portion of the dive, the diver may be
able to reduce workload during a dive abort and continue in closed-circuit
mode for a potentially substantial period of time. If the partial canister
failure occurs at a low workload, the diver will likely need to either
periodically flush the breathing loop with diluent and/or oxygen in a manual
semi-closed mode (as outlined above), or resort to an open-circuit bailout.
Once again, only first-hand experience will help guide the diver towards the
appropriate course of action. However, if ample breathing gas supplies are
available (a they should be in all cases), it is certainly more prudent to
complete the dive in open-circuit mode.
Unrecoverable Loop Failure
The "worst-case scenario" for any rebreather dive is a catastrophic
unrecoverable loop failure. This can be caused by a severed breathing hose,
badly torn counterlung, or completely failed (e.g., flooded) absorbent
canister. In such cases, if a diver does not have access to a secondary
rebreather system, a bailout in open-circuit mode is inevitable.
Without Required Decompression Stops
If there is no "required" decompression time, an open-circuit bailout is
the simplest solution. If the diluent gas supply was monitored properly,
there should be plenty of breathable gas to conduct a slow, controlled
ascent to the surface. If the rebreather system allows open-circuit access
to the oxygen supply, a "safety" stop can be conducted at a depth of 10-20
ft/3-6 m to reduce the probability of DCS.
2. Dives With
Required Decompression Stops
As stated earlier, the most logistically difficult aspect of any
rebreather dive requiring substantial decompression is accommodating the
possible need for completing the full required decompression in open-circuit
mode. Two general scenarios that I have developed are outlined below. In
both cases, divers carry a total of 80 cf of diluent and as much as 27 cf of
oxygen (as described above in the "System Configuration and Equipment"
a. Drift Dives
Our most frequent diving method involves a "live" boat following
free-drifting divers. There are many advantages to this method, a discussion
of which is beyond the scope of this article. Herein I will describe our
standard protocol for open-circuit bailout from this type of dive.
Figure 3a illustrates the normal dive plan: divers pull a "tow
line" (made from thin but strong, brightly-colored line) that is
attached to small but highly visible "surface float". The boat
captain follows this float throughout the course of the dive,
keeping a watchful eye for any "emergency floats" that come to
the surface. A normal ascent from such a dive (assuming no
rebreather failures) involves divers commencing their ascent
along the tow-line. At a pre-determined time, the
surface-support crew clips a "decompression line" (as described
above in the "System Configuration and Equipment" section) to
the tow line via the carabineer (or other similar clip) at the
weighted end of the decompression line (Fig. 3b). The weight of
the decompression line slides down the tow line until the divers
rendezvous with it. The divers then detach the decompression
line from the tow line (the tow line is either pulled in by the
surface support crew, or left to drift until all divers have
surfaced), and complete the decompression on the decompression
line.Depending on wind and swell conditions, the boat may or may
not be physically attached to the decompression line via a
"tether" (Fig. 3c). If one or both divers are forced to conduct
a bailout in open-circuit mode while the pair is still together,
both divers commence the ascent together. The diver conducting
the bailout inflates the "emergency float" that he or she has
carried throughout the dive, clips it to the tow line, and
allows it to slide along the tow line back to the surface.
Depending on the particular parameters of the bailout situation,
the diver may attach a note of explanation written on a slate
that is attached to the emergency float (Fig. 3d).
As soon as
the float reaches the surface, the surface-support crew responds by
deploying the decompression line as described above. In this situation,
however, the surface-support crew also attaches a pre-determined
configuration of open-circuit breathing gas supply (usually air or EAN) to
the weight of the decompression line (Fig. 3e).
If both divers are simultaneously conducting an open-circuit bailout,
both emergency floats are sent to the surface, and the surface-support crew
attaches an appropriate volume of open-circuit gas supply. In either case,
the float or floats are usually deflated and returned to the divers along
with the open-circuit gas supply by attaching them to the weight of the
decompression line and allowing them to slide down the tow line to the
divers (Fig. 3f). When the divers rendezvous with the bottom of the
decompression line, they detach the tow line as described above, and
continue decompression. A additional supply of oxygen is then sent down the
decompression line by the surface-support crew to a depth of 20 ft /6 m.
If weather conditions allow the boat to be tethered to the decompression
line, a surface-supplied oxygen rig (as described above in the "System
Configuration and Equipment" section) may be deployed instead of a
self-contained oxygen supply (Fig. 3g).
The ultimate worst-case scenario involves a separated pair of divers who
both independently and simultaneously require open-circuit bailout. If the
first emergency float to the surface is attached to the tow line, then the
procedures as outlined above are followed, just as if the divers were
ascending together (the only difference is that in this case, the diver
might not detach the tow line from the decompression line). If a diver
becomes separated from the tow line, he or she will commence an ascent to
the surface and will deploy an emergency float to the surface, attached to
the line of the reel that the diver has carried (as described above in the
"System Configuration and Equipment" section). If the diver does not require
open-circuit bailout gas supply, he or she writes a note to that effect on a
slate, and attaches the slate to the emergency float.
When the second
emergency float is spotted by the surface-support crew, they deploy a
self-contained open-circuit oxygen supply down the first decompression line,
and deploy a second decompression line to the isolated diver. If there is no
note on a slate to the contrary, the surface support assumes the second
diver is also engaged in an open-circuit bailout, and supplies gas
accordingly (Fig. 3h). In general, the surface-supplied oxygen system is not
deployed whenever a diver pair is decompressing separately – it is better to
allow the boat freedom to move back and forth between the decompressing
divers. If possible, the surface-support crew communicates to each diver the
direction of the other diver, so that the divers may swim towards each other
and complete decompression together. If the separated diver sends his or her
emergency float to the surface first, or if the two divers are both
separated (independently) from the tow line, the response procedure is
similar, but in the reverse order (i.e., first come, first served).
b. Fixed Station Dives
In cases where the reef extends nearly vertically from the surface to the
depth of operation (i.e., a "drop-off" or "wall"), the primary
surface-support vessel may anchor on-site. In this case, divers run a
continuous guide-line from the anchor to the point at which the dive is to
be conducted, and set staged emergency gas supplies at various appropriate
intervals along the guide line.
In these conditions, general cave diving protocols are followed in terms
of returning to the surface along the same path that the descent was made.
Ideally, both divers will carry emergency floats and extra reels with line,
and a secondary "chase" boat will be onsite to accommodate a bailout
situation as described above (in case a diver becomes separated from the
VII. System Maintenance
Specific rebreather maintenance procedures will be defined by individual
manufacturers for their particular units. Described below are some general
considerations for basic rebreather maintenance.
A. Absorbent Canister
Methods for calculating remaining absorbent canister life were described
above. Whether or not the absorbent should be replaced between dives depends
on a variety of factors, including how much use the canister has previously
been subjected to, how much time has elapsed since the previous dive, what
sort of profile is anticipated for the subsequent dive, and various other
factors. A general rule of thumb is: "absorbent is cheap, lives are not."
Nevertheless, it is not always necessary to replace the canister between
every single dive. In all cases, however, a canister should be removed from
the breathing loop if the surface interval exceeds a few minutes. If the
surface interval exceeds a few hours, the canister should be sealed and
protected from ambient air if the absorbent is not going to be changed prior
to the next dive.
In any case, if a canister has not been used for more than a few days,
the absorbent should be changed. When packing the canister with absorbent,
it is important to ensure that all the absorbent material has completely
settled. This usually involves filling the canister, sealing it, vigorously
tapping it, topping-off the absorbent level, and repeating the process
several times. If the absorbent is not properly packed, a bumpy car or boat
ride could lead to subsequent absorbent settling, which may allow
of gas through the canister, and a greatly diminished canister life-span.
B. Breathing Loop
The breathing loop should be opened and ventilated and dried as much as
possible at the end of each diving day. The entire loop (including
mouthpiece, hoses, counterlung(s), canister, etc.) should be disinfected
with an appropriate disinfectant periodically (as often as every dive day,
but no less-frequently than once per dive week).
C. Oxygen Sensors
Oxygen sensors should always be kept as dry as possible. The life-span of
the sensors can be extended if they are sealed in an anoxic environment
(i.e., nitrogen or helium) during long inter-dive periods. Sensor
calibration should be verified frequently (before every dive) and
re-calibrated as needed. Sensors should be replaced according to
manufacturer specifications, and spares should be kept on hand (it is
strongly inadvisable to conduct a closed-circuit rebreather dive with two or
fewer oxygen sensors). As with all aspects of rebreather diving, common
sense mixed with a healthy dose of discipline is the best protection against
Below I describe several incidents from which I have learned valuable
lessons. Although these by no means represent all of my experiences, they do
underscore a few of the points made previously in this article.
1. Over my Head.
Here’s What Happened: After about 35 hours of
practice dives in shallow water, I felt ready for the "big leagues", so I
decided to make a dive to 85 ft /26 m. The rebreather had proven so reliable
that I decided I didn’t need to use the heads-up display, so I pushed it out
of my field of vision. The current was strong, so I made a rapid descent to
the bottom, manually adding gas to the breathing loop to compensate for the
increasing pressure of depth. Once on the bottom I found myself down-current
of the dive site, so I immediately started swimming against the current
without checking any of my gauges.
I fought hard for at least 5 minutes, and I wasn’t quite experienced
enough to notice that the oxygen injection solenoid had not fired since my
initial descent. Only after I finally arrived at the dive site, huffing and
puffing, did it occur to me to check the gauges. The PO2 was 3.5
atm! I later realized that I must have been manually adding oxygen, rather
than diluent, during the initial descent. If, after 5 minutes of heavy
workload, the PO2 in the breathing loop was 3.5 atm, I can only
imagine what it was when I started swimming against the current. That I did
not convulse from CNS oxygen toxicity under those circumstances can only be
described as miraculous. I was not wearing a full-face mask.
Take-Home Messages: 1) Distinguishing manual
diluent addition from manual oxygen addition valves should be as reflexive
and intuitive as breathing; such mistakes should simply
2) One must know the PO2 in the breathing loop at
times; besides disabling the heads-up display, I made the mistake of not
checking the PO2 displays after a substantial depth change. 3)
Had I convulsed, a full-face mask would have saved my life; chalk one up in
favour of the use of full-face masks with rebreathers.
Lesson learned: rebreather divers should not let
their confidence exceed their abilities. [Nor should any other divers –ed.]
2. Between a Rock and a
Here’s What Happened: My rebreather partner John
and I descended on our first deep dive of our expedition to Papua New
Guinea. We followed the slope down to a depth of about 330 ft /100 m, and
found a rock with some interesting fishes.
About 10 minutes into the dive, John caught my attention and showed me
that his PO2 had fallen to about 0.7 atm. His solenoid had been
firing correctly, but the PO2 was not being maintained at
set-point. He tried to manually add oxygen, but when he pressed the valve,
nothing injected into the loop.
Although his primary oxygen cylinder gauge indicated that it was full, he
switched over to his backup oxygen cylinder (also full) – but he was still
unable to inject oxygen into the breathing loop. At about this time, I began
to notice that the PO2 in my breathing loop had also fallen below
set-point. When I tried to inject oxygen into my breathing loop, I had the
exact same set of failures as John. Four different oxygen supply systems had
independently and simultaneously failed! By that time, the PO2 in
John’s breathing loop had fallen to 0.5 atm, so we aborted the dive. As we
started to ascend, the PO2 in John’s breathing loop fell sharply
as the ambient pressure dropped, until we reach 275 ft /83 m when it was 0.2
atm – dangerously close to hypoxic. John’s only option at this point would
have been to abort in open-circuit mode. He tried one last time to manually
add oxygen to his breathing loop, and finally it began to trickle in. I also
noticed that the PO2 in my breathing loop had returned to
set-point. Perplexed, but nevertheless relieved, we completed our
decompression with perfectly functional rebreathers.
Only after the dive were we able to figure out the cause of the problem.
All four oxygen first-stage regulators (primary and backup on both
rebreathers) had environmental protection systems that included a rubber
diaphragm sealing the ambient pressure balance chamber of the first-stage
regulator. Unbeknownst to me, this chamber was supposed to be filled with a
fluid (such as alcohol), but the fluid had long-since evaporated out.
Because this chamber in all four oxygen regulators was gas-filled, the
rubber diaphragms stretched inward in response to increasing ambient
pressure until they had "bottomed-out" on the adjustment nut for the
inter-stage pressure spring. Once the diaphragms had "bottomed-out", the
inter-stage pressure was no-longer compensating for increasing ambient
pressure. At 330 ft /100 m, the inter-stage pressure was equal to the
ambient pressure, so there was no movement of gas from the regulator to the
breathing loop. Back in shallower water, the regulators had returned to
Take-Home Messages: 1) Know the functional design
of every component of rebreather, inside and out; I should have been
familiar with the oxygen regulator first stages and should have known how to
maintain them properly. 2) It is important to intimately understand gas
physics and physiology; it should have been obvious to us right away what
the problem was, and how best to solve it. 3) Different people work at
different rates; John’s body burns oxygen about twice as fast as mine does
at low to moderate workloads, which is why this particular problem was much
more acute for him than it was for me. 4) Understand the bailout options;
the diluent regulators were functioning correctly; we could have injected
diluent into the loop to maintain a safe PO2.
Lessons learned: Rebreather divers should have an
intuitive understanding of the mechanical aspects of the rebreather, gas
physics, rates of oxygen metabolism, and bailout options.
3. Know Thy Mix
Here’s What Happened: John and I descended on our
way to 220 ft /67 m. At about 115 ft /35 m, we switched our diluent supplies
from air to heliox, and continued our descent. Shortly before reaching the
bottom, John noticed that the PO2 in his breathing loop had
climbed to 1.6 atm. He correctly responded by flushing the loop with heliox,
but the PO2 escalated to nearly 1.8 atm. Additional diluent
flushing had no effect on the PO2. Both primary and secondary PO2
displays were giving identical readings, and there was no indication of
sensor malfunction. He switched back to air as a diluent and flushed the
loop, and the PO2 dropped down below 1.5 atm (but the narcosis
level increased). We immediately aborted the dive. I had filled both of our
heliox cylinders more than a month earlier, and at the time, I confirmed
that both contained 10% oxygen. I had not re-analyzed the heliox cylinders
prior to this dive, but after the dive we discovered the FO2 of
the heliox on John’s rig had increased to 25%.
Take-Home Messages: 1) Always analyze, label and
log your gas mixture, and know what you’re breathing prior to the dive; had
we done this, we never would have encountered a problem. 2) Don’t bypass the
brain when solving problems; although John had believed the oxygen content
of the heliox was 10%, and although his training was to automatically
respond to high PO2 by flushing the loop with diluent, he was
still savvy enough to realize what had happened, and cleverly switched back
to air to bring the PO2 back down (under the circumstances, he
regarded narcosis as the lesser of two evils compared to the high PO2).
Lessons learned: It is imperative that diluent
gas supplies be mixed properly and analyzed immediately prior to the dive;
the brain should not be bypassed when responding to a problem; an intuitive
grasp of the causes and effects of rebreather operations is critical; laws
of physics don’t lie.
4. Starved for Breath
Here’s What Happened: While in Papua New Guinea,
I rushed to assemble the rebreather for a dive on which Bob Halstead was to
take photographs of John and me. I quickly calculated (in my head) how much
dive time I had used on that particular canister of absorbent, and decided
it was about 8 hours. Because I was typically getting 11 hours out of a
canister, and because this was to be a short dive, I decided not to spend
the time to re-pack the canister with fresh absorbent.
We fought a strong current down to a depth of 130 ft /40 m, where we were
to take the photographs. I found it extremely difficult to catch my breath
once we were down. Although I had worked hard against the current, I was
unusually short of breath. When I was still starved for breath after about 5
minutes of posing for the camera (low exertion), it was obvious that I
should abort the dive. During the ascent, the symptoms subsided slightly,
but then quickly re-appeared with a vengeance during my safety decompression
stops. I flushed the loop with air, and was soon able to breathe normally
again. Within a few minutes, however, the shortness of breath returned.
After I surfaced (with a splitting headache), I looked over my dive logs
and discovered that I had actually used that particular canister of
absorbent for thirteen previous hours of dive time.
Take-Home Messages: 1) Managing rebreather
expendables must be done carefully; I should not have calculated a variable
as critical as remaining absorbent life so flippantly. 2) Carbon dioxide
absorbent is cheap, lives are not; regardless of my miscalculation, I should
have changed the absorbent long before.
Lessons learned: Knowing the remaining life of a
canister of carbon dioxide absorbent is critical.
5. Slow Down There, Young
Here’s What Happened: This involves two incidents
which occurred on the same day. One morning in Papua New Guinea, I was
rushed to get the rig ready for a deep dive. I had prepared the rebreather
the night before, so I just climbed into it, did a quick pre-dive check of
the system, and decided to forgo the "positive pressure" loop test.
Tightening the straps on my full face mask, I deflated my BC and made a
"giant stride:" entrance off the dive platform. My first inhalation filled
my throat with water, and I began to cough and choke. Because I was
negatively buoyant, I had to struggle to ascend the two or three feet to the
surface, and then hastily rip the full face mask off.
Gasping and coughing at the surface, it occurred to me that I had very
nearly drowned. I assumed the water had leaked into the mask’s oral cup when
I jumped into the water, so I carefully replaced the mask, started
descending, took a breath, and inhaled water down my throat again! Once
more, I struggled back to the surface, ripped off the mask, and gasped for
After I climbed back aboard the boat and removed the rebreather, I saw
the source of the problem: I had neglected to connect the inhalation
breathing hose to the rebreather -- it was just dangling free! Not only had
I almost killed myself (twice!), but I had completely flooded the rebreather
Later that same day, I neglected to replace the plug over the data
download jack on the main electronics housing. Within seconds of the
rebreather entering the water, the main electronics completely flooded with
salt water and were destroyed.
Take-Home Messages: 1) Pre-dive check routines
are very important and should not be bypassed; conducting a
positive-pressure loop test would have alerted me to the fact that the
breathing hose was disconnected. 2) Pre-dive checks should be thorough; my
routine previously did not include checking to see that the plug is replaced
on the data download jack -now it does.
Lesson learned: Haste makes waste, and can
potentially lead to costly, and even deadly consequences.
6. A Long Way on Two
Breaths of Air
Here’s What Happened: It was the last day of our
Papua New Guinea expedition, and we had time for only one more dive. My
advisor, Jack Randall, had seen what he believed represented a new genus and
species of fish at a depth of 80 ft /24m. Because he had been diving all day
using conventional air scuba, he had no remaining bottom time left at that
depth. I had been using the rebreather all day (optimized gas mixtures), so
I had plenty of remaining bottom time.
We decided that Jack would bounce down with me to show me the spot where
he had seen the fish, then I would look for it and try to collect it. We
rushed to gather our equipment together in the chase boat, had our guide
motor us out to the correct spot, and Jack rolled over the side. Just as I
was about to follow, I noticed that my diluent cylinder was completely
empty. Jack was already gone, and if I had returned to the
more air, I never would have found him again, and he would never be able to
show me where the fish was. I manually flushed the loop with air using my
mouth and rolled over the side to follow Jack.
During my descent, I had to add oxygen to the breathing loop to
compensate for the drop in loop volume. By the time I caught up with Jack at
60 ft /18 m, the PO2 in my breathing loop was 1.6 atm (too high
already, and it would have been way too high at 80 ft /24m). The only way I
could get more air into the breathing loop was to get it from Jack’s
cylinder. I motioned to him that I needed to buddy breathe, and he assumed I
needed to abort the dive. I did my best to explain to him that all I wanted
to do was to take a few breaths of his air and exhale them into my
rebreather (to add more nitrogen to the breathing loop), but I wasn’t
getting the message across. After two breaths of his air, I gave up trying
to explain, and simply motioned that everything was O.K. He pointed to where
he had seen the fish, and headed back to the surface. When I got to 80 ft/
24m, the PO2 in the loop was just over 1.4 atm. However, if I
exhaled any gas from the loop, I would have lost nitrogen, which would have
been replaced by oxygen, and the PO2 would have been too high.
Thus, I had to be very careful managing my loop gas.
Jack had said the fish was light brown with a black spot near the tail.
All of a sudden, a small light-brown fish with a black spot near the tail
swam by. I spent nearly an hour chasing the fish, all the while being very
careful not to loose any gas from the loop. Remarkably, I was able to stay
the whole hour without any increase in the PO2. Even more
remarkably, I managed to catch the fish! I completed the dive, proud of my
accomplishment (both for catching the fish, and for stretching so much dive
time out of only two breaths of air).
Then the error of my ways suddenly dawned on me: what if I needed to make
an open-circuit bailout from the dive? I would have been screwed. To add to
my failure, when I showed the fish to Jack, it was the wrong one! Apparently
there is another light-brown fish with a black spot near the tail at 80 ft/
24 m off Papua New Guinea.
Take-Home Messages: 1) Always ensure that at any
time during the dive, at least one gas supply is safe to breathe in
open-circuit mode; had I needed to abort from the dive on open-circuit, I
would have had to breathe pure oxygen at a depth of 80 ft /24 m. 2)
Rebreathers really can go a long way on only a small quantity of diluent!
Lessons learned: Always make sure there is enough
gas to make a safe abort to the surface; and make sure you have a more
specific description of a new genus and species of fish than "light brown
with a black spot near the tail", especially if it’s the last dive of an
In this article I have described my reasons for using closed-circuit
rebreathers, some of the lessons I’ve learned from my experience with this
equipment, and an outline of the procedures and protocols I have developed
for diving with rebreathers in the sorts of environments and conditions that
I do (deep coral reefs). While this article may contain some useful tid-bits
and "words of wisdom" of general applicability, in no way is it intended as
a template for generalized rebreather standards.
Military divers have used closed-circuit rebreathers for many decades,
and represent the single largest experience-base for closed-circuit
rebreather operations. Certain commercial divers and other individuals also
have independent experience that spans many years to decades. Specific
rebreather designs are many and varied and will likely continue to change in
the years to come. No single user or user-group has all the answers for all
Present and future rebreather divers will continue to experiment with new
combinations of equipment, environments, and diving objectives; and new
procedures will need to be invented and refined. Perhaps the single most
important step to take in minimizing the number of accidents involving
rebreathers is to create and maintain an open exchange of information
between past, current, and potential future rebreather divers. Expanding the
collective body of knowledge, experience, and wisdom to its maximum scope
can only enhance the progression of our individual levels of safety and
productivity with this evolving technology.
Allen, G.R. and J.E. Randall.
1997. Two new wrasses (Perciformes: Labridae) from Papua New
Guinea. Rev. fr. Aquariol.
Comper, W. and W. Remley.
1996. Rebreather round-table: DeepTech and seven industry experts
take a hard look at rebreather safety issues and training standards.
Earle, J.L. and R.L. Pyle.
1997. Hoplolatilus pohlei, a new species of sand tilefish (Perciformes:
Malacanthidae) from the deep reefs of Papua New Guinea.
Gill, A.C., R.L. Pyle, and J.L. Earle.
In press. Pseudochromis ephippiatus, new species of
dottyback from southeastern Papua New Guinea (Teleostei: Perciformes:
Pseudochromidae). Accepted for publication in Revue fr. Aquariol.
Halstead, B. 1996.
Hi-Tek Adventure. Scuba Diver, September/October 1996: 61-64.
Montres Rolex S.A.
1996. Richard Pyle, United States. Project: Investigate
biodiversity in the undersea Twilight Zone. Pp. 146-147. In:
Enterprise: The 1996 Rolex Awards. Secretariat of the Rolex Awards for
Enterprise, Geneva, Switzerland. 191 pp.
Pyle, R.L. 1991.
Ein schatz aus der zwielichtzone. Unterwasserfotografie (UWF)
Pyle, R.L. 1992a.
The Twilight Zone. AquaCorps: Mix. 3(1):19.
Pyle, R.L. 1992b.
Deep reef set. AquaCorps: Mix. 3(1):17-21.
Pyle, R.L. 1994.
Rare and unusual marines: The Narc Angelfish
Centropyge narcosis Pyle
and Randall. Freshwater Mar. Aquar. 7(4):8-19.
Pyle, R.L. 1995.
High PO2 symptoms my experiences.
Pyle, R.L. 1996a.
How much coral reef biodiversity are we missing?
Pyle, R.L. 1996b.
The Twilight Zone. Natural History, 105(11):59-62.
Pyle, R.L. 1996c.
Section 7.9. Multiple gas mixture diving, Tri-mix. In: Flemming, N.C. and
M.D. Max (Eds.) Scientific Diving: a general code of practice, Second
Edition. United Nations Educational, Scientific and Cultural Organization
(UNESCO), Paris; and Scientific Committee of the World Underwater Federation
(CMAS), Paris, pp. 77-80.
Pyle, R.L. 1996d.
Adapting to Rebreather Diving. Immersed 1(2):12-21.
Pyle, R.L. In press.
The use of nitrox in closed circuit rebreathers for scientific purposes. In:
Bozanic, J. and G. Stanton (Eds.) Proceedings of the American Academy of
Underwater Sciences Nitrox Diving Workshop, Catalina Island.
Pyle, R.L. and J.E. Randall.
1992. A new species of
Pomacanthidae) from the Cook Islands, with a redescription of
Revue fr. Aquariol. 19(4):115-124.
Randall, J.E. and P.
Fourmanoir. In press.
rubrovittatus, a new genus and species of labrid from New Caledonia and
New Guinea. Bull Mar. Sci.
Sharkey, P. and . R.L. Pyle.
1993. The Twilight Zone: The potential, problems, and theory behind
using mixed gas, surface based scuba for research diving between 200 and 500
feet. In: L.B. Cahoon (Ed.) Diving for Science...1992. Proceedings of the
American Academy of Underwater Sciences Twelfth Annual Scientific Diving
Symposium, American Academy of Underwater Sciences, Costa Mesa, CA. pp.
Breath taking stories here:
I would like to thank Dr. Richard L. Pyle for
this contribution to the rebreather diving community and his
permission for adding this article to my website. Now in June
2010 this page has been read over 10.000 times!