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31
doi:10.3723/ut.34.031 Underwater Technology, Vol. 34, No. 1, pp. 31–38, 2016
www.sut.org
Technical Briefing
Abstract
The closed circuit rebreather (CCR) is not a new diving tech-
nology. From the late 1990s CCR units were commercially
available in Europe, and increasingly more divers, and
among them scientific divers, have been trained to use
them. Even if many benefits exist for using CCR for all diving
depth ranges, it is in the deep diving zone ranging from
50 m to 100 m of sea water where the main advantages to
using this equipment exist. Using rebreathers does carr y
additional risks, and these must be mitigated to ensure safe
usage. A standard for CCR scientific diving has existed for
many years in the USA, and the levels of expertise within the
European scientific diving community are now sufficient for
a European standard to be established. National legislation
for occupational scientific diving in many cases excludes
CCR diving, which can limit its use for scientific purposes.
This paper suggests that, where possible, legislations
should be allowed to evolve in order to include this type of
equipment where and when its use has direct advantages
for both the safety and the efficiency of scientific diving. This
paper provides a brief description of the fundamentals of
closed circuit rebreather diving and outlines the benefits
that its use offers diving scientists. Special attention is given
to safety issues with the asser tion that the CCR concept is,
if strictly applied, the safest available technique today for
autonomous deep scientific diving purposes.
Keywords: CCR scientific diving, mixed gas diving, diving
safety, deep diving
1. Introduction
The closed circuit rebreather (CCR) is not a new
diving technology. The concept of rebreathing gas
underwater has been traced back to at least 900 BC
(Bozanic, 2010) and the modern-day design
remains based on pre-WWI models, an example
being the Fleuss rebreather that was made in 1879.
During both World Wars, many improvements were
made to rebreathers based on their use for covert
military actions.
The rst electronic closed circuit rebreather,
known as the Electrolung, was marketed in 1969.
However, it was not until the late 1990s when elec-
tronic CCR started to be sold into the mainstream
scuba diving markets, with the introduction of the
BUDDY-INSPIRATION (now renamed the Ambient
Pressure Diving’s Inspiration CCR range). Modern
CCRs for the European market are made by a small
number of manufacturers, and their design and
construction must follow the European Normative
for rebreathers, EN 14143. The requirements con-
tained within NBN EN 14143 (Bureau voor Normal-
isatie, 2013) are that rebreather technology using
air as a diluent gas can be used to a depth of 40 m,
while Trimix/Heliair/Heliox diluents should be
used below 40 m to the maximum depth covered by
the EN standard of 100 m. The technologies associ-
ated with CCRs continue to improve their function-
ing and use, with the latest developments including
CO2 sensors in the breathing loop, bailout valves
and solid state oxygen sensors (Sieber, 2014).
CCRs do not produce bubbles except for very
few during the ascent phase of the dive. Their main
advantage for diving physiology is that they permit
the diver to breathe a constant partial pressure of
oxygen during the dive. A sodalime lter removes
the carbon dioxide produced by human metabo-
lism while an electronic feedback system controls
the partial pressure of oxygen (ppO2) available in
the breathing loop controlling oxygen addition
into the loop automatically if required.
The three main advantages that CCRs offer the
scientic diver are the signicant lack of bubbles,
gas efciency and the optimised decompression
that constant partial pressure of oxygen permits.
The lack of bubbles has been shown to reduce the
The closed circuit rebreather (CCR): is it the safest
device for deep scientific diving?
Alain Norro*
Royal Belgian Institute for Natural Sciences, Operational Directorate Nature, Gulledelle 100, B-1200 Brussels-Belgium
Received 12 August 2016; Accepted 20 September 2016
* Contact author. Email address: alain.norro@naturalsciences.be
Alain Norro. The closed circuit rebreather (CCR): is it the safest device for deep scientic diving?
32
impact the diver has on the marine life being stud-
ied and improve the quality of science being under-
taken in environments where the diver’s bubbles
would physically disturb the ecosystems being stud-
ied. Moreover, and from a safety perspective, CCR
technology (when used according to the rules) is
based on built-in redundancy and operational pro-
cedures that can enhance the safety of the diver.
However, CCR technology can add new risks,
oxygen and/or carbon dioxide toxicities can occur
very rapidly when the rebreather is not working
properly or if the diver did not setup the equip-
ment according to manufacturer specications.
Gas choice is of primary importance and proper
training is the key factor for mitigating these risks.
The equipment must be handled with care, and it
is important that the diver adopts new approaches
to how they undertake their diving when moving
from using open circuit to CCR.
Much of the scientic use of CCR technology
with mixed gases has been based on extending the
underwater exploration range to limits that far
exceed those possible when using typical scuba
equipment. One of the rst researchers to take
advantage of the new technologies was the ichthy-
ologist Richard Pyle from Honolulu, who used CCR
equipment to study sh found in the mesophotic
zone. It is not the purpose of this paper to provide a
full list of all the studies that have employed CCRs,
but many applications exists in behavioural sciences
such as: Collette (1996) looking at sh behaviour;
Lobel (2009) studying underwater acoustic ecology;
and Tomoleoni et al. (2012) and Tinker et al. (2007)
who used CCRs to facilitate the capture or recap-
ture of sea otters. Moreover, Hinderstein et al.
(2010), Sherman et al. (2009) and Rowley (2014)
used the advantages provided by CCR deep mixed-
gas diving to study mesophotic coral ecosystems.
In Europe, it is widely accepted that diving for
occupational scientic purposes should be limited
to a maximum depth of 50 m when diving open
circuit scuba using air. Beyond that depth, mixed
gas technology is used in order to overcome the
problems generated by nitrogen narcosis and to
achieve acceptable gas densities that reduce the
work of breathing (Mitchell and Doolette, 2013).
The current situation in Belgium is that the scien-
tic diver is advised to use rebreathers for diving as
soon as there is a demonstrable added value for the
scientist or for the quality of the science undertaken.
Nevertheless, in many European countries, the use
of mixed gas diving in support of underwater sci-
ence is still in its infancy, and in some cases the use
of rebreathers for occupational diving may not be
permitted by law. In France, the capability to use
rebreathers in scientic diving was initiated in 2014
(L’Agence nationale de sécurité sanitaire de
l’alimentation, de l’environnement et du travail
(Anses), 2014). However, the administrative and
human resource challenges will probably be numer-
ous and will more than likely be similar to the chal-
lenges outlined by Dokken (2006), who described
how rebreathers became accepted for use in sci-
ence diving in the USA.
For deep scientic diving, the CCR technology
brings additional benets over open circuit, such as
reduced mixed gas requirements because of signi-
cantly higher gas efciency leading to much lighter
equipment to carry on expedition or use during
the dive. In closed circuit diving, the breathing
mixtures are different to open circuit and are usu-
ally set to deliver a lower equivalent narcotic depth
(END), which allows better quality underwater work
to be undertaken. CCR diving also brings a lower
risk of making errors during decompression since
the units alter the gas mixtures internally, negating
the need for any physical gas switches by the diver. The
negative aspects of CCR diving are the high costs of
the rebreather unit itself, as well as the nancial and
time costs associated with the training required to
be able to use them for the scientic research diving
(Lang and McDonald, 2012). Careful planning is
key to ensuring a safe diving activity, and special
attention is needed when considering the bailout
gas strategy for all aspects related to oxygen and
carbon dioxide toxicities, gas density, inert gas
narcosis decompression stresses.
The following section focuses on the safety issues
related to the training, dive planning and opera-
tional use of mixed gas CCR technology when
applied to scientic dives between 50 m and 100 m
depth. The discussion also considers the practical
application of dive planning rules, including gas
choice and bailout strategies.
2. Methods
2.1. Training
All rebreather manufacturers require that training is
taken prior to the purchase and use of their units by
the diver. This training is the most important step to
ensure the efcient and safe use of CCRs by a diver.
If rebreather diving is being considered for a group
of divers who will then work together in the future,
then the group should consider undertaking the
same training courses together. This approach may
be time-consuming, as planning for a minimum of
two years of preparation for a team prior to any sci-
entic diving projects starting may be advisable.
Rebreather training is commonly divided into a core
course that is generic to all rebreather diving, and
33
Underwater Technology Vol. 34, No. 1, 2016
then a unit-specic course dedicated to the particu-
lar make and model of the rebreather that will be
used. The diver is, therefore, only certied to use
one type of rebreather. Should the diver change unit
type, they would be required to undergo additional
training that is specic to that new unit.
In addition to being unit-specic, rebreather
training is also limited to a given depth of operation.
Most training agencies have three levels of rebreather
qualication; these tend to be dened by the maxi-
mum operating depth (MOD) that the training sup-
ports. The actual MOD limits differ slightly between
training agencies but generally MOD-1 training sup-
ports rebreather diving where the diluent gas is air
diving to maximum depths of 40 m, the MOD-2 level
uses trimix gas mixtures as the diluent to maximum
depths of 60 m, and the MOD-3 level uses a trimix
diluent to a maximum depth of 100 m. Once quali-
ed at one level, the diver must usually achieve at
least 50 hours of diving on the unit before starting
the training for the next level.
Training usually begins with an initial introduc-
tion to the theoretical considerations of diving phys-
ics and gas physiology before the diver can begin to
learn to use the diving unit in actual underwater
operations. During the practical training, the diver is
taught how to safely assemble and test the unit before
diving. Because of the relative complexities of a CCR
unit, evidence suggests that the diver is less likely to
make mistakes during the setup if checklists are used
to guide them through the process (Mitchell, 2014).
In fact, many modern CCRs have checklists pro-
grammed into the display units with the diver having
to follow them when preparing for a dive.
Once in the water, the diver is rst trained in the
normal use of the rebreather before being trained
on actions to be taken in case of malfunction of vari-
ous parts of the equipment. A rebreather is a more
complicated piece of gear than normal scuba, and
so equipment malfunction may be more likely to
happen. Therefore, all rebreather diving should
have an alternate source of gas – known as bailout
gas – available. For the advanced MOD-2 and MOD-3
training levels, more consideration is given to ade-
quate gas planning. This includes learning to con-
trol the psychological issues related to deep diving
and escape procedures in case of rebreather mal-
functions, and it may include training that is based
on bailing out to open circuit diving. At the
advanced training levels, more emphasis is given to
considering the various possibilities to continue
breathing from the main CCR breathing loop while
safely solving problems that have occurred.
Following Lang and McDonald (2012), the
nature of occupational scientic diving could mean
that the training undertaken should consider
including some of the more common science-
related tasks to be undertaken underwater. Ideally,
the training should be delivered by a scientic diver
who holds the appropriate CCR instructor certica-
tion. Some of the more important aspects of CCR
training are dive planning, gas choice and bailout
strategy for mixed gas diving.
2.2. Dive planning
The selection of the diluent gas is the rst step
when starting dive planning but is inuenced or
driven by knowing the dive site location and the
planned maximum depth. The diluent gas is usu-
ally based on a mix of oxygen, nitrogen and helium
(trimix) or oxygen and helium (heliox). The dilu-
ent gas could theoretically be a single inert gas or a
mixture of inert gases, but it must, in practice and
for safety reasons, contain some oxygen. The frac-
tion of helium is dened when the END is known
and the fraction of oxygen is dened by the maxi-
mum ppO2 acceptable at the MOD of the dive. The
computation of the END in the breathing loop of a
rebreather is somewhat more complicated than for
open circuit and will always result in a shallower
END than when using open circuit for a given frac-
tion of nitrogen. In the CCR sector, this mix is often
blended as heliair, which is a mixture of just helium
and air but is always hypoxic (i.e. containing a frac-
tion of oxygen that is less than 21%). This is mainly
because of operational simplicity, but also because
oxygen control is provided anyway by the CCR.
After computing the END the resulting gas den-
sity must be taken into account in order to minimise
the work of breathing. The work of breathing on a
rebreather is inuenced by its design (loop, CO2 can-
ister, position of the counter lung) as well as by the
gas density. Assuming that the diver has not modied
the design of the breathing loop, the present recom-
mended values for gas density in the loop are below
5.7 g L-1 (Antony and Mitchell, 2016). This corre-
sponds to breathing air at 30 m, with an absolute
maximum limit of 6.7 g L-1 (air at 40 m). Maintaining
the gas density below these limits will mitigate the
risk of CO2 retention and therefore hypercapnia.
It is a basic safety factor that bailout gases are
always carried during a CCR dive. These are dened
both in terms of the gas fractions of the three gases
(O, He and N) and in the overall quantity of breath-
ing gas required. To do this accurately, it is neces-
sary to have estimates of the breathing rate of the
diver expressed as their respiratory minute volume
(RMV). Two different gases are usually planned:
a ‘bottom’ gas and a ‘decompression’ gas. The frac-
tion of oxygen on the bottom and decompression
bailout gases are computed knowing the maximum
ppO2 allowed at the MOD of the dive and the depth
Alain Norro. The closed circuit rebreather (CCR): is it the safest device for deep scientic diving?
34
at which a decompression gas will be required. The
fraction of helium in the bottom gas is computed
using the permitted END, which also takes into
account that the partial pressure of nitrogen should
not build up at the moment of the gas switch and
that an acceptable gas density is achieved. Similar
calculations are needed for the fraction of helium,
if any, in the decompression gas.
The volume of bailout gases to be carried is com-
puted iteratively based on the basic dive parameters
of planned bottom time and the resulting decom-
pression obligation. However, the eventual volumes
can be moderated depending on the choice of bailout
strategy, which could be determined by a requirement
that all divers are to dive completely self-sufciently.
Alternatively, some reliance could be allowed for a
dive team bailout where gases could be shared, or
even on gases that could be available at a decom-
pression station deployed by the surface vessel.
After the selection of the diluent gas and bailout
gases, the diver then needs to plan the amount of
decompression that will need to be made and how
the stops are staged. To do this, the CCR mixed-gas
diver can use either dive tables or planning soft-
ware that include decompression algorithm(s) for
constant partial pressure of oxygen diving. Except
the work done by the US Navy (Johnson and Gerth,
2001), there are not many tables that exist to support
diving using constant partial pressures of oxygen in
helium. VPLANNER or, more recently, MULTI-
DECO (Vplanner +Bulhman GF) developed by
HHS Software are the most commonly used soft-
ware for determining decompression. Some CCR
manufacturers provide decompression computers
that measure the breathing loop partial pressure of
oxygen in real time and continuously compute
decompression for a given diluent gas composi-
tion. A further alternative is to use unlinked mixed-
gas dive computers that allow set-points for constant
ppO2 computations to be made. Doolette and
Mitchell (2013) evaluated the present-day use of
decompression algorithms by technical divers.
They concluded that even though the commonly
used decompression algorithms were not validated,
unlike the US Navy tables (Johnson and Gerth,
2001), the technical diving community is perform-
ing many thousands of dives safely, even though the
incidence of decompression sickness remains
unknown. Doolette and Mitchell (2013) further
concluded that it remains unknown if these unvali-
dated decompression procedures are optimal.
2.3. Diving operations
Deep diving is always challenging because of the
many aspects to be considered during the planning
process. Examples include dive location and weather;
local administrative requirements; length of the
proposed operation (a single dive or a series of
dives); accommodation and catering related to the
length of the operation; the management of the
quantity and quality of the breathing gases; the dive
team; the safety diver; underwater communications;
and the decompression support both underwater
and onboard, if required by the diving regulations
or if surface decompression (SurD) is going to be
used. Some of these aspects are addressed in Euro-
pean Scientic Diving Panel of the European Marine
Board (ESDP, 2011).
Planning and executing CCR diving at work will
vary considerably with the diving location. For
example, planning and operations for cold water
CCR diving will be different to that carried out in
moderate or warm waters (Bardout, 2016). The tar-
get dive site could be, for example, a natural rock
wall, a wreck or isolated rocks on the sea bed, and in
each case the diving procedures will differ. This, in
turn, could inuence the type and size of support
vessel. The support vessel should be able to provide
enough gas in quantity and quality for the diving
operation to be completed safely. Basing the work
on CCR diving only will reduce the quantity of
required gas drastically, permitting the use of
smaller vessels and lighter loads. All breathing gases
supplied should full the European norm UNI EN
12021:2014 especially for the oil content in the air
that is used in any oxygen-clean apparatus, includ-
ing at the blending stage. Finally, it is extremely
important to verify the actual nal gas mixes that
have been blended; best practice is to do this using
more than one oxygen and/or helium meter.
The level of competency qualication required
of the members of the CCR diving team is usually
linked to local regulations. At European level, there
are scientic diver qualications overseen by the
ESDP (2009). Unlike what exists through the Amer-
ican Academy of Underwater Sciences (AAUS,
2013), there is currently no specic competency
level or standard recognised by the ESDP for
rebreather diving in Europe. At the national level,
an occupational scientic diving organisation may
be responsible for establishing the acceptable stand-
ards. For example, in Belgium certication from
known training agencies that are recognised by the
manufacturer of the rebreather is accepted. The
same approach could be adopted by the ESDP when
a future standards supporting rebreather use in sci-
entic diving across Europe are being considered.
2.4. Safety of CCR mixed-gas deep scientific
diving operations
Scientic diving activities are known to be safer
than any other kind of occupational diving, at least
35
Underwater Technology Vol. 34, No. 1, 2016
where decompression sickness is concerned
(Dardeau et al., 2012). The study of Dardeau et al.
(2012) was based on a dataset from the AAUS for
the period 1998–2007; CCRs were in use by AAUS
members during that period (Sellers, 2016). There
is not much literature concerning the diving acci-
dents resulting from the use of CCR, or any other
types of rebreather outside the military sector
(Louge et al. 2009). Trytkjo and Mitchell (2005),
Lippman et al. (2011) and Fock (2013) examined
the matter at different levels of approach. Fock
(2013) examined deaths resulting from CCR dives
within the period 1998–2010 and concluded that
the risk of dying when using rebreathers appears to
be 10 times what would be expected when using
open circuit. The majority of the reported deaths
were during what Fock dened as ‘high risk dives’
or which included ‘high risk-behaviour’. Examples
were entering the water with partially functional
equipment or carrying insufcient bailout gases for
an emergency.
Recently, Sellers (2016) extensively described
the use of rebreathers in scientic diving opera-
tions at a number of American institutions. The
study showed that rebreather dives represent less
than 0.7% of the total numbers of dives operated.
Based on the dataset examined, the non-fatal acci-
dent rate for rebreather diving was 6 for 15 767
dives. Moreover, it was possible from those data to
isolate dives that were deeper than 58 m (the AAUS
maximum depth limit for diving on air only) but
undertaken using mixed gases. No accidents were
reported for those types of dive and, since 2011,
these deep mixed-gas scientic dives were operated
more using CCR than open circuit scuba.
Some rebreather models can log data during the
dive (Parker, 2014). The information that tends to
get logged is: ppO2, time, depth, voltage of the bat-
teries, scrubber temperature (if measured), and the
decompression obligations in addition to any set
points selected by the diver and any alarms occur-
ring during the dive. These data are valuable when
examining what occurred in the case of any accident
and are used to inform future training priorities.
3. Discussion
There are several rules or recommendations that
CCR divers use when considering the correct frac-
tions of oxygen and helium that make up the dilu-
ent gas to be used during a deep CCR dive. For
instance, Lombardi and Godfrey (2011) recom-
mend having a partial pressure of oxygen with a
maximum PO2 of 1.30 bar at the MOD in the dilu-
ent, while Mount and Dituri (2009) recommend a
maximum of 1.00 bar at MOD. The idea behind
this maximal ppO2 in the diluent gas at MOD is
simple. In the case of a hyperoxic loop, a diluent
ush must be able to reduce its ppO2. Therefore,
having a lower ppO2 in the diluent than the normal
loop values of between 1.20 and 1.30 bar helps to
reduce the resulting ppO2 quickly while also using
a lower volume of gas. Having less oxygen in the
diluent also reduces the overall density of the gas
mixture.
The composition of the inert gas fraction of the
diluent gas is a source of discussion that lacks any
denitive conclusions. Lombardi and Godfrey
(2011) chose END values that ranged from 15 m to
50 m. Some of those values exceeded the recom-
mendations of some training agencies, which pro-
pose an END value of 36 m (Mount and Ditury,
2009), or of some manufacturers, such as setting an
END of 24 m for the depth of 100 m (Parker, 2016).
Moreover, an END of 50 m results in a gas density
that is well over the acceptable limit.
The narcosis effect of the diluent mix will fur-
ther affect the judgment of the diver at depth, and
this is certainly not desirable when both diving
deep and working underwater. Not only will narco-
sis reduce the quality of the work, but in cases of an
emergency the diver suffering narcosis will also
have an increased reaction time with possible unde-
sired outcomes. Diving with a diluent mix that does
not satisfy the manufacturer’s recommendations is
dangerous behaviour and increases the risk associ-
ated with the dive.
The same simple rules also apply to the behav-
iour of the rebreather diver in relation to the oxy-
gen cells used in the rebreather oxygen control
system. The cells must be tested during any
rebreather start-up and dive to conrm that they
are working correctly. Making an oxygen ush at
6 m depth will give a good indication of the status
of the cells. In the case of outdated cells (more
than 18 months from their manufacturing date) or
cells found to be out of working limits, the dive
must be terminated and the cell(s) replaced before
diving the unit again. Otherwise, there will be an
increase in the risk taken for the dive.
The last point to be discussed is the correct
choice and strategy for bailing out of a dive in an
emergency. Bailout strategy can vary in two ways:
the diver may choose to be fully self-sufcient on
bailout gas, or the diving team of two or three
divers may choose to share the bailout gas within
the group, resulting in a lighter load during the
dive for each individual.
In the rst situation, the diver must carry through-
out the dive a minimum of two extra cylinders – one
bottom gas and one decompression gas. The size
and number of bailout cylinders will depend on the
Alain Norro. The closed circuit rebreather (CCR): is it the safest device for deep scientic diving?
36
planned dive prole. The quantity of gas required
must address the worst-case scenario of a bailout.
This will be when the failure of the rebreather
occurs exactly at the end of the bottom time section
of the dive – in other words, when the decompres-
sion time is maximal. In the case of a complete fail-
ure, for example where the breathing loop becomes
ooded with water or an inefcient CO2 absorber,
the diver must stop breathing from the rebreather
loop and instead move onto open-circuit bailout.
In less extreme events, such as total or partial fail-
ure of the electronics or the loss of a gas, the
rebreather loop can still be dived in semi-closed
mode, either on the diluent or on the bailout bot-
tom gas. This mode type means that the dive can be
safely completed using only a third of the gas quan-
tity needed for the equivalent open circuit bailout.
When the self-sufcient bailout strategy is cho-
sen, the diver can select a conguration that would
include breathing the diluent gas as the rst bail-
out gas, followed by an intermediate gas mixture.
This would be then followed by breathing a decom-
pression gas that could be used until reaching 6 m
depth, where the loop could be breathed in pure
oxygen mode, if not ooded or if the CO2 absorber
continued to work properly; this is because elec-
tronic control would not be required at those
depths. There would still be an option of breathing
pure oxygen in open circuit mode in the case of
complete failure of the breathing loop. This con-
guration could make use of 6.8 L carbon 300 bar
dive cylinders for the intermediate and decompres-
sion gas mixtures, and 7.0 L aluminium cylinders
for the diluent and pure oxygen gases. However,
when planning the volumes of gases required to
support this conguration, consideration must be
given to the fact that gases compressed to 300 bar
do not follow the ideal gas laws. The Van der Waals
interactions cannot be ignored above pressures of
240 bar and, instead, real gas laws apply and have
the effect of reducing the assumed available volume
of breathing gas.
Where it is planned that the bailout gases would
be shared within the dive team, there is the obvi-
ous requirement that the divers remain together
during the complete dive. Following Mount and
Dituri (2008), a team bailout could be planned
based on the three divers having sufcient bailout
gases to support the ascent of 1.5 divers to the sur-
face in open circuit mode. The three divers would
each carry a pair of 11 L S80 cylinders: one cylin-
der would be lled with a bottom gas that should
always be available to any of the three divers; the
second would contain a decompression gas for two
divers; and the third would be an intermediate gas
for the third diver. The team would have to swap
the cylinders between them during the ascent to
support the diver having to bail out. The team bail-
out strategy is more optimal in terms of the num-
ber of cylinders that need to be carried per diver,
but it does assume that only one unit during the
dive has a problem requiring a full open circuit
bailout.
The bailout ascent prole will depend on the
composition of the bailout gases.
These are chosen in CCR deep mixed-gas diving
following strict guidelines. The guidelines are
based on the CCR diver having a bottom gas avail-
able that, when breathed on open circuit, would
neither be hyperoxic or narcotic at the MOD of the
planned dive. For example, a CCR dive that used
air as the diluent to a maximum diving depth of
40 m, could use air as the bailout gas at that MOD.
When diving deeper and using a mixed gas the
ideal is to have the maximum possible oxygen frac-
tion in the bailout gas to enable a maximum ppO2
of 1.40 for the bottom gas and 1.60 for the decom-
pression gas. For the remaining inert gas composi-
tion of the gas mixture, the strategy followed is to
minimise the increase of ppN2 to a target amount
while lowering the fraction of helium in the gas.
The ideal situation is to replace the helium with
oxygen while keeping the same amount of nitrogen
at the gas switch. This is usually not practical for
something like a 100 m depth dive with a bottom
time of 20 min while only using two ascent gases. It
would, however, be possible with three ascent gases.
In practice, for a dive of 20 min bottom time at
100 m depth, the gas choice should be as follows.
The initial diluent being used for the dive could be
8/67 (8% is the fraction of oxygen while 67% is the
helium fraction, with the remaining part 25%
being the nitrogen fraction of the mix; this mix
would have an END of 23 m at 100 m). On initiat-
ing a bailout, the diver could switch to a bottom gas
of 13/65, which during the ascent is then changed
to a 25/45 mix at 45 m. At 20 m, the diver could
change to a 50/20 triox mixture (triox gases that are
a trimix with an oxygen fraction higher than 21%),
followed ideally by a last switch at 6 m from triox to
pure oxygen. In this case, the total decompression
time during a bailout situation would be similar to
the CCR time without bailout. Mount and Dituri
(2008) published a table that can be used to com-
pute the composition of bailout gases.
The scientic diving sector has the lowest inci-
dence of decompression accident rates of all the
industry sectors (Dardeau et al., 2012). This may, in
part, be because of the education levels of the pop-
ulation in the sector, in addition to their ability to
recognise when to not attempt or to terminate
dives that are considered to be unsafe. There is, at
37
Underwater Technology Vol. 34, No. 1, 2016
present, no evidence to support that the accident
rates will change as or if the mean diving depth
increases. Fock (2013) suggested that CCR diving
risk is reduced signicantly when all the rules are
respected and the use of the rebreather is well
understood by the user. In most of the accident
cases reported, the causal factor was human error
and not rebreather failure.
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