| The Ouroboros Rebreather.
A Technical Review.... By Kevin Gurr
My first encounter with rebreathers was
in the late 1980’s and consisted of a 200m chamber
dive on an experimental unit. It failed. This brief experience
and a range of deep open circuit diving projects convinced
me that rebreathers were the future. Almost eight years
ago, having decided the choice was limited, I decided to
build my own. This is a brief summary of what I have learnt
and how combined with a review of recent incidents, I have
finalised on the current design.
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Fundamentals
In order to understand what makes a good rebreather,
firstly one must understand the fundamental properties
that govern a rebreather design. This can be divided
into 4 areas.
1. The resistive work of breathing
(WOB) within the rebreather
2. The hydrostatic WOB of the unit when submerged
3. The absorbent duration
4. The oxygen control dynamics
Looking at each in
turn.
The Resistive Work of Breathing
This is purely a result of the gas flow restrictions
within the unit. In other words how much the size
of the pipes and orifices generate a resistance to
breathing. Such things as small mouthpiece mushroom
valves, small hoses, counter lungs with insufficient
volume or room to expand and long absorbent paths
within a canister are common elements which go to
make up a resistive breathing circuit.
Restive WOB is also a function of
gas density and hence depth. The deeper the dive and
the higher the gas density the greater the WOB. WOB
is also a function of ventilation or breathing rate.
The more gas flow (higher breathing rate) the mores
resistance is generated. A rebreather that breaths
OK on the surface may well not at 40m on an air diluent.
This why current CE (Central European test standards)
and military test standards insist on a resistive
WOB measurement at depth and with different ventilation
rates and normally in two orientations (swimming positions).
The Hydrostatic WOB
This is the result of the resistive WOB and the effects
of the position of the counter lungs about the body
when the rebreather and diver are submerged in water.
For example, a back mounted counter lung rebreather
may have a good resistive WOB but when in a horizontal
(face down) swimming position the distance, hence
pressure difference between the counter lungs and
the lung centroid, may when combined with the resistive
WOB create an excessive pressure which the diver has
to suck against in order to take a breath. In this
case the inhale pressure would be excessive (because
the diver is inhaling gas from a lower pressure) and
the exhale would be easy having breathed out into
a lower pressure. Chest mounted
counter lungs have the reverse affect in the same
swim position.
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Above; Electronic
guts at the heart of the Ouroborus Rebreather
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It would seem a perfect solution is an
over-shoulder counter lung upon which any hydrostatic effects
have little result. However if a rebreather has a poor resistive
WOB anyway, the combination of this and any minimal hydrostatic
WOB can still mean the unit has a highly restive breathing
circuit and hence have a laboured breathing ‘feel’.
In summary a rebreathers WOB can only be
quantified under a range of hydrostatic (rotating) positions
with additional measurements at depth. Furthermore, assessing
a unit under a range of surface conditions at undefined
ventilation rates is inadequate as it is only under stressfull
conditions (and normally at depth) that we generate high
work-rates and this is precisely when the rebreather needs
to have a low WOB.
Absorbent duration
This is a greatly misunderstood area. Many manufacturers
quote durations based on simple surface trials which is
insufficient and potential dangerous. Absorbent life is
primarily affected by the following;
Above; Electronics
team working the units central pod in the factory.
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1. Amount
(Kg or Lbs) of material
2. Type of absorbent material
3. Water temperature
4. The absorbent canisters ability to insulate against
the water temperature
5. The amount of CO2 generated by the diver
6. The gas density/depth
7. The style and design of canister |
The standard CE
test for a canister is done at 40m with air as a diluent
in 4 degrees centigrade water temperature at a CO2 generation
rate of 1.6 l/min and a ventilation rate of 40l/min. Some
navies test canister at around 18m and with as low as 0.5l/min
CO2 generation. The CO2 breakthrough figure is referenced
to 5mb.
The range of depths, gas densities, CO2
rates and water temperatures used in each case can, dependant
on the canister design, give markedly different durations.
What is certain from the data available is that canister
durations measured at the surface are dangerously inaccurate
for predicting overall dive durations. It can be easily
shown that for most axial and even radial canisters the
efficiency of the canister decreases significantly with
depth. One canister measured went from 77% efficient in
15m to 49% efficient in 40m. The 15m duration was 3 hours
while the 40m duration was 1 hour and 50 minutes.
The ‘saving grace’ of currently
available designs is that most people cannot maintain rates
of 1.6 l/min CO2 or seldom dive in 4-degree water. However
data suggests that as a method of specifying canister duration,
a single hourly rate independent of primarily depth and
gas density, is insufficient and a range of diving parameter
curves may be better employed to specify a unit’s
duration. In other words in order to assess a rebreathers’
suitability for one type of diving over another, it would
seem appropriate to test a unit at a range of depths with
at least air and trimix as gases. To provide a safety margin
water temperature and CO2 generation rate should remain
constant.
Oxygen control system
This can be a mechanical or an electronically controlled device.
Which ever is chosen it is important that the control of the
oxygen level is maintained within certain limits. If decompression
is to be conducted using tables or a fixed PO2 dive computer,
the limits must be accurately maintained. Rapid excursions
to and from depth must also not generate excessively low or
high PO2’s. Ideally any such limits reached should generate
an alarm, as it cannot be relied upon that the diver will
notice especially in a multi-tasking situation. Some rebreather
designs, due to high flow rates within the oxygen circuit,
can generate massive PO2 spikes sufficient to cause convulsions
in a short space of time if left unchecked, this is an undesirable
design feature should an addition valve fail. Within the CE
and most Navy tests is a PO2 tracking control test as well
as a upper and lower limit test after rapid depth changes.
With units employing a constant minimum feed of oxygen, which
is supplemented by the diver, then this minimum should not
generate the upper and lower test limits specified.
Primary and secondary Po2 displays |
Compact and tidy electronics pod and
pneumatics. |
The Ouroboros
With all of the above in mind and a review of rebreather accidents
I started designing my unit. Rebreather
problems people had in the past
were because;
1. They started the dive with their
electronic control system off
2. They started the dive with their oxygen turned off
3. They descended with diluent off and then panicked when
they could not find the manual addition
4. They did surface swims on hypoxic diluents
5. They did not pack the absorbent canister correctly or
the design of the canister allowed CO2 to bypass if O rings
where incorrectly greased or assembled.
6. With insufficient guidance on canister durations, people
exceeded the duration limits
7. Temporary floods made the breathing loop unusable
8. Insufficient filtering produced oxygen solenoid failures
9. Hose attachment systems produced stress points which
perished the hoses
10. Electronics in the loop was affected by moisture
11. Gas supplies were accidentally switched
12. Failures in the electronics made the unit unusable
13. They become stressed at high work rates
14. DCI occurred as a result of the units inability to maintain
a near constant PO2
15. They did not follow pre-dive procedures
The Ouroborus is designed with
the above in mind and as a result has the following features;
1. A system that protected all ‘soft
parts’ and pressure lines
a. As a result, back mounted counter lungs are employed
in the primary design. All internal pipe work is stainless
steel, breathing hoses are internally sprung with a rubber
and nylon covering to prevent abrasion and crushing.
2. A low WOB
3. A canister that would auto pack with
little or no chance of channelling due to mis-packing
a. Axial and tower radial canisters suffer from large amounts
of ‘pack down’ due to the long length of absorbent.
Short ‘doughnut’ radial canisters have less
of a problem. Axial canisters especially, when not packed
correctly, can produce a bypass channel when tipped flat.
The Ouroboros uses a doughnut radial design
4. Efficient water removal systems
a. The split counter lungs provide an efficient water trap.
Water can also be dumped from the exhale counter lung automatically
5. A canister with high efficiency even at depth and with
high CO2 rates
a. The unit has been tested at and will be rated for a range
of depths
6. A ‘no tools’ unit for general maintenance
a. Daily maintenance requires no tools. Even electronic
parts can be ‘field stripped’ with a multi-tool.
No specialist tools are required.
7. Electronics with manual overrides
a. In the event of an electronic failure, a separately powered
and isolated means of monitoring PO2 is vital. In units
where dual electronics provide all the control it is advisable
that two different software writers are employed to avoid
common ‘bugs’. The Ouroboros has a seperatly
powered, triple PO2 meter display, even cable severance
will not interfere with the main electronics.
8. On board decompression
a. This reduces the affects of units not accurately tracking
PO2 when hard tables or fixed PO2 computers are used
9. No electronic lockouts
a. Given that units will be used for cave and deep diving,
there should not be an instance where the diver cannot over
ride the electronics and complete the dive. All automation
on the Ouroboros is de-selectable to allow manual over rides.
10. Ease of assembly with no chance of misplacing parts
or gases
a. All primarily maintenance is ‘tool free’
with no possibility of connecting gases incorrectly. External
gas supplies are also coded to avoid swapping.
11. A modular system for different diving styles
a. The primary design is a back-mounted in a carbon fibre
case. Different canister durations are available. A ‘travel’
system with a soft pack design and even a chest mounted
counter lung format is available
12. Intuitive alarms
a. Alarms available through a head up display (HUD)
are backed up by ‘on screen’ detail on the current
alarm
b. A rear facing display is available for ‘buddy’
diving and instruction
c. Alarms are visual and also via a vibration system in
the HUD
d. Digital HP transducers ensure cylinders that are turned
off generate alarms as do any leaks in the LP or HP systems
13. Automatic turn on systems once in-water
a. Wet turn on contacts, a pressure activated turn on and
turn on via switches provide redundancy to ensure the chance
of an hypoxic incident is reduced
14. Minimum life support
a. Once turned on, irrespective of the ‘set point’
a minimum PO2 of 0.4 is always maintained
15. No non hermetically sealed electronics in the breathing
loop
16. Low flow gas paths to reduce PO2 spiking but high enough
for good diluent flushes
a. All gas flow lines are flow matched to avoid ‘spiking’.
The automatic diluent addition valve can be user adjusted
for smooth descents
b. Both the Auto diluent addition and oxygen solenoid are
protected by in-line filters
17. Easy interchangeability with open circuit safety equipment
a. The unit will accept a range of back plates and wing/harness
system. Designed especially for instructors that often have
to switch back to open circuit.
18. Data logging
a. Data logging provides useful feedback in training and
incident scenarios. Every parameter is logged including
when the user aborts a pre-dive sequence
19. Isolation of automatic circuits which then allows manual
over rides
a. Automatic polling of sensors as well as manual isolation
in the event of failure can be achieved. Closed circuit
decompressions can be disabled in favour of an open circuit
decompression.
20. A simple but effective pre-dive sequence with short
set-up and breakdown times
a. An electronic pre dive sequence is available. Simple
canister packing and ‘no tools’ assembly ensure
rapid preparation for diving.
The Ouroboros is a modular design. The
primary layout is a back mounted system in a hard carbon
shell. Lightweight cordura covers are available with differing
canister sizes as well as a chest mounted counter lung configuration.
The unit will be in production first quarter 2005.
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