Canadian Forces CUMA Rebreather
A special thanks to Dave Tomblin, and CPO2 Rob Mackay of
FDU and his crew, for this article, which would not have been possible without
their help.
FDU’s Rebreather prep room.
Recently Leon Scamahorn of ISC, and I, were invited to
the Fleet Diving Unit (Pacific) of the Canadian Forces base at
CUMA is an acronym for Canadian Underwater Mine
countermeasures Apparatus and is a specifically designed unit to minimize
magnetic and acoustic signature and allow divers to approach and work on ordnance
in an underwater environment safely.
“Safely” being a relative term.
The rig, ready to dive, is 78 lbs, holds about 5 lbs of
scrubber and is rated to 270 fsw for a maximum run time of about 150 minutes.
This endurance figure is based upon the supply and flow of oxygen. Diluent endurance is depth dependent. All fittings are machined and of excellent
quality, mostly brass or plastic, most of the weight seems to be in fittings
and valves. No shortcuts here. The rig has a hard backplate
and was not very comfortable while standing, but if I was busy working on a live
mine, I think I would have other worries.
Front and back view of me wearing the CUMA with the
back fairing removed.
SHOW ME THE MINES!
I hope this thing comes with diapers.
The unit utilizes front mounted split counterlungs and
has a left to right gas flow direction. The
divers exhaled gas leaves the DSV (dive surface valve)
into the exhale hose through a mushroom valve and into the divers right
(exhale) counterlung via an elbow fitting.
That counterlung is fitted with an adjustable BCV (Buoyancy control
Valve) that is equipped with a gas bubble diffuser and allows dumping of loop
volume continuously while diving. (remember the unit
is SCR) The gas then leaves the CL and
enters the backpack housing, made of vacuum formed polyethylene, through a
cutout and into the scrubber canister assembly.
The scrubber basket is a radial design and is very similar to the
scrubber used in the PRISM CCR. Scrubber
endurance is rated to 180 min with 2.2 kg of granules. The flow into the scrubber basket is outside
to inside and the gas then enters the canister lid where the fresh pre mixed
gas is injected, following that through the housing out to the inhale
counterlung which has a quick dump valve as well as a fitting for bailout gas
to be plumbed. More on
bailout later. The gas then comes
out of the inhale counterlung to the diver with another elbow fitting and to
the second mushroom valve in the DSV.
Scrubber in the housing, and the injection point on
the scrubber lid.
The way this unit is designed to operate is rather
interesting and unique, in my experience.
It uses two inconel flasks, or spheres (inconel, of course, is
non-magnetic and corrosion free) to provide a gas supply to the diver, one
filled with pure oxygen, the other with pure helium, each with its own DIN
valve and regulator. The spheres are
wrapped in a neoprene bag for protection from dings and scratches. The oxygen regulator is a constant IP unit,
which does not change its 290 psig output pressure with changes in depth and it
provides gas through an orifice to the loop at 3.6slpm (standard liters per
minute) regardless of depth, whenever the supply valve is turned on. The supply valve is situated just inside a
cutout in the back fairing, a difficult place to reach for the diver, but
prevents the diver from inadvertently shutting off the O2 during a dive. Gas is
situated conventionally, with diluent on divers left and oxygen on the right. The diluent valve has a knob that reaches
through the fairing, making it accessible. The Helium regulator is a standard
scuba reg that has its intermediate pressure fixed at
155-200psig above ambient. Both gasses
are fed into a ratio regulator which mixes and feeds them into the loop at the
lid of the canister. The ratio regulator
keeps the helium at zero flow when at less than 20 fsw depth, so above that
depth, the unit is an oxygen only rebreather.
Once the diver descends past 20 fsw the ratio
regulator ramps up the helium injection to match the flow to the depth. The target mix is equal to 1.5-1.7 PO2 and
that is what is fed into the loop. This
mixture, combined with the divers rebreathing, drops
the loop mix to about 1.0, and all decompression calculations are figured on
that PO2. the
flow rates are calculated to keep the loop mixture between .29 and 1.68, of
course the low figure is with the diver working hard, and the high figure while
at rest. Max VO2 is calculated to be 3 lpm. Rarely will the
loop fall below .7 in actual diving. So,
even though this unit is semi closed in operation, it is still a constant PO2
system, with the only variance being the divers work rate. A normal mass flow SCR like the Draeger dolphin, is a constant FO2
rig, basically just a long duration Nitrox cylinder.
The diver is provided with an SPG for each flask that
is plumbed over the shoulders, outboard of the counterlungs, and secured to
them.
The ratio Regulator just
below the spheres. Hoses are
color coded, green=oxygen, yellow=dil.
PO2 Diagnostic Analyzer.
Nestled just above and between the spheres is the PO2
diagnostic analyzer, where the gas flowing from the ratio regulator is analyzed
prior to being dumped into the loop. The 8 pin connector plugs into a diagnostic panel (see photo below)
to allow a complete “dirt dive” and checkout prior to a real “wet” dive. At the bottom of this device is a single
galvanic oxygen cell and its data is processed by the
analyzer and a simple two color LED provides the data to the diver. This LED is mounted directly into the side of
the Cressi-Sub FFM the diver wears. The cell is in its own pressure compartment,
which keeps it dry and clean, not in the loop where it can be assaulted by
moisture. The assembly has a battery
also, just below the circuit board. The
LED provides only Go/NoGo info to the diver, a green
light means the PO2 is within acceptable tolerances, the red light means it has
deviated and a “Red Light Drill” is necessary.
The diver is provided no more data than that,
there is no PO2 monitor for the loop.
This assembly is activated by being plugged into the mask LED, which has
a 6 conductor, wet mate plug, shown below.
Connector for HUD LED, and the LED mounted into the
mask.
The ratio regulator is fitted with two additional hoses,
one tapped into the helium supply and one tapped into the O2 supply, they feed
to a bypass valve, which allows the diver to flow mixed gas into the loop, to
flush or provide for a fast descent, basically giving loop volume. Above 20 feet of depth, the bypass only
provides oxygen. This valve is located
by the divers right hip.
The bypass valve.
There is also a bailout system, which consists of a
butt mounted cylinder with 160 liters of a premixed 30/70 heliox,
a regulator with a fixed metering orifice that is plumbed to an on/off rotary valve
on the divers chest and the output of this is plumbed into the divers inhale
counterlung. This provides approximately
9 minutes of gas flow for the diver to make a controlled ascent to the
surface. If the diver has accumulated
decompression, depending on the severity of the unit failure, he may be able to
go back on the main system to do stops on O2 at 20 fsw,
otherwise it’s a trip to the chamber.
The bailout mix can be breathed OC if the unit is flooded, but rotating
left shoulder up and activating the bailout and breathing open loop. Doesn’t sound like my happy place.
Quoting from the manual, “At a depth of 81msw
(270fsw), 30/70 heliox has a PO2 of 2.7ATA. As the bailout gas is diluted in the
breathing loop and the diver leaves the bottom shortly after operating the
bailout, this does not pose an oxygen toxicity problem. The endurance of 30/70 heliox
bailout gas is approximately 9 minutes, which is sufficient for a controlled
ascent to the surface or to 12msw to undertake the RED LIGHT Drill.”
YIKES! Sounds
like a submarine escape drill.
Some more detailed pics below.
The Cressi-sub FFM and the
DSV, note the green and red dots on the DSV, these match up with the Green and
red dots on the loop hoses. The DSV
operates from the horizontal, which is breathing on the loop, to straight up,
which opens a slot in the DSV allowing the diver to breath ambient air. UP for atmosphere, down for
loop gas.
The bailout valve, off. A 90 degree turn brings fresh 30/70 into the
loop from the bailout cylinder.
Exhale counterlung with the Buoyancy valve and bubble
diffuser.
Here is the diagnostic console used to pre-dive the
CUMA. The pre-dive is quite a process.
This is quite a purpose designed unit and is certainly
not for the recreational diver, the build quality is awesome and the price tag
reflects that. I like how compact it is
and the ratio regulator is a neat bit of engineering. Personally I would rather have more info than
red or green, but the guys diving this have more on their mind than just play
diving. The clearance divers we met
during our show and tell were all just straight up guys, the hospitality they
provided was second to none and we have been invited back to check out some
more dive toys and maybe get in the water to try them out. Stay tuned for more info…
Ron Micjan
25JUN06