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 Victoria, BC Canada, for a little show and tell.  We showed off the ISC Megalodon, and they showed us the CUMA and CCDA.  Both the CUMA and CCDA use the same loop, DSV, counterlungs, scrubber canister, bailout system, and FFM (cressi-sub).  The units are both Semi-Closed Rebreathers (SCR), but there the similarities end.   This article will focus on the CUMA.


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