OMG C-96 mixed gas conversions

The OMG Castoro C-96 Pro is a fully closed oxygen only rebreather manufactured in Italy. (Castoro is Italian for Beaver). Dave Sutton has a teardown of this unit on his webpage, as well as a description of a conversion of this chest worn rebreather to full mixed gas closed circuit operation. This webpage provides an alternative configuration for modifying the C-96 for cave diving use with the usual sidemounted cylinders used by British cave divers.

The general photograph above shows neoprene hose covers, primarily for abrasion protection. The oxygen cylinder is reversed compared to the original, in order to maintain the system of 'Rich on Right' and 'Lean on Left'. The gas nipple is the standard item moved to (diver's) bottom right, the original connecting fitting just unscrews and can be replaced with a standard male quick connect as it is the same thread.

Details

• O₂ Supply
• Diluent Supply
• Oxygen Meter
• Assembled

O₂ Supply

Oxygen is delivered betwen 0.65 and 1 litres/min (approximately metabolic rate) using a KISS valve from Gordon Smith. The Apeks DS4 first stage is fitted with button pressure guage and overpressure relief valve. The Apeks first stage has been modified to set the interstage pressure to a constant value (ca. 10 bar) so that the flow rate to the counterlung stays fixed. The KISS valve has a particle filter on the inlet side. The pipe is a standard BC hose cut to size, using O-clips for security (jubilee clips are a snag hazard and could cut the bag). A loop of runner inner tube secures it up against the side of the bag.

Diluent Supply

Diluent is added manually via a drysuit inflator assembly from AP valves.

O₂ meter

A three sensor remote powered display from Mark Munro is used to provide an indication of the oxygen conent in the counterlungs. A Bob Howell Draeger cell holder holds three type K1D cells sideways and a 9V battery in a 32 mm diameter rod of Delrin, using a cheap woodworking router and a drill.

O2 monitoring fits the Draeger port (true left) and an Inspiration dump valve is fitted on the true right. These positions are out of the way, under the twin support straps. The dump vents on overpressure and can be manually operated by a cord suspended from the support strap.

Assembled

The C-96 should always be donned facing Mecca!