
Dan's Rebreather Brainstorms
Passive Addition Semiclosed Rebreather
The RMV-keyed passive addition semiclosed concept used in the Halcyon rebreather has always struck me as an effective method of insuring a relatively steady FO2 over a range of breathing rates, compared to a constant mass flow system. The original Halcyon's "Rube Goldberg" implementation of the concept, with its weighted accordion bellows, pulleys and cables, however, led me to dismiss it as the basis for a homebuilt project. The introduction of the Halcyon RB-80, designed by Dr. Reinhard Buchaly, with its relatively compact design and lack of convoluted depth compensation mechanism, inspired me to give the concept a second look. Take a look at a pictorial teardown of an RMV-keyed SCR which may or may not be the RB-80 (only the Czech Navy knows for sure!) at the Golem Gear website.
Dave Sutton's photos of the inner workings of what may be the original passive addition SCR, the DC-55, gives a good idea of how elegantly simple this type of rebreather could be. Ĺke Larsson’s page gives a nice overview of another demand-controlled SCR, the Interspiro DCSC, as well as an analysis of the passive gas addition system on the DC-55.
STDE (Swiss Technical Diving Equipment) was selling a virtual RB-80 clone but apparently has stopped distribution because of intellectual property conflicts with Halcyon. There is another passive addition prototype being advertised, the AirWay CO.RA (COnstant RAtio), which appears to be a robust RB80 clone. The CO.RA can be seen at Taucher.net (in German). There is also a German homebuilt unit that is claimed to be depth-compensated, the BK2. Italian Carlo Marcheggiani has built an RB-80 clone called the EDI 2001 which he has dived to 76 meters. Cave Explorer Frédéric Badier has built the "Joker" rebreather, another passive addition SCR uniquely configured for redundancy in cave diving. More photos and discussion of the Joker can be found (in French) at "Les recycleurs par Frédéric Badier" and at A.H.R.
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My homebuilt SCR will be a near-clone of the Halcyon RB-80 (or at least as far as I can deduce what's inside - I haven't seen one in person.) The main housing will hold a scrubber in the upper section, a gas injection valve below the scrubber, and a cylindrical dual bellows counterlung / metered discharge unit in the bottom of the shell. Breathing gas bottles will attach to either side of the housing, which can be mounted to a standard BCD with the same hardware as a standard scuba bottle.
A mouthpiece with DSV and large diameter hoses feed exhaled gas to and take scrubbed, replenished gas from the main unit. Exhaled gas travels through an inlet tube down to a small plenum below the scrubber at the top of the metered discharge/counterlung assembly. It then passes freely into the outer bellows, and via a check valve (scuba exhaust valve) into the smaller inner discharge bellows. Upon inhalation, gas is drawn up through the scrubber, contracting the outer bellows. As the bellows contracts, the gas in the inner bellows is forced out through another check valve in the bottom of the bellows and dumped. As the bellows reaches the end of its travel, it activates the lever of a downstream valve (the guts of a normal scuba second stage), making up the volume needed for a full breath.
I've hear from a couple of homebuilders who tried the neoprene-coated nylon bellows from McMaster and found them too stiff for counterlung use. The polyurethane bellows from the same source are said to be much more pliable and might work well.
The discharge valve from the inner bellows will have to be spring loaded - if a simple exhaust mushroom were used, an unmetered discharge would occur on anything more than a gentle exhalation. The slight spring pressure needed to keep the valve closed during exhalation would be easily overcome by the 5:1 mechanical advantage of the bellows on inhalation, perhaps aided by the lower position of the counterlung relative to the diver's lungs. A more involved solution would be to counterbalance the exhaust valve with a diaphragm similar to a scuba second stage, with the outer side of the diaphragm ported to the breathing loop, and the inner (exhaust valve) side vented to ambient. During exhalation, the positive pressure in the loop would hold the valve closed, while on inhalation the negative pressure would allow the diaphragm to retract, releasing the exhaust valve.