PASCR

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. Åke 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.

Pros:

No electronics - no worries about water leakage.

No 100% O2 addition - less risk of O2 toxicity.

Counterlung enclosed inside shell.

Intuitive recognition of gas addition failure. ("I can't get a breath!" alarm mode)

Integral water / condensation removal pump.

Cons:

No electronics - no direct monitoring of PO2.

Decreasing PO2 on ascent - need to vent expanding gas to allow makeup with fresh gas.

Deep dives require one or more gas switch.

Lower back counterlung location may cause high work of breathing.

Single counterlung requires a larger scrubber than split CL.

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.

Mouthpiece/DSV:
1" (1 1/4"?) PVC ball valve drilled axially to ~1-1/16" to accept 3/4" thinwall PVC adapter to SCUBA mouthpiece. Optionally, the valve shell would be drilled 90° radially to connect to a SCUBA second stage to provide OC bailout - OC reg or loop would be exclusively valved to mouthpiece via DSV ala Halcyon. Inlet/outlet check valves ~1-3/16" diameter from a dust respirator (How do these stand up in water? Maybe wiser to use large diameter SCUBA exhaust valves?). Take a look at Patrick Duffy's Home Built DSV .

Scrubber:
Intergranular space should be » tidal volume - discharged gas. This allows the entire recycled gas volume to be in contact with the lime for one full respiratory cycle. At a tidal volume » 3 liters, intergranular space = 40%, and a discharge ratio of 17%, a 6.23 liter scrubber would be called for. At 7.25" I.D. with a 1.375" O.D. inlet tube, cross sectional area = 39.8 sq. in., calling for a scrubber bed about 9.5" long. At 8" I.D., the same scrubber layout would require a bed about 7.7" long.

Counterlung / Metered Discharge:
McMaster-Carr sells a range of neoprene-coated nylon bellows ranging from 1 3/4" O.D. to 12" O.D. If I could find a suitable tube for the main housing (approximately 7" I.D.), I'd use the 6 3/4" O.D. X 5" I.D. bellows for the outer bellows. This would need to be about 9" long to give a maximum counterlung volume » my vital capacity » 4 liters. An inner bellows measuring 3" O.D. X 2" I.D. would give a discharge ratio of about 17%. An 8" X 6" outer bellows would allow a shorter counterlung (at about 6 1/2") and would give a choice of 3 1/2" X 2 1/2" or 3" X 2" inner bellows for a discharge ratio of 18% or 12%.

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.

Gas Addition:
Since a fixed percentage of the exhaled breath is discharged through the inner bellows, the unit will need to make up this lost gas. As the counterlung reaches the end of its travel on inhalation, the end plate of the bellows operates the lever of a downstream demand valve (simply the innards of a conventional SCUBA second stage), which replenishes the breathing gas in the loop. The demand valve is mounted through the side of the main housing between the scrubber and the counterlung, with its demand lever extending through a slot into the counterlung. For redundancy, a second demand valve could be included.

Gas Switching:
Because of the fixed discharge ratio, dives beyond about 100 feet will require gas switching to provide breathable levels of PO2 both at depth and near the surface. One arrangement would be to have a quick-connect / valve block arrangement similar to the Halcyon, with the ability to plug in quick-connect whips from stage bottles or back gas and to direct gas flow to either demand valve. My current thinking is to use a valve similar to the Sartek RSV-1 to select bottom or travel gas.

Water Extraction:
The inner bellows along with its check valves is really nothing more than a positive displacement pump powered by the outer bellows. As such, it can be made to serve double duty as a pump to evacuate any water which may enter the loop from condensation or from mismanagement of the DSV / mouthpiece. The inlet to the inner bellows is simply connected via a small tube to the lowest point in the plenum below the scrubber. If there's any water present, it will be drawn into the bellows along with the exhaled air as the bellows expands, then expelled with the discharged air on the inhalation stroke.
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