O2CCR

Dan's Rebreather Brainstorms

Closed Circuit Oxygen Rebreather
(First Generation)

My other rebreather brainstorms - Electronically Controlled Closed Circuit Rebreather & Passive Addition Semi-Closed Rebreather - so far remain just concepts. I've started to put together that simplest of devices, an oxygen closed circuit rebreather. As usual, I'm borrowing heavily from everybody who's already been here, done this. Many will of course notice a strong resemblance to Dr. Bob's FEOR line of oxygen units.

My main purpose for this rig, other than just building one so I'll be qualified to dive an RB, is to have a source of warm, moist oxygen for those long, cold deco stops we occasionally make here in the Big Lake. As such, I wanted it to be wearable in addition to my usual dive gear, thus the front-mounted design.

Mouthpiece/DSV: DSV & hoses

1" PVC ball valve drilled axially to ~1-1/16" to accept 3/4" thinwall PVC adapter to SCUBA mouthpiece. Inlet/outlet check valves ~1-3/16" diameter from a dust respirator. No, I didn't copy Dr. Bob's or Patrick Duffy's - I thought I had invented it until I saw their sites. I guess form follows function. I expect that anything deeper than oxygen depths would require a somewhat larger bore.

Hoses:
1-3/8 I.D. plastic shop vac hoses. The size is appropriate for my use, but these seem awfully fragile for the usual dive-boat environment. I may have to sew up some protective sleeves for these. Dr. Bob may have found the perfect affordable homebuilt rebreather hose, but none of the rebreather suppliers - er, hardware stores around here sell them.

Scrubber:

An axial flow scrubber with a bed 4 inches in diameter and 12 inches long is utilized. The scrubber dimensions are scientifically calculated to be aesthetically and egonomically matched to the pony bottle I happened to have available for an O2 source. (Actually, it's a medical "D" oxygen bottle fitted with a SCUBA valve, formerly used as an argon bottle, now being used for oxygen again - how's that for the worm eating its own tail?) Coincidentally, the dimensions are also nearly exactly those specified for 170 minute duration with Baralyme in the U.S. Navy Experimental Dive Unit's Research Report 3-64, "Carbon Dioxide Systems for SCUBA, Quantitative Considerations of Design and Performance of Cylindrical Canisters" (available from the References section of Mark Munro's Web site.) The scrubber is made from a piece of PVC sewer pipe with a handmade plug glued into one end, with the other end stopped with a removable plumbing test plug. Internal PVC baffles connected by spring-loaded through rods keep the scrubber material from channelling. The oxygen supply tank mounts below the scrubber, held in place by a custom stainless steel band (also known as a hose clamp).

Counterlungs: 'Jason Bag' Counterlung

Dual chest-mounted "Jason bags" constructed of 4.13" inside diameter PVC lay-flat hose ~14-1/2" long each. Tee fittings between DSV and scrubber are attached to counterlungs via PVC unions to allow breakdown for cleaning. The first set of bags I glued together with PVC cleaner and cement, but it took a couple months before I dared breathe out of them - it took that long for all the solvents to evaporate! The current lungs are constructed in the classic "Jason Bag" manner with screwed-together aluminum strap clamps. They mount to the neck strap at the bottom and in the middle. These counterlungs are long enough that the metal clamps might be a pain in the neck (literally), so I'm thinking about using either a pair of 2 liter Dromedary Bags in the over-the-shoulder configuration or a single 4 liter bag across the chest. A single counterlung would give up the advantages of the split counterlung: slower gas velocity across the scrubber lime and consequently lower apparent breathing resistance. It also begs the question of the position of the counterlung - in the inhale or exhale limb? At least a couple of commercial oxygen CCRs (Lar V & Norge II) have the CL in the exhalation side, apparently as a water trap ahead of the canister. Some homebuilders have recommended placing the CL in the inhalation side, since it is easier to blow against a high resistance scrubber than to suck gas through it. I haven't decided.

Oxygen Addition:
Rather than inventing a manual valve to inject oxygen into the loop, I decided to go with a professionally designed combination automatic demand valve and manual addition valve - an old SCUBA second stage with the exhaust blocked. It's mounted on the scrubber outlet and can be operated by the right hand, which also operates the cylinder valve.

In Action: Test driving on the bike

After getting some sodalime (4-8 mesh, 14-19% moisture) from KeoMed, it was time to charge it up & give it a test drive. Reading on the couch worked fine, so it was off to the weight room and a ride on the stationary bike. After a few minutes, the loop gas got warm, and after 15 minutes, it was almost uncomfortably hot. Most of the heat seemed to be in the inlet end of the scrubber, so I guess the cross-sectional area must be sufficient.

Since the bike test, I've modified the rig slightly. I was using aluminum window screen inside the PVC baffles to hold the sodalime in the scrubber. BAD IDEA! As part of the chemical reaction, sodalime and water produce sodium hydroxide, which reacts strongly (read: eats!) aluminum. It's been replaced with fiberglass screen. The tee fittings are now mounted directly to the scrubber body, eliminating the short hoses (and four potential failure points.) This also simplified the mounting of the counterlungs.

In-water testing began with a dive on the wreck of the DeSoto in 8 feet of fresh Lake Superior water. I wore the RB in its deco-rebreather mode - clipped to the harness on my regular OC gear. I learned a few things about it - the counterlungs are a bit too small - they'd bottom out AND overinflate on a normal diving breath. The demand valve was set too light - that is to say, it freeflowed. Well, duh! It's at waist level, so it's some 16 inches below the mouthpiece when I'm vertical in the water. To get through the dive, I operated in manual mode, turning the tank valve on and off. The "Jason Bags" are annoying in the chest-mounted configuration - the metal clamps are too close to my face for comfort. I'm eager to try the Dromedary bags. It's also a pain to get at my drysuit inflator valve and keep my BC inflator hose clear with the RB in place. The mushroom valves I use are very close to the inside diameter of the mouthpiece fittings, so they vibrate with a moaning sound that kind of detracts from the silent breathing that rebreathers are supposed to be about. The warm, moist air is great, though - OC SCUBA air seemed cold, dry, and noisy by comparison.

Closed Circuit Oxygen Rebreather
(Second Generation)

Gen II - Version 1

After the test dive on the first generation rebreather, it was put it out to pasture and the Gen II rebreather constructed, building on the lessons learned with Gen I. Gen II has several improvements in comfort and durability over the earlier test bed, aiming at making this a practical, comfortable rebreather for routine use on the decompression stages of deep dives in the Great Lakes.

Scrubber:

Gen I employed a scrubber canister made of 4" PVC drain pipe with a wall thickness of ~1/16 inch. Gen II uses Schedule 40 PVC DWV pipe with a wall thickness of more than 1/4 inch. This has the advantage of being a much better insulator, keeping the scrubber bed warmer for higher efficiency, as well as being structurally much more robust. The same general dimensions and layout are used, with the addition of a water trap which doubles as a neck strap bracket. The trap can be drained underwater by opening a drain cock and pressurizing the loop to blow the water out.

Counterlungs: Dromedary Bag

Even using the lightest-weight lay-flat hose I could find, the "Jason Bag" counterlungs were rather stiff; and the metal clamps were too close to my face for comfort. I got a buddy to machine a couple of O-ring fittings to fit into the necks of a pair of Dromedary Bags. The 2 liter size actually holds ~2.2 liters, and a pair makes a a good match for my ~4 liter vital capacity. They are also much more flexible and better shaped for counterlung duty.

Mouthpiece/DSV:
The same DSV has been retained, with the addition of a small purge valve à la Patrick Duffy's. This makes it much easier to switch to the RB underwater without having to swallow the water in the mouthpiece (or blow it into the loop and soak the scrubber!)

Hoses:
Although the shop-vac hoses used in the Gen I were about the right size and flexible enough, they seemed rather fragile. Patrick Duffy at OxyCheq sells genuine rebreather hoses OEMed for Gordon Smith's KISS rebreather. They're robust, flexible rubber hoses that fit perfectly over the 1" PVC pipe favored for homebuilt breathing loops, and are tough enough to stand up to the abuse routinely inflicted on dive gear. I modified a PVC compression union for use as a hose connector, allowing the hoses to be easily removed from the scrubber for post-dive cleaning and drying.

Oxygen Addition: Gen 2 O2 Addition Valve

The oxygen addition system for the Gen II is a refinement of the Gen I glorified SCUBA reg, this time using an adjustable second stage to give me some degree of control over the inhalation resistance. A custom hose and a 90° swivel keep the danglies to a minimum.

In Action:
The rig has been dived a couple of times so far, just to give it a wet test and to photograph it in use. Both dives were on the wreck of the Desoto, a wooden schooner wreck from 1869 in Marquette Harbor. The DeSoto provides some scenic photo sites within a short swim from shore in rebreather-friendly depths.
With winter coming on and the boat put away, it might be next summer before I get to try the Gen II in its intended "deco-breather" mode. There's always ice diving, though...

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