Absorbent canister/Electronics housing

In the Electrolung the CO2 absorbent and electronics were housed in a transparent acrylic (Plexiglas) cylinder which together with the gas bottles was worn as a backpack. This cylinder was 24" long x 4" I.D. with 1/4" thick walls. It was divided into two pieces. An 18" section contained the absorbent bed with a 4" space at the top which accommodated the sensors and O2 solenoid. A 6" section at the top contained the electronics. A small knob on the electronics section operated a double throw, center off, switch controlling power to the electronics via either set of batteries. This was reachable behind the head if development of your biceps didn't prevent it. Actually geeks do make the best RB divers anyway but out of respect for the temporary cease fire I will refrain from further comment along that line.

The ends were sealed by 1/2" thick PVC O-ring sealed plug type closures. A thick PVC double plug type bulkhead joined and partitioned off the electronics section from the absorbent section. The entire assembly was held rigidly together by a central 1/4 SS tie rod running from top to bottom. It had a large external knob at the bottom for tightening and loosening which was effected by screwing into a metal socket on the top closure. O-ring seals were used to seal the tie rod penetration of the bottom closure, the metal tie rod socket on the top closure and where the tie rod passed through the electronics bulkhead. This last was only to prevent capillary action from possibly drawing any water from the absorbent section into the electronics section. The electronics section atmosphere was vented to the absorbent section through a small canister of silica gel via a separate bulkhead penetration and a small standpipe as mentioned earlier in the description of the Electronics.

The O-ring seals for the end closures and the join at the electronics bulkhead were all radial type seals which automatically effect a proper seal when they are plugged into the cylinder. Sealing is effectively independent of how tightly or loosely things are clamped together. With this type of seal and operating at ambient pressure the possibility of leakage around a seal is vanishingly small.

A brief aside for homebuilders. Although O-rings are marvelously effective seals and are universally used in all types of underwater equipment it is remarkable how often manufactures use them improperly. O-ring suppliers have various free pamphlets and data sheets on proper application of O-rings which includes data on the correct shape and tolerances for the grooves which accommodate them. It is well worth while to avail yourself of this information.

Common errors in O-ring usage which you often see in marine equipment are: Grooves too deep resulting in inadequate sealing pressure. Too shallow, resulting in too much compression of the seal leading eventually to fine radial cracking and consequent leaking. Too narrow, which interferes with proper compression and sealing at low compression, and distortion toward a square cross section under full compression, this leading again to radial cracking of the seal. Finally, and most ignorant of all, is the use of rounded U-shaped grooves which defeats the whole principle and advantages of the circular cross section and turns it effectively into a flat gasket.

The absorbent canister portion of the cylinder was a 12" section toward the bottom defined by two 1/4" thick acrylic or PVC internal bulkheads perforated with an array of holes. Plastic screen was used to keep the absorbent from falling through the holes. The top bulkhead was fixed in place. The bottom one was free to move but held in place against the absorbent column by a large spring. This served to keep the absorbent compacted without channeling despite any minor settling of the granules after filling the canister.

We used Barylyme as an absorbent. This is a National Cylinder Gas trade name for Barium Hydroxide. It was widely used in hospitals and came in hermetically sealed one quart cartons of the type used for milk. The electrolung canister held two cartons which would be sufficient for six hours of moderate activity. We changed them after four hours. Barylyme came with a color indicator, pink when fresh, blue when expired. It is less caustic than soda lime and worked well for us.

As described earlier gas was drawn from the counterlung to the space at the bottom of the canister down a central 1" I.D. tube leading from the inlet hose attachment down to the bottom end of the canister section. From there it passed back up through the absorbent into the sensor/solenoid chamber and on via the inhale hose to the mouthpiece.

The gas supply cylinders were mounted on either side of the absorbent canister/electronics cylinder using spacer blocks conforming to the curvature of the respective cylinders. The three cylinders were secured rigidly in place by two large SS hose clamps. One was adequate for the purpose so there was backup in the event of one breaking. The spacer blocks also served as attachment points for the harness which was a U.S. Divers one of the type widely used before B.C.'s took over this function. There was a wide vinyl strap for each shoulder plus a waist strap. The overall configuration of the Electrolung backpack was similar in many respects to that of the small triple tank OC rigs favored by the French at that time. It rode well on the back and was quite comfortable. All up weight of the Electrolung was about 30 pounds.

Some people expressed concern about the use of acrylic fearing the possibility of breakage. This is one of those things which is more apparent than real. In this case it is protected on one face by the wearers body, on either side by steel gas cylinders and at top and bottom by thick PVC ends. The only real exposure to any possible impact was the curved surface of a 4 1/2' O.D. 1/4 thick cylinder which would be extremely hard to break. We did look at using polycarbonate (Lexan) which is literally bullet proof but found it crazed and crumbled into small pieces when exposed to hydroxides. It would of course be easy to make the whole thing of PVC but I feel the advantage in being able to see condensation, water, and the condition of your absorbent more than outweighs the non-problem of smashing heavily into things while going backwards. Beckman offered a fiberglass fairing for those who might be concerned with this but of course some then bought it because they likeed the way it looked and others did so because they felt that if it was offered they probably should get the complete set up.

A final aside for homebuilders. The Electrolung was really a homebuilt which became a commercial product. It was built entirely with a drill press, lathe, and jigsaw; plus a bench grinder for shaping and sharpening lathe tools as the only power tools. For anyone attempting to build any kind of underwater equipment a metal lathe is really a must. With one you can easily make all kinds of cylindrical housings, O-ring sealed fittings, ports and closures, and any kind of threaded fitting you might need. A small lathe with a five or six inch swing over the bed will enable you to make housings and ports up to 10-12" in diameter. Good quality Chinese made lathes suitable for this kind of work are now available for about U.S. $ 1500. For another few hundred dollars you can also get a milling attachment as well which is a useful addition. Teaching yourself how to use it is not hard. Good text books which cover this kind of machine tool work are readily available and easy to follow.

The Electrolung patent is online at http://www.patents.ibm.com/details?patent_number=3727626 The online material is a summary but you can order the full patent there. It contains much more detail including various drawings.