Winner: Poseidon Discovery

The world's leading designer of scuba gear brings closed-circuit rebreathers to the masses

photos: Top, Poseidon Diving Systems; Bottom, Karl Shreeves

First breath

Closed-circuit rebreathers, long the scuba technology of choice for professionals, may soon be available to amateur divers.

This month, Swedish scuba manufacturer Poseidon Diving Systems plans to introduce an electronically controlled closedâ¿¿circuit rebreather (CCR) called Discovery, which promises to change mainstream sport diving the way Microsoft Windows changed computing. Rebreathers, which have been around for decades, greatly increase dive time but at an enormous cost in complexity, training, and setup time. Poseidon’s new system is designed to do away with all that. In October, the company gave me the system for a 30-minute test splash during a trade show in Las Vegas.

To see what sets Discovery apart, let’s review some scuba basics. The conventional scuba equipment that Jacques Cousteau introduced in 1943 has an open circuit. The regulator supplies air from the tank, and your exhaled breath vents into the water as bubbles. Your body uses only a fraction of the oxygen in each breath; the rest is wasted. At higher pressures, each breath takes more from your cylinder, so you use up your air faster just when you need it the most—on your deepest dives.

A rebreather with a closed circuit gets around the problem by salvaging oxygen in your breath. You exhale into a counterlung, a gas bladder that expands and contracts as you breathe. The air then passes through a chemical-absorbent canister that removes carbon dioxide. Sensors analyze the oxygen content and direct the system to add either pure oxygen or regular air, whichever is needed to keep the oxygen concentration within its proper boundaries. The resulting gas goes to a second counterlung, then back to you when you inhale.

Closed-circuit rebreathers can give you seven times as much dive time as standard scuba equipment, regardless of depth. They also make it easier for your body to adapt to changes in water pressure, because they adjust the ratio of nitrogen to oxygen as you go up or down. The downside is complexity. It can take up to an hour, even for experienced divers, to set up and check a CCR compared with a few minutes for an open-circuit system. Then, underwater, CCR divers must decipher multiple readouts—including sensor readings, supply pressures, decompression status, oxygen exposure status, oxygen partial pressure, and scrubber life; recognize the combined effects of all those variables on dive time; and make what are in effect tens of thousands of implicit decisions. Even with electronic systems, problems aren’t always obvious.

So until now, sport use of CCRs has been limited to a relatively hard-core cadre of ”tec” divers who are willing to deal with these hassles and risks. Even many professionals have stayed with the simplicity of open-circuit scuba.

Discovery may change all that. It is the culmination of 20 years of design and development by Bill Stone, arguably the most innovative and experienced CCR engineer alive, famous for leading several milestone cave-diving projects.

Since the mid-1980s, Stone and other divers have used limited-production rebreathers that use computer controls to improve operation and make the device easier to use. He has taken his Cis-Lunar Mark III, IV, and V models, designed for science and exploration, to depths in excess of 120 meters—edgy stuff.

The Discovery Cis-Lunar Mark VI, though, is the first to use sophisticated predictive modeling to make many of the complicated decisions that divers have had to make themselves, after undergoing many hours of training; it is designed for mainstream divers going no deeper than 40 meters. Though the Discovery is vastly simpler to use than its limited-production predecessors, its inner workings are much more complex—just as even the earliest Windows software had many more lines of code than MS-DOS. The result is a CCR design that’s an order of magnitude more sophisticated than what came before it.

The heart of Discovery is a set of 8â¿¿bit microprocessors distributed around the rig to provide computing power where it’s needed. Discovery not only monitors all subsystems but also collects breathing and environmental data to update its models in real time. That data is applied against a 160 000â¿¿entry decision table, gathered from thousands of hours of human test dives. The upshot is a system that takes on the decisions that burden conventional CCR divers—and, at times, Stone says, makes better decisions than humans.

Consider an emergency requiring a quick ascent or descent. Maybe you need to get to the surface quickly or dive down to a fellow diver in trouble. These rapid changes are problematic with a conventional CCR because you can’t change your oxygen-nitrogen ratio or breathing-gas volume quickly enough. You’re left with either too much or too little oxygen—and either situation is life-threatening. According to Stone, however, Discovery’s predictive modeling recognizes the abnormal descent or ascent and adjusts the mix of breathing gases based on where the diver will be rather than trying to keep up with where the diver is.

Under Stone’s guidance, I tried a beta unit in a swimming pool. Setup took a mere 10 minutes, compared with an hour for conventional rebreathers. You just change out the scrubber canister, attach two filled gas cylinders, and turn it on.

Underwater, the harness and weight distribution felt comfortable and tight immediately; normally it takes one or two dives to get any scuba rig ”tuned” to an optimal fit. Dry, the unit weighs around 15 kilograms, which is a bit lighter than a typical open-circuit rig. I understood the display without any real instruction. Stone told me to expect some alarms due to some software revisions they’re working on. This allowed me to see the warning icons, feel the mouthpiece vibration, and switch to open-circuit bailout. No issues here.

Discovery also did well at handling rapid breathing, a common problem in all scuba gear, open or closed, especially at greater depths. At its worst, you feel like you’re breathing through a soda straw while running a marathon. I huffed and puffed as hard as I could, simulating rapid breathing, but Discovery kept up.

When I surfaced, Stone asked me how the hoses felt, referring to the tendency for many CCRs to restrict comfortable head movement. I realized I hadn’t even thought about the hoses.

My dive lasted only 30 minutes and didn’t go deeper than about 2.5 meters, so I wasn’t able to see how Discovery performed at depths that would truly test the system’s capabilities. Still, I’m left with the impression that this may indeed be the first CCR for casual recreational divers.

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