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January 17, 2007

Thoughts about rebreathers

A year or so before I got into scuba I saw a remarkable, though rather corny, movie that contributed to my desire to see the world underwater, and also led to another interest. The film was "The Cave," a weird combination of gorgeous diving and underwater cave photography and a B-movie horror plot of evil creatures decimating a team of divers in spite of their modern technology. One such piece of technology was a "rebreather" that enabled them to be underwater much longer. For all I knew back then, that rebreather was just fiction. Now, of course, I know that it is not. It's a scuba unit that tackles the challenge of breathing underwater in a different, and arguably more elegant, way.

Simply put, a rebreather recycles the air we breathe, taking advantage of the fact that humans only use a small fraction of the oxygen that is in every breath we take. It scrubs out the carbon dioxide and uses various mechanisms to keep things in balance. The result is a smaller scuba unit that nonetheless allows much longer bottomtime, and also makes for a far less intrusive diver because, depending on the type of rebreather, there are no bubbles, or far fewer. (The picture on the left shows a Draeger rebreather unit I saw during a dive at Lake Tahoe. Note the very small Nitrox tank.) It's easy to see how such a device can fascinate. I hate waste in any shape or form, and the rebreather wastes nothing. I am also not fond of needless work, and shlepping around far more gear than a more elegant technology allows seems nonsensical. With all those advantages, why aren't rebreathers everywhere?

That is a good question. They've actually been around far longer than conventional scuba units. Long before Cousteau and friends tested the first demand valves and the "Aqua Lung," divers had been using rebreathers. In fact, they've been used for almost 130 years. And not just in theory but in actual commercial work, going back to the end of the 19th century (in 1880, a rebreather design by Englishman Henry Fluess was successfully used to close a sluice door in a flooded tunnel). Of all the diving books I've read that touch upon rebreathers in any way, half seem to view them as some sort of specialized technology that will never be viable for sports diving. The other half see them as the future of scuba.

So why aren't we all using rebreathers? The simple answer is safety. It's not that rebreathers aren't safe if used properly, it's just that they are infinitely more complex than conventional scuba, and when it comes to the air we breathe underwater, there is no margin for error.

See, scuba, despite all the remarkable technology that goes into all those sophisticated first and second stages, is really a simple concept. Compress a lot of air into a tank, then breathe it. Sure, there are dive physics which affect how we breathe air underwater and there are physiological aspects to consider -- like the impact of nitrogen on our bodies -- but for that we have dive computers and training. But by and large, scuba is a simple process: use compressed air. It is a passive technology.

Rebreathers, on the other hand, are complex as there are many more factors to consider. Yes, you use the air you bring underwater with you much more efficiently, but that comes at a price. If the chemicals used to "scrub" the carbon dioxide out of the air fail, you can die without even noticing that anything is wrong. If something in the oxygen delivery system malfunctions you may either get too much or too little oxygen and, again, you can die. If the whole complex rebreather mechanism isn't maintained meticulously, it may fail or become contaminated, again with ominous consequences. So there. Rebreathers are an elegant technology but just about the opposite of the famous "fail-safe" ("fail-safe" meaning that when a component fails it defaults to safe). If a scuba regulator fails and starts free-flowing, you're "safe" because you continue to get air (albeit not for long); if a rebreather fails, it fails in a bad way.

So that's all bad news, but not enough to write the technology off. We humans routinely deal with dangerous technology that is safe because we made it so. If an airplane fails, the consequences are usually deadly, but we accept that risk because the technology has advanced to a point where the risk is very small. Heck, even home heating systems are essentially deadly, but we all have one. The difference is that millions use air travel and billions a heater, whereas only very few use a rebreather, hence the development of the technology is far slower.

Another issue with rebreathers is that there are different rebreather technologies. With some there are no bubbles at all. That's because with them you breathe pure oxygen and so there is no waste. The body uses part of the oxygen gas, the rest is cleansed and used again. Problem is that those can only be used in very shallow depths. Go deeper and the higher partial oxygen pressure makes oxygen toxic to our bodies and the result is a seizure. So this technology is primarily used for military purposes where the total absence of telltale bubbles is mandatory.

Most rebreathers are semi-closed, which means you do use some other gas, nitrogen. That way you can go deeper, which means pressure differences and therefore the need to be able to expell some gas. The advantage here is that oxygen and nitrogen can be combined any which way, or come from different tanks, which means the breathing gas can be mixed to the best advantage at any given depth. With less nitrogen to worry about, yet rebreathing technology that makes maximum use of oxygen, bottom times can be much longer. More complex units may add helium to the equation so that you can go deeper yet without experiencing nitrogen narcosis.

The problem is clear: how to add oxygen properly, sufficiently and reliably at all times, and how to measure its presence. So we're dealing with an inherently more complex technology that can turn deadly in an instant, one that requires extensive understanding and training and meticulous maintenance, and one that costs a whole lot more. That is the reason why rebreathers are the exception and not the norm, for now. I've only begun to learn about rebreather technology. There will undoubtedly be more entries on this topic. And I may end up using one at some point.

Posted by conradb212 at January 17, 2007 11:05 PM