Rebreathers

Rebreathers are underwater life support systems that permit you to stay longer at deeper depths. It’s not hard to make a normal scuba tank last for an hour (or more) very near the surface. At 60 feet, this duration drops to less than half an hour. At a hundred feet, maybe 15-20 minutes. A recreational-grade rebreather, however, will give you several hours at any depth in the 0-100 ft. range. So the deeper you dive, the more advantageous the rebreather becomes. Rebreathers always give you more time, but especially give you more time at greater depths.

You would need hundreds of lbs of conventional scuba tanks to get as much breathing gas supply as you get with a 60 lb. rebreather with a small tank of oxygen. The equivalent of one scuba tank’s worth of oxygen could provide as much as 30-40 hours of sport dive time. This extension also plays into the psychology of a dive. A flustered scuba diver might huff and puff when exerting and burn though the contents of a scuba tank in no time. A rebreather diver, however, can huff and puff all he or she wants, but because none of the exhale is wasted, there’ll usually be more oxygen left than they probably have time to use — they’ll get cold or hungry first. A rebreather can be much more relaxing to dive therefore because your margin for error in breathing gas supply is measured in hours, rather than minutes.

How it works: Most scuba divers breathe in air from a pressurized scuba cylinder, through a regulator, then breathe out the exhaled gas into the surrounding water (resulting in a column of bubbles). The gas in those exhaled bubbles includes a significant amount of oxygen; thus, scuba is inherently inefficient. The idea of a rebreather is to recapture some or all of that exhaled gas, process it, and return it back to the diver, with little or no waste. Instead of breathing through a regulator, rebreather divers breathe from a “loop” that directs the exhaled gas into a “counterlung” (a flexible bag that expands to receive the diver’s exhaled breath, and collapses when the diver inhales again), through a “scrubber” (a canister containing a granular chemical such as calcium hydroxide, that removes the carbon dioxide from the exhaled gas), and back to the diver to be inhaled again. At some point in the loop, oxygen is added to replenish that which is metabolized by the diver. In short, a diver consumes oxygen and expires carbon dioxide; and a rebreather chemically removes the carbon dioxide, replaces the oxygen that was removed, and returns the gas to the diver. One advantage of these loops: it is much more pleasurable to breathe warm moist gas (recycled) than to inhale cold dry gas.

The three main advantages of rebreathers are: 1) Better gas use efficiency (especially down deep, where they can be more than 100 times more efficient than scuba); 2) Better decompression optimization in the case of fully-closed systems; and 3) Quieter operation (useful for observing or photographing marine life). In short, rebreathers allow for deeper, longer, and quieter diving. The “quieter” part is not just nice; it can be quite important because of the absence of the usual noisy and visually startling exhaust bubbles allows a diver to observe underwater life much less obtrusively.

There are two different kinds of rebreathers: “semi-closed” rebreathers, which are entirely mechanical but waste some gas; and “fully-closed” rebreathers, which use sophisticated electronics to control oxygen levels in the breathing mixture and waste almost no gas.

Thousands of “semi-closed” rebreathers are currently used by recreational divers. They are designed for shallow use (i.e., less than 130 feet deep). These usually cost less than $5,000. If you are mostly interested in doing quieter dives at “normal” scuba depths, for no more than 1 or 2 hours at a time, you’re better-off getting one of these. The two most popular are built by Drager: the DragerDolphin and the DragerRay.

Then there are a series of mid-range units, These allow for more dynamic breathing gas mixtures, and incorporate electronic control systems. Most of these are used at shallow water diving as well, but some intrepid divers have modified them in ways to get down several hundred feet. These usually cost in the range of about $8,000-$15,000. The 4 most popular units on the market include:
- The “Inspiration” at Ambient Pressure Diving
- The “Megalodon” at Custom Rebreathers
- The “PRISM Topaz” at Steam Machines
- The “AURA CCR2000″ at Rebreather

Another class of semi-closed rebreathers built by Halcyon have been used successfully on some of the world’s most extreme deep cave exploration dives. This particular kind of rebreather can be thought of as a “gas extender” for conventional scuba cylinders, and costs around $7,000-$8,000.

At the high-end are models designed with ultra-reliable components and incorporate multiple layers of equipment redundancy, to assure the highest probability of continued function even in extreme circumstances. Only the military and a few members of the lunatic-fringe (like me) are willing to fork over the $15,000-$50,000 for this class of rebreather. The model I use is built by Cis-Lunar Development Laboratories, and is manufactured only in limited batches when sufficient demand warrants it.

There are downsides to rebreathers, which should be stated clearly. Whereas on scuba, the most potentially life-threatening problems are very-much self-evident (can’t breathe, hose burst causing bubbles, etc.); on rebreathers the big potentially life-threatening problems are extremely insidious (i.e., too little oxygen, too much oxygen, too much CO2). This means that on scuba, you mostly need to know how to *solve* problems when they arise. On rebreathers, you have to not only know how to solve them, but also you need the discipline and awareness to recognize that you have a problem that needs solving in the first place. When a regulator fails on a scuba diver, it’s obvious to the diver that he/she has to go to a backup regulator or borrow air from a buddy. When an oxygen addition system fails on a rebreather diver, he or she can very easily drift off to unconsciousness without ever knowing anything was wrong (I’ve seen it happen).

But this technology is changing fast. The possibilities for underwater exploration using rebreather technology are amazing. Bill Stone, an engineer who has designed some of the world’s most sophisticated closed-circuit rebreathers, famously conducted a 24-hour non-stop dive using one of his early prototype designs. In truth, he used up less than half of the total capacity of the unit, meaning that he could have gone for at *least* another 24 hours, and perhaps as much as an additional 48 hours. Yup, that’s 3 days of life support underwater from one self-contained pack.

The web is the best place to start for more info. One of the best sites on the web is here. Mastering Rebreathers is the most comprehensive book on rebreathers, and is also the most recently published (important for a field that is almost as dynamic as the computer industry).protorig.web.jpg“The late Sheck Exley tests the Cis-Lunar MK1 prototype rebreather, December 11, 1987. Weighing a whopping 205 pounds (93 kg) the unit was not considered “portable” and had to be wheeled into the spring on a dolly, yet underwater it was neutrally buoyant. The unit had outrageous “range” and was also fully redundant, meaning it contained two complete rebreathers within the one backpack. Shortly before this dive Bill Stone used the rig to conduct a 24 hour dive in Wakulla Basin. Less than one half of the rig was used for this dive, meaning that a 2-day underwater mission could have been carried out, with perhaps as much as an extra day’s worth of life support held in reserve. This was the first rebreather to utilize multiple, redundant computer systems for electronic control of the life support backpack. It was also the first rebreather to use lithium hydroxide to remove carbon dioxide from the breathing loop and contained novel concepts to permit the safe use of that material [photo 1998 Wes Skiles].”

rebreatherdiver.web.jpgWearing a PRISM Topaz rebreather

-- Rich Pyle  

[This book appears to be out of print, as the cost on Amazon is now 10x what it was in 2003. We recommend you use WorldCat to locate a copy at a library near you. Or try eBay. -- SL]

Mastering Rebreathers
Jeffrey E. Bozanic
2002, 548 pages
$225+*

Available from Amazon