Rebreathers: Boon or Bane?

"Rebreathers are the greatest thing since sliced bread!" "Rebreathers are boxes of death!"

It's safe to say rebreathers often elicit strong emotions in divers, but what is the true story? Among the anecdotes, opinions and conjecture, what are the real benefits and hazards of diving on rebreathers?
Function
There are many different types of rebreathers, but they all perform the same basic functions: They retain a diver's exhaled breath, remove metabolic carbon dioxide (CO2), provide safe oxygen levels and maintain an adequate breathing volume. Different designs do these things in different ways, but mixed-gas closed-circuit rebreathers (CCRs) are the most common type used recreationally today.




When you exhale while diving a rebreather, instead of venting gas as bubbles, your exhaled breath fills a breathing bag, or counterlung. This gas then flows through a chemical absorbent that removes CO2. Two cylinders supply oxygen and a diluent gas such as air, mixing them to provide an optimal nitrox mixture for your depth. Sensors measure the partial pressure of oxygen (PO2) so the diver or electronics can prevent oxygen levels from getting too high or too low.

Mechanical or electrical malfunctions are rare, but a failure in any one of the tasks the rebreather is supposed to do can lead to a serious incident, or even death, while diving. Flooding of a rebreather can lead to a caustic cocktail that can cause chemical burns (see sidebar). Too much oxygen may result in convulsions and unconsciousness, and too little will result in unconsciousness from hypoxia. Either usually results in the diver's death by drowning. Carrying an open-circuit "bailout bottle" is standard practice in rebreather diving.
Hazards
About 200 rebreather divers have died since 1960, most of them in the last 20 years. Approximately 100 open-circuit scuba divers die in the United States each year, but it is difficult to compare the two numbers as there are a great many more open-circuit dives done than closed-circuit.

Some of the rebreather divers who died were exceptionally experienced. Among them were people who had been diving for 10, 20 or even more than 30 years. They had been to very deep depths, explored the far recesses of complex cave systems and visited little-known wrecks in remote locations, all without incident using open-circuit scuba. What happened?

We have limited data on rebreather fatalities, but several trends are apparent. One is that rebreather divers with little experience tend to have more difficulties than those with many dives. This is similar to data seen in open-circuit scuba: New divers are more prone to trouble than experienced divers. An experience level of 20 to 25 dives seems to be a turning point at which the incident rates begin to decline.

Open-circuit dive gear is very robust; rebreathers are less so. Traditional scuba equipment has been evolving, improving and becoming more reliable through extensive use and development over the past 50 years. Rebreathers are also progressing down this developmental road, but at a slower pace.

It takes more time to prepare a rebreather for diving than it does open-circuit equipment. It also takes more time to clean a rebreather after a dive. Some people, by their innate temperament, are simply unable to consistently exercise the discipline necessary to properly maintain a rebreather. These are the same people who do not rinse their gear after diving, continue to dive their equipment despite inadequate maintenance or poor function and basically fail to exercise due care for their equipment. This approach is never a good idea, but it's generally possible to get away with this attitude in open-circuit diving more consistently than it is while using a rebreather.

Two major factors contribute to the deaths of very experienced divers using rebreathers. The first is that the type of diving they do (caves, wreck penetration, exceeding recreational depth limits) involves more risk. Because of the increased risk we expect to see higher incident rates regardless of the type of equipment used. The second issue is that because of their vast experience with open-circuit gear, these divers may quickly feel at ease on a rebreather. They are tempted to pursue the same types of diving using their rebreather they did while using a scuba cylinder. The problem is they have not yet built the muscle memory and patterning they need before extending their diving into more advanced environments. Events that would be simple for them to handle on open-circuit equipment spiral out of control when they use rebreathers simply because they do not yet have a comparable level of experience on the newer gear. Essentially, they push their diving too far too soon.

The role of complacency must also be considered. When people become more comfortable in the water, they may be more likely to bend the rules. This might manifest in equipment preparation, such as not performing required predive checks; dive environments, such as entering caves before truly ready; or dive practices like diving alone. Any of these or many other careless behaviors might lead to a fatality while using a rebreather.

If this is so, why do people use them? For me, there are two primary reasons: time and animals.
Benefits



I learned to scuba dive because I wanted to explore under water. Using air, my dive times were constrained primarily by nitrogen loading. I couldn't stay too long, as no-decompression limits restricted time at depth. Even when I began exceeding those limits, venturing into the realm of decompression diving, I still could not stay as long as I wished.

Nitrox eased those time constraints a bit, but it was still fairly restrictive. I had to match the gas I planned to use with the depth of the dive. If my dive plans changed (with regard to depth) it was very difficult to change the gas mix to suit — each change meant a trip to the dive shop to empty and refill a cylinder, which was impractical.

A rebreather is essentially a self-contained nitrox-mixing machine. It is able to provide the optimal nitrox mix for your depth, changing the oxygen content as you move up and down in the water column. By doing this it is possible to double, triple or even quadruple your no-stop dive times compared to air.

And you can do this while carrying only 40 cubic feet of breathing gas, half diluent (air or nitrox) and half oxygen. Because you are not wasting gas by expelling it into the environment, 20 cubic feet of oxygen can last as many as 10 hours underwater, regardless of depth. Just think, a 20-cubic-foot pony bottle could provide enough gas to stay at 100 feet all day, without refills and without going to the surface. This does not account for decompression considerations, of course, but the gas volume could last. For me, that was a magnetic attraction.



Another advantage to no bubbles is the possibility of improved interaction with wildlife. We look at fish in different ways: Snorkeling, freediving, open-circuit scuba, surface-supplied hookahs and rebreathers are all just tools that allow us different ways to see the underwater world. I see more fish when I dive with a rebreather, but to me the quality of fish is more important than the number I see. Using a rebreather, I see rare fish more frequently. I see more unusual behaviors and fish acting more naturally. The most fun is seeing animals I have never seen while using a scuba tank: dolphins, whales, tuna, wahoo and other species that shy away from the noise bubbles generate. As a result, rebreathers have vastly expanded the variety of species I've been able to capture in underwater photographs.
Other advantages
In warm-water locations like Palau, the Bahamas and the Caymans, water temperature is never the reason I have to surface; it's always nitrogen loading that limits my dive time. When I travel with a rebreather and dive with groups of divers using a variety of equipment, I typically get to spend about twice the amount of time underwater as the open-circuit divers. The rebreather can even be set so I can do my "surface interval" while I am still at depth, shooting pictures while the others sit on the boat or beach waiting to reenter the water. As long as the dive operator is comfortable with the protocol, I love doing that.

Because rebreathers can be set to provide a high-percentage oxygen mix, they can be used at shallow depths without incurring any inert-gas (e.g., nitrogen) loading at all. No nitrogen loading means no decompression-sickness risk and no mandatory delay to fly after diving. This has allowed me and other rebreather divers to add an entire extra day's diving at some travel destinations.

Gas efficiency is a great benefit to technical divers because it gives them far more time to pursue their objectives while underwater. Additionally, because many rebreather units may also be used with helium-based breathing gases such as heliox or trimix, they are particularly well suited for deep-diving applications. A technical dive that might cost an open-circuit diver $200 in gas might cost a rebreather diver only $15 to $20. However, the initial investment for rebreathers is relatively high. A typical unit might cost two to six times that of an open-circuit system. In general, training is also more expensive than traditional scuba. Cost of both training and equipment has been decreasing over the years, but it will be some time before there is parity between the two modes of diving.

Is a rebreather the right tool for you? Only you can answer that. Rebreathers do not necessarily require a lot of dive experience, but they do require special training, and their use does demand great attention to detail and a willingness to spend a little more time on care and maintenance. They may be costly to own and operate, but they have the potential to allow you to do and see more.

If the benefits a rebreather provides appeal to you, you might wish to consider using one. If not, stick to the tools you know. Either way, I wish you continued enjoyment and safety in your underwater explorations.
Want to learn more?
What I Learned About Diving from a Caustic Cocktail

© Alert Diver — Spring 2012