I remember the first time I saw an ad for an ElectroLung. I was leaning on the glass counter at LA County Skin Diving School reading a copy of Skin Diver Magazine. In the page margin there was a black and white ad offering a device called a mixed-gas, closed-circuit rebreather. The ad claimed the device was totally silent, produced no exhaust bubbles, and could support a diver at 1,000 feet for six hours. It was illustrated with a drawing of a diver swimming silently through a kelp forest carrying a speargun. The cost was $3,000 plus training. It was the summer of 1968 and I was eighteen years old. I immediately wanted an ElectroLung in the worst way. Problem was, I made $1.25/hour, and I inevitably spent $.50 each hour at the soft drink machine. Owning an ElectroLung was the most fanciful of dreams. But it was a dream that never went away.

The ElectroLung did not enjoy success in the consumer market place. Within a very short time several divers had died using the device. The ElectroLung was withdrawn from the market and the remaining units in production were literally smashed up with sledge hammers (by company lawyers, the story goes). I had to wait more than twenty years for my chance to dive in silence.

Opportunity presented itself in 1989 as I began pre-production on a film about marine life in the Sea of Cortez. During the research process, I started exploring the feasibility of acquiring military rebreathers for filming the schooling behavior of hammerhead sharks. At that time, no commercial mixed-gas closed-circuit rebreather was available to civilians. But due to some spectacularly good timing, I was allowed to lease two Biomarine Mark 15.5 rebreathers from the maker. Biomarine had just lost its contract to produce rebreathers for the US Military. They had two prototype units left over from their failed prototype development program. Bob Cranston and I spent a week at the Biomarine factory in Philadelphia learning how the system worked by talking with Biomarine engineers and sales people. At the end of this indoctrination, we were given two rebreathers, a couple of military manuals and told to go home and try not to kill ourselves.

As Bob and I suited up to make our first closed-circuit dive in the Sea of Cortez, our paranoid minds filled with stories of ElectroLung deaths. We nervously descended twenty feet and stood on the bottom repeatedly scanning the bewildering array of electronic instrumentation designed to forewarn of imminent death due to hypoxia, oxygen toxicity, or asphyxia. Slowly we began the long process of teaching ourselves to dive closed-circuit.

Today more than 90% of my dives are made using a highly modified Biomarine Mark 15.5 rebreather. During the last ten years I've logged 879 open water hours on the system including a few trimix dives below 300 feet. The hours that I've logged do not necessarily qualify me to teach anyone how to dive a closed-circuit mixed-gas rebreather. But I have developed some strong opinions concerning rebreathers, rebreather diving protocols and diver attitudes. Some of these opinions are not always popular with those who have just purchased a rebreather or plan to purchase one.

THE MYTH OF INVISIBLITY

Most people believe the main advantage to diving closed-circuit is silence. A closed-circuit rebreather produces no bubbles, eliminating the cacophonous column of air that rises above open-circuit divers. As a wildlife filmmaker I coveted the idea of being able to approach marine life silently, without bubbles. I dreamed of hiding unseen behind sea fans and filming humpback whales as they performed mysterious social behaviors, previously unknown to science, only a few feet away from my hidden camera. I dreamed of approaching fish that no longer bolted every time I exhaled. I dreamed of seeing creatures and behaviors no open-circuit diver had ever seen. I dreamed of stealth.

Well, dream on. After several hundred open-water hours on closed-circuit, I've learned two things about wildlife. Most marine vertebrates are not stupid, and most can see just fine. Just because I no longer exhaled bubbles didn't mean I was invisible. I first began to reach this conclusion when Bob and I attempted to use our rebreathers to approach singing humpback whales in the Caribbean. As I approached a singing humpback by moving stealthily from gorgonian coral to gorgonian coral across the bottom of the Silver Bank, I suddenly realized the animal was watching me. I got the distinct impression the animal was thinking "who does that idiot think he's fooling?" A moment later it swam away.

The best shots of whales I've ever captured on film, in fact all the shots of whales I have captured on film, have been done using standard scuba. It's true that you can approach some animals more closely with a rebreather than you can with open-circuit. This is especially true of schooling hammerhead sharks. But the difference is far less dramatic than you might guess. It certainly seems that fish tend to bolt away when you exhale bubbles. But I've learned that exhaling usually triggers that response once you've reached a minimum tolerable distance. On closed-circuit the animal will usually allow you to approach slightly closer (between ten and twenty percent) and then will bolt anyway. Remember, the animals aren't blind. Certainly, there is some advantage to closed-circuit. You do get closer. But this minor difference in marine life approachability, in my opinion, does not justify the enormous logistical hassle and expense, of diving closed-circuit. If silence was the only advantage to a rebreather, I'd go back to standard scuba full time. Most new and many experienced rebreather divers disagree with me on this point. I believe these divers also dreamed of stealth. And dreams die hard.

SO WHY BOTHER?

Although becoming invisible was the reason I began diving closed-circuit, I soon learned a much more compelling reason. Bottom time. A closed-circuit mixed-gas rebreather is simply the most efficient underwater life support system in the world. My rebreather has the capacity to support me on dives longer than twelve hours! Not only can a diver stay underwater almost indefinitely, but decompression is greatly minimized because the system automatically produces the optimum gas mixture for whatever depth the diver descends to.

For me, the almost unlimited bottom time and minimized decompression has revolutionized the art of wildlife filmmaking. I no longer rush through a roll of film in pace with my falling tank pressure gauge. Instead, I stay with the animals until they perform the behaviors I want to capture on film or until it's time for lunch. Gas supply no longer determines how long I stay, how long I wait for an animal to perform, how deep I go, or how far away from the boat I swim. If I wish, I can literally stay down all day. For an underwater wildlife filmmaker, this advantage more than justifies the hassle and expense of rebreathers.

There is another, perhaps darker, advantage to closed-circuit. Using this technology it becomes possible to dive deep using exotic gas mixtures. On open-circuit, self-contained deep dives become impractical due to the huge volumes of breathing gas a diver must carry. But with a Mark 15.5 closed-circuit mixed-gas rebreather a diver can stay underwater for twelve hours regardless of depth. Add helium to the gas mixture and divers can descend to five hundred feet or more completely self-contained. Of course, it's not that simple. A thirty-minute dive to 350 feet is going to require about 3 ½ hours of decompression. Deep trimix dives are highly technical and fraught with risk even for experienced, extremely well trained divers.

REALITY CHECK

Before rushing out to put a deposit down on a rebreather consider the logistical hassles I mentioned earlier. Rebreathers can be a major logistical pain. Closed-circuit mixed-gas rebreathers are large, cumbersome, and heavy. My Mark 15.5 weighs about 75 pounds. Some designs are a bit lighter, but they allow more limited bottom times.

There are two compressed gas bottles in a mixed-gas closed-circuit rig. One is filled with air and the other with pure oxygen. If you want to use trimix, you add another cylinder filled with heliox. Getting the air tank filled is no problem. But getting the oxygen tank filled can be problematic. Few dive boats offer pure oxygen fills. So when you go diving you must either bring your own oxygen or make special arrangements with the boat you plan to use. Of course taking oxygen with you on an airplane is out of the question. If your charged oxygen tanks were discovered in the cargo hold of an airliner the FBI would lock you up and throw away the key. Even if the boat has bottles of pure oxygen on board, cascade charging your rebreather tank won't fill it to capacity. Oxygen storage bottles are filled to about 2,000 psi. Rebreather tanks usually require 3,000 psi. To fill your rebreather tank, you need also have an oxygen-clean booster pump.

Some boats use oxygen generators to make oxygen for their nitrox systems. Although this oxygen is fine for that purpose, oxygen made from most popular generators contains about 4 percent argon gas. This small amount of argon makes generated oxygen unsatisfactory for closed-circuit rebreathers.

When traveling, you will also need access to carbon dioxide scrubber chemicals. This stuff comes in five-gallon pails. A single rebreather diver uses about one-half pound of scrubber per hour.

When I travel to a location where I want to use my rebreather, I must take the rebreather in its case which weighs about 85 lbs, the booster pump (another sixty lbs), several pales of carbon dioxide absorbent, and a fifty pound box filled with spare parts, tools, and accessories. All this extra stuff makes diving from small boats really inconvenient. And most live-aboard owners will be less than ecstatic to see you come aboard with so much stuff.

Maintenance is also a major pain with mixed-gas closed-circuit rebreathers. It takes me about an hour to get my system assembled and calibrated to dive on the first day of a trip. This process is often repeated for the first couple of dives until all the bugs are worked out. After the first day or so, the system can be made ready to dive in about fifteen minutes.

Another fifteen minutes are required for cleaning and maintenance at the end of each dive. Minor glitches and malfunctions occur on every expedition. These problems often require hours of diagnosis and repair. If you are not comfortable working on mechanical and electronic equipment, you will probably find rebreathers frustrating if not dangerous.

THE DANGERS

Most divers consider closed-circuit mixed-gas rebreathers extremely dangerous. This impression is both true and false. In many ways diving mixed-gas closed-circuit is much safer than diving standard scuba. Diving rebreathers can also be terrifyingly dangerous. How can both statements be true? Consider a Boeing 747 on approach to LAX international airport. For most of the 300 or so people on board, being thrust into the cockpit and given the controls would not only be terrifying, but would almost certainly result in panic, a high speed collision with planet Earth, and instantaneous death. For the few private single-engine aircraft pilots on board, there might be hope. But because these pilots lack the proper training, they too would most likely end by augering into the tarmac and killing everyone on board. Fortunately, airlines companies require two or three qualified 747 pilots aboard these aircraft. For these people, the bewildering array of instrumentation, the complicated controls, even the minor malfunctions that regularly occur on these aircraft, do not cause panic, fear, or even an increased heart rate. The obvious difference between the 747 pilots and their passengers is the exquisite training, flight simulation, and practice they receive.

Like aircraft, rebreathers require considerable training, simulation, and practice. The extent of which far exceeds that of standard open-circuit scuba. In addition to a dive computer that allows me to switch between ten different gas mixtures on the fly, my rebreather has four other instruments that must be regularly monitored. Make no mistake, failures and malfunctions do and will occur often. But these problems are indicated by the instrumentation or easily recognizable changes in the system's breathing characteristics. Unlike flying an aircraft, diving a rebreather does not require fast reflexes or quick thinking. If the diver is monitoring his instruments and can accurately interpret the information, nothing happens fast. Even a complete system flood only requires that the diver reach down and replace his mouthpiece with a regulator attached to his open-circuit bailout. However, if a diver fails to monitor his instruments regularly, does not interpret the information accurately, or has not practiced the proper emergency drill when a malfunction occurs, his rebreather may very well kill him. For inadequately prepared divers, rebreathers are unquestionably dangerous.

The good news is that many experienced rebreather divers actually feel safer diving their rebreather than diving open-circuit, just as 747 pilots feel and are safer in their aircraft than in their, far less complicated, automobiles. There are two major safety advantages to mixed-gas closed-circuit rebreathers. One is the perfect gas mixture these systems maintain. No matter your depth, the rebreather delivers a perfect nitrox mix. During decompression, whether precautionary or required, the rebreather can deliver pure oxygen. This significantly reduces the chances of decompression sickness.

The other reason mixed-gas closed-circuit rebreathers can be safer than standard scuba is time. Because the rebreather allows the diver to stay down literally all day, all time pressure goes away. No longer need you calculate how far from the boat you swim, how long you stay, how deep you go, how much decompression time you incur, all based on your constantly diminishing air supply. You take your time. You never run out of air nor do you ever fear running out of air. This major cause of scuba diving accidents is eliminated with a closed-circuit rebreather.

CASUAL STUPIDITY

Perhaps the single most valuable piece of advice that I can give a prospective rebreather diver is this: Never under estimate your capacity to do something really stupid. Now you're probably saying to yourself, "No way. If I ever dive a rebreather I'm going to be insanely careful." But after you've logged thirty or forty hours and have developed some confidence you will probably surprise yourself. If you've never accidentally jumped into the ocean without your fins or without your weight belt, then maybe you are incapable of casual stupidity. But don't count on it. Between Bob Cranston and I, we've made about every stupid mistake possible on a rebreather from simply forgetting to turn the unit on to jumping in with all the electronic cables disconnected. The good news is that casual stupidity is survivable, assuming you are well trained and have well practiced diving protocols and emergency procedures. Again, the most serious of malfunctions can ultimately be dealt with by switching to open circuit bailout. Most rebreathers are designed to allow the diver to breath the on-board gas supplies via a standard scuba regulator. If the on board gas supplies (usually 20 to 40 cubic feet) are not sufficient to allow the diver to reach the surface, then additional bailout bottles should be carried. The key to surviving your own stupidity is training and self-preparation.

TRAINING

Most people have more common sense than to purchase a rebreather and begin diving it without training. Acquiring training seems an almost laughably obvious necessity. In fact, most manufactures require that you get training before selling you a rebreather. But consider this. To become a certified flight instructor requires a minimum of 250 hours of flying experience. To become a certified rebreather instructor requires less than ten hours of rebreather diving experience. There is no way for you to evaluate the quality of your dive training except in retrospect (at which point it's too late). No matter how good your instructor seems to be, you must consider the possibility that your instruction will be inadequate.

This doesn't mean that instruction will be valueless and so you should just skip it. It means that a certification in rebreather diving will probably not give you all the safety procedures and diving protocols you will require to dive a rebreather safely. After becoming certified, you should begin training yourself. Study all the possible things that can go wrong during a rebreather dive then use logic to develop reactions to those situations. Regularly practice those procedures both physically and mentally.

ARE SEMI-CLOSED REBREATHERS SAFER?

The most common rebreathers available today are semi-closed systems. These are popular for two reasons. First, closed-circuit mixed-gas rebreathers have, until recently, been unavailable and are still not widely available. Second, ElectroLung history has resulted in a diving industry fearful of electronically controlled rebreathers. The main fear is that an electronically controlled system may malfunction and inject too much oxygen into the breathing loop causing oxygen poisoning, the consequences of which are convulsions, drowning, death, and multi-million dollar lawsuits. This danger is eliminated by the fundamental design of semi-closed systems. Ironically, malfunctions that can lead to dangerously high levels of oxygen are among the least worrisome risks when using a mixed-gas closed-circuit rebreather. Except during descent, it takes considerable time for the rebreather to inject enough oxygen to cause toxicity. Any responsible diver will notice the high levels on his instruments. Even if the instruments are not monitored, the diver will usually notice the increased gas in the breathing loop and the venting of the over-pressure relief valve. During descent the situation is different since oxygen may be injected while the breathing loop is being compressed. But with a closed-circuit mixed-gas rebreather, oxygen levels are never predictable during descent and so even irresponsible divers tend to monitor oxygen levels closely when going down. Dangerously high levels of oxygen are one of the less formidable problems when diving a closed-circuit mixed-gas rebreather.

Low oxygen or hypoxia, on the other hand, is the major danger with all rebreathers, closed or semi-closed, and is the cause of most rebreather fatalities. Since most semi-closed systems have no electronics for measuring oxygen partial pressure, divers using these systems are, arguably, at even greater risk of hypoxia than closed-circuit divers. The danger of hypoxia cannot be overestimated. When oxygen levels drop to the danger point, the diver will feel light headed and experience the onset of tunnel vision. Within five seconds of noticing these symptoms, you can be unconscious. This may not be enough time to switch to open circuit, or do anything else for that matter. With a closed-circuit mixed-gas rebreather, this danger is eliminated by regularly and religiously scanning the instruments.

The functional differences between mixed-gas closed-circuit rebreathers and semi-closed rebreathers are enormous. Semi-closed systems share almost none of the advantages of closed systems except silent operation. And bubble free operation is only done semi-well with semi-closed rebreathers. Even if semi-closed systems were completely bubble free, I wouldn't find this advantage valuable enough to warrant the additional hassle. If there is one reason to learn to dive a semi-closed system, it may be as a stepping-stone toward a fully-closed mixed-gas rebreather.

Closed-circuit mixed-gas rebreathers, on the other hand, may forever change the way you dive and enormously enhance your appreciation of the marine wilderness. Good or bad, this technology is here to stay.