Bill Stone will never forget the day he began to make diving history by immersing himself in Wakulla Springs, a network of submerged caves in northern Florida. On the early afternoon of December 3, 1987, he swam to a depth of about 30 feet and settled in, carrying only two 30-cubic-foot oxygen tanks and a sack of "blood and gore" war novels. Had he been using normal scuba gear, he would have had to surface after 30 minutes. But Stone, an automation engineer at the National Institute of Standards and Technology in Washington, D.C., was wearing a homemade rebreather. He didn't come up for 24 hours - the longest anyone has ever survived underwater with a self-contained breathing device. "When I got out of the water and checked, I found I'd used only half of my consumables. That was the big shock. I could have stayed under for another 24 hours," he says.
Stone's rebreather recycles his exhaled air, scrubbing it of poisonous carbon dioxide and squeezing out every last molecule of oxygen in his tanks. His inventiveness has transformed the ability of humans to carry out scientific exploration underwater. With rebreathers, divers can linger for half a day without thinking about their air supply, exploring previously undivable undersea caves, surveying shipwrecks, or just passing time with the fish.
The concept of rebreathers is not new. In the 1870s, Henry Fleuss, a British merchant seaman, developed the first primitive oxygen rebreather - an unwieldy device handy for rescuing mine workers. For the next hundred years or so research into rebreathers was carried on primarily by navy engineers, who could overlook many problems of cost and convenience. Convinced that rebreathers could be small, cheap, and reliable, Stone designed his first rig in the mid-1980s and dubbed it the MK1. He eventually founded a company, Cis-Lunar Development Laboratories in South Lancaster, Massachusetts, which has developed four more models, each of increasing complexity. The latest one, the MK-5P, features sophisticated gas and depth sensors, three microprocessors, computer displays, and a $17,500 price tag. Although that may seem expensive, a diver who can afford a second automobile can afford a state-of-the-art rebreather. Dozens of other companies have hopped on the bandwagon.
With a rebreather, as with scuba gear, a diver inhales through a mouthpiece, drawing compressed gas from a tank. But the expired air is saved, shuttled through a valve into a balloonlike bag, and then back into the breathing loop. With each breath the concentration of oxygen in the bag drops and carbon dioxide rises. The main job of the rebreather is to get rid of carbon dioxide while replenishing the oxygen in the diver's air supply. Canisters of soda lime or lithium hydroxide do the first part by scrubbing carbon dioxide out of the air. In the fanciest rigs, such as the MK-5P, electronic controls take care of the second part, automatically sensing the oxygen content and injecting more of the gas as needed.
The process is not nearly as foolproof as it might sound. "Sooner or later, your scrubber canister is going to run out and you are going to start inspiring a high, potentially dangerous level of carbon dioxide," says John Clarke, scientific director of the Navy Experimental Diving Unit in Panama City, Florida. If carbon dioxide concentrations reach about 10 percent, inhaled air becomes lethal, no matter how much oxygen is present. "Right now, we have no way of letting the diver know if that is happening. With a rebreather, you can die without even knowing that you are dying," Clarke says. Divers must rely on carefully calculated tables that let them know how long their scrubbers should last if everything is functioning perfectly.
One solution being studied is a sensor that measures carbon dioxide levels by detecting the distinctive frequencies of infrared radiation emitted by the carbon dioxide molecule as it vibrates in the air. Engineers also are developing films that monitor changes of pH, or acidity, in exhaled air. Dissolved carbon dioxide makes air more acidic, so a sudden drop in pH means carbon dioxide levels are spiking. "The films could be placed directly on the body, to measure carbon dioxide intake," says Martin Heerschap, project manager for Cochran Undersea Technology in Richardson, Texas, which manufactures an electronic, fully closed rebreather.
Other advances may allow the machinery to respond still more intelligently to its wearer's needs. For instance, as depth increases, so does the pressure on the counterlung - the flexible bag that holds exhaled gas - making it increasingly difficult for the diver to exhale. Clarke and his colleagues have patented a device, similar to a hospital ventilator, that reduces the load on the counterlung to keep breathing free and easy. Farther ahead, Clarke and engineer John Mittleman of the Navy's Coastal Systems Station in Panama City envision placing micro-machines in the rebreather, on the diver's skin, or even inside the diver's body. These tiny devices could collect precise data on oxygen and carbon dioxide levels and signal any trouble. "That could be his warning to do something different - maybe start breathing harder, or come up into more shallow waters," says Clarke.
In the farther future, engineers are working on devices that pull oxygen directly from the water, much as a fish does, so a diver might never run out of breath. Japanese engineers recently found an effective way to separate the hydrogen and oxygen molecules in water using light. Heerschap says rebreathers might someday incorporate a version of this technology: "If so, the rebreather could be reduced to the size of a small backpack, with only scrubbing material and sensors and no oxygen tanks."
Explorers of the deep briny blue aren't the only ones who will benefit from advances in rebreathers. Mountain climbers also waste a lot of oxygen each time they exhale. New technology could drastically cut the number of 30-pound oxygen tanks climbers have to haul up a mountain, not to mention the amount of trash they leave behind. "Those poor guys climbing Mount Everest ought to be using rebreathers," says Richard Nordstrom, president of Cis-Lunar. "Instead of taking 500 oxygen tanks, they could take maybe 10." Similarly, rebreathers could lighten the load of firefighters, search-and-rescue teams, and hazardous-materials workers. "Firefighters could survive 100 to 200 times as long on an oxygen tank one third the size of what's used now," says Nordstrom.
And rebreathers might be the key to conquering outer space. The challenge of life support in a vacuum is not much different from that of survival 300 feet underwater. In fact, the space shuttle's extravehicular-activity suit is essentially an extremely fancy, modified rebreather but, in true NASA style, it costs tens of millions of dollars and cannot be fixed while in orbit. Stone predicts that a decade from now commercial space explorers will don drastically cheaper suits containing off-the-shelf replaceable parts derived from commercial rebreathers. The suits will have flexible exoskeletons and low internal pressures, giving tomorrow's astronauts the dexterity of today's divers. When that day comes, says Stone, "space suits will simply be another item of gear from the orbital equipment supply catalog."
The name says it all: The Rebreather Web Site, at www.nwdesigns.com/rebreathers, has a glossary, index of manufacturers, list of frequently asked questions, and articles about the technology. Cis-Lunar's Web site (www.cis-lunar.com) describes its innovation products in details.