2 Dive 4 Scuba

Use Less Air

You think your air consumption is low? Maybe, but compared to the experts at the game, the whales, porpoises and seals who make their livings where air is scarce, you are a disgrace. You blow off that precious resource like a teenager with a sudden inheritance.

Consider the documented record for the longest dive on a single lungful of air: one hour, 52 minutes, set on Aug. 25, 1969, by a sperm whale. If that whale's lungs were average, they were about half the size of yours in proportion to its body. As if you had only one lung. How long do you think you could stretch one lung of air? A minute?

OK, not a fair comparison. After millions of years of practice holding their breath, whales should be better at it than you and I. Among its natural advantages, a whale has lungs able to exchange 80 to 90 percent of their volume with each breath while humans exchange only 10 to 15 percent. Having more myoglobin, whales are able to store three to 10 times as much oxygen in their muscles as humans can. With bigger red blood cells, and twice as many of them per liter, their blood can store much more oxygen, too. Their mammalian diving reflex is more pronounced, so they are better able to shut down parts of their body that would otherwise waste oxygen. By contrast, we humans are like cars designed for a world of free gasoline.

Though whales have a long head start, you can take a few baby steps down their evolutionary road and use your limited air supply more efficiently. Not only will a tank last longer, you'll feel less tired and more relaxed at the end of a dive. To begin with, you can extract more value from each breath by taking deeper breaths when you inhale and forcing out more carbon dioxide when you exhale. You'll never reach the whale's ability to replace 80 to 90 percent of its lung volume with each breath, but you can do a lot better than you are now.

Breathe Deeply

It's probably not obvious why taking deep breaths should make your limited air supply last longer. Whales take their deep breaths directly from the atmosphere, after all. If 80 cubic feet is all you have to draw on, won't it last longer if you just sip at it? My bank balance certainly seems to.

Some divers try to save air by deliberately taking short, shallow breaths, breathing from the top half of their lungs. But they end up wasting air, not saving it. The reason is that they're retaining and building up carbon dioxide, and it's too much carbon dioxide, not too little oxygen, that triggers the urge to take the next breath.

"When you're breathing at a higher lung volume and taking a fairly shallow breath, you retain more carbon dioxide, which gives you more of a ventilatory drive," says Dr. Richard Vann, vice president for research at Divers Alert Network. "You'd be better off ventilating a higher tidal volume and exhaling closer to your residual volume. In other words, blow out the carbon dioxide."

Short, shallow breaths leave more than 85 to 90 percent of your lungs filled with carbon dioxide-rich "dead air." All that carbon dioxide itches to get out, so you're forced to take another breath before you actually need the oxygen. You draw air you don't need from your tank, cycle it through the top half of your respiratory system just to pick up the carbon dioxide most urgent to escape, and dump it into the ocean.

Don't believe it? I wasn't sure I did either, so I tried an experiment. I measured my surface air consumption using a digital pressure gauge and a stopwatch. I tried it first breathing as slowly and deeply as I could, and then taking short breaths on half-full lungs. Both times, I tried not to take another breath until I felt the urge to do so, as if I were trying to conserve air. The result? When breathing slowly and deeply, 50 psi lasted 30 percent longer. Short, shallow breathing burned through the air faster. Admittedly, this was a "shade-tree" experiment that might not pass muster with the National Science Foundation, so take it for what you think it's worth. It suggests to me that short, shallow breaths don't save air and that you can almost certainly stretch that 80 cubic feet by inhaling deeply, by holding that air in your lungs for a moment or two for maximum gas exchange (use your diaphragm instead of closing your throat), and then exhaling as completely as you can so you expel as much carbon dioxide as possible.

Your carbon dioxide level can rise before you know it. Maybe because you're kicking hard. Or maybe you're just breathing hard. One of the dangerous aspects of short, shallow breathing is that it's a vicious circle. The shallower and faster you breathe, the more carbon dioxide you generate just sucking on the reg. So you suck harder. "Basically, it's suffocation," says Vann. "It's a scary experience. You're hyperventilating, but you're always ventilating your upper airways and not exchanging gas." It takes constant attention to keep your breathing slow and deep because it's not natural under stress. If your pattern of breathing changes, try to slow down and calm down.

Two warnings, however. There's such a thing as too little carbon dioxide, too, so don't exaggerate the slow, deep breaths to the point of hyperventilation, which can lead to blackout because you suppressed too far the urge to breathe. And slow, deep inhales followed by slow, complete exhales may cause you to rise and fall a few feet in the water column. When pinpoint depth control is important--when you're hovering over fragile coral for a photo, for example--you'll have to take shorter, quicker breaths not to disturb your buoyancy.

Don't Bully Mother Nature

You think of whales as gentle creatures, and they are in an unexpected way. They've figured out in their millions of years of shouldering aside the ocean that it pays to be patient. They don't try to rush through the ocean. Though a baleen whale can swim faster than you can run, most of the time it cruises slower than you would walk.

Water is not only much heavier than air, it is not compressible. You can afford to bully the air, because when you force your body through it, the individual molecules in your path will squeeze closer together without much complaint. But water molecules are already as close together as they can get. Whenever a whale or a human swims through the water, it has to move its own volume in water out of the way. That's hard work.

If you are neutrally buoyant, you displace your weight in water--including tank and gear, say 200 pounds. A gallon of water weighs about eight pounds, so that's 25 gallons of water you have to push aside.

Put another way, that's five of those jerry cans you see mounted on the backs of four-wheel drives. And even when you swim at a slow 1 mph, you have to push aside all of those 40-pound jerry cans every four seconds. Walking the same speed on land, you're pushing aside only a few ounces of air.

Doing that kind of work requires a lot of energy, and that uses up a lot of air, because your body makes energy by burning glucose. Glucose plus oxygen yields energy for muscles plus carbon dioxide. So working hard hurts your air supply two ways, by using oxygen faster and by producing more carbon dioxide, which promotes wasteful breathing.

Whales produce energy the same way and have learned to shove aside the water slowly, by moving and swimming slowly, because going slow saves energy and air. Actually, the problem is not speed, it's acceleration. Moving your body forward means pushing aside the water that's in your path. As you swim forward, you accelerate every molecule of water in your path from zero to whatever speed it needs to get out of your way. And as every motorhead knows, it's those zero-to-60 times that suck up the horsepower. For example, Consumer Reports says a 3,200-pound Chevy Malibu gets to 60 in 8.1 seconds with 144 horsepower. The same weight with a Ferrari nameplate gets there twice as fast, but needs almost four times as much horsepower to do it. The Ferrari is also sucking down fuel and air at a phenomenal rate, and you will too if you make those water molecules jump like Ferraris to get out of your way.

If going faster gets you to your goal sooner, you might think you'd come out with about the same overall air consumption per mile. Afraid not. That's the "drive faster when you're almost out of gas" theory, and it doesn't work under water either. Swimming or driving faster consumes not just more air and energy but a lot more. In theory, the energy and air cost is proportional to the square of the speed. That means going twice as fast costs four times as much energy and air; going three times as fast takes nine times as much.

At least that's what the physics books say. To test the theory, I tried another experiment. I gave four test divers underwater speedometers, digital tank pressure gauges and stopwatches and had them measure their air consumption at speeds from 0.5 to 2 mph. Admittedly, this is another shade-tree experiment, but the numbers did show that when they doubled their speed, all four divers more than doubled their air consumption. They were wasting air by going faster.

What's your most efficient speed in water? Everybody moving through water has a unique speed limit beyond which the energy cost of going faster skyrockets. It's a function that is generally proportional to the square root of the length. So a 100-foot whale (square root = 10) can go about four times as fast as a six-foot diver (square root = 2.45) with the same general energy efficiency. Whales have apparently found their most efficient speed by experience. Though baleen whales can sprint faster than 20 mph, they cruise long distances at only 2 to 4 mph, according to Monterey Bay Aquarium Senior Marine Biologist Dr. Steve Webster. Your most efficient speed would be about one-fourth as fast, no more than 1 mph. That's only 1.5 feet per second, like a very slow walk.

Whales also gain by showing Mother Nature a neat appearance. Next to their streamlined shape, we divers are as sloppy as, well, that improvident teenager again. Whales, porpoises and fish have smooth skins and no instrument consoles dragging in the water. A snorkel dangling from a mask strap, a hose that sticks out, a couple of unnecessary gadgets snapped to your BC, all stir the water as you swim through it. That wastes energy and air. Likewise, keep your arms at your sides or behind your back--inside your slipstream.

Don't Sweat What You Can't Fix

Several years ago, we put six divers side by side on a measured course. All swam exactly the same speed at the same depth using the same fins. Their other equipment was not identical but very similar. All six were very experienced, relaxed divers. We expected to see nearly the same air consumption for all six.

To our surprise, however, their air consumption ranged from 38.3 to 85 psi--while swimming the same distance at the same speed and depth. Even excluding the best and worst, the differences were amazing. The middle four divers ranged from 41.2 to 60.3 psi.

What accounts for these huge differences? The air hog was by far the heaviest, so he had to push aside the greatest amount of water with every foot of travel. A small woman had the lowest air consumption, nearly tied by a tall, thin, torpedo-shaped male. But even divers who were fairly similar in size had very different air consumption figures that couldn't be explained by differences in fatigue, stress or skill.

Why? They were, like all of us, different people with different lungs, different cardiovascular systems, different metabolisms, different genes. The lesson here is that there is no ideal air consumption rate and that you shouldn't be ashamed for using more air than your buddy. You can only do what you can.

So let's have less comparing of gauges and less chest thumping over which of us can make a tank last longest. Conserving air is a means to an end. A large cushion of extra air is useful only if it translates into a longer, safer or more enjoyable dive. But if all you're after is bragging rights to having the most air left over at the end of a dive, maybe you're missing the point of diving in the first place.

Having the most air at the end of the dive is not the proof of diving excellence that we sometimes make it. Unless your name is Willy.

The Small Stuff

Several easy fixes will reduce your air consumption, and though none of them is as important as slowing down and breathing efficiently, they do add up.

Check your gear. An obvious one is to check your equipment for air leaks--or ask your buddy to check. Often, you can't see the leaks yourself. A little bubbling from your tank O-ring or your BC inflator can add up to several hundred psi over an hour's dive. An octo that free-flows occasionally will dump air a lot faster. Detune it if you can, and mount it with the mouthpiece facing down. Don't detune your primary reg, however. You'll increase the work you have to do to suck in air, which will increase your carbon dioxide production, which will accelerate your breathing rate and, in the end, waste air.

Streamline and simplify. Minimize the "hole in the water" made by your body. The less water you have to shove aside, the less energy and air you have to consume. One way is to reduce how much lead you carry because extra lead needs extra BC inflation to support it. An inflated BC pushes aside more water. Another way to shove less water is trim your body to horizontal, so your legs are following through the hole made by your shoulders and not enlarging it. Many divers do, in fact, swim with their heads up and fins down.

Watch your kick cycle. Finning with short, rapid strokes also causes less turbulence and drag. When you spread your legs wide for a big stroke, your fins are actually slowing you down. Rapid finning takes getting used to, however. It may be tiring and even waste energy and air until your legs are trained to it.

Go slow, stay shallow. Depth and current both waste energy and air. Doubling your depth doubles your air consumption, and if you double your speed to compensate for a current you may nearly quadruple your air consumption. So dodge both when you can. Often, a wall looks the same at 40 feet as it does at 60. On the other hand, there are trade-offs: a current may be weaker along the bottom than near the surface.

Stretch your boundaries. Consider taking up yoga for more control of your breathing. Don't laugh. Free divers swear by it.

By John Francis

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