Scuba diving

Now that winter has descended upon us in earnest, many Canadians are planning a trip south to Florida or the Caribbean, where they’ll bask in the warm sun, eat exotic foods–and maybe even try a little scuba diving.

“Scuba” is a word in its own right now, but originally it was an acronym for “Self-Contained Underwater Breathing Apparatus.” Attempts to develop such apparatus began in the early 20th century, but it wasn’t until 1943 that what we think of as the typical scuba, called the Aqualung, was invented by Jacques Coustau and Emil Gagnan. It was the aqualung that opened up recreational diving to the likes of vacationing Snowbirds.

Scuba divers wear tanks that carry a supply of pressurized “breathing gas”–which can be either air or a mixture of oxygen and other gases. The diver breathes through what’s called a regulator; there are also mechanisms for reducing the pressure of the compressed gas before it’s delivered to the diver’s lungs. Every time the diver takes a breath, the slight negative pressure caused by his or her intake signals a valve to open and release gas. The valve closes when the diver stops inhaling. A separate, one-way valve lets the diver’s exhaled breath bubble away into the water.

In more sophisticated scubas, that breath is recycled, passed through a canister that absorbs carbon dioxide, then through a breathing bag, where more oxygen is added–then breathed again. Scubas that use this method let divers carry smaller tanks but still stay underwater for a long period of time.

There’s more to scuba diving than just strapping on some tanks and jumping in the water, though. You have to know what you’re doing to dive safely–and one of the things you’d better understand is just how different the underwater world is from the land-locked one in which we spend most of our days.

Even diving to the bottom of a swimming pool will give you practical experience of one of the major factors of being underwater: pressure. Water is very dense. As a result, water pressure increases about half a pound per square inch (or about 35 grams per square centimetre, if you must have it in metric).

At a depth of approximately 10 metres, the pressure on a diver is twice what it is at the surface. At 30 metres, it’s four times as much. At the deepest depths divers have ever reached, around 600 metres, the pressure is a whopping 70 kilograms per square centimetre–which means the total pressure on the diver at that depth is equivalent to 1,135 tonnes!

Nevertheless, it’s worth remembering that air pressure alone at sea level is about 14.7 pounds per square inch–a little over one kilogram per square centimetre. Since the average man has about 2,500 square inches of surface area–about 16,350 square centimetres–even at sea level, he’s subjected to almost 19 tonnes (or about 16 1/2 tonnes) of atmospheric pressure! We don’t think about it, because the pressure is equalized inside and out. And fortunately for divers, our body is mostly water, and so pressure remains equalized in our cells and tissues as we descend into the depths, even though it increases.

Unfortunately for divers, we do have a few organs that tend to pockets of air–and if that air is still at sea-level pressure when the pressure on the body has doubled, trouble ensues. Scuba diving equipment solves the most obvious source of this problem, the lungs, by providing air at exactly the same pressure as the water surrounding the body. (The lungs don’t care how dense the air is, just so long as they can draw a full breath. The effect of this on the diver is that the deeper he goes, the less time he can stay down. At 30 metres, where the pressure is four times that at sea level, for example, it takes four times as much air from his tanks to fill his lungs as it did at the surface.) There are other areas of the body, however, and sometimes parts of the diver’s equipment, where air tends to linger, and that can cause problems, known as “squeeze.” The most common is probably “ear squeeze.” When the diver plunges into the water, his middle ear contains air at atmospheric pressure. The pressurized air being breathed from the tanks does make its way into the middle ear through the eustachian tube, equalizing the pressure–but if your ears have every popped as you drove up into the mountains, or on board an airplane, you know that the process isn’t perfect, and if you have a cold, sometimes it doesn’t happen at all. When you’re diving, if the pressure doesn’t equalize, the greater pressure outside the ear presses in on the ear drum and can eventually rupture it.

There are other types of “squeeze,” all caused by similar phenomena: sinus squeeze, when because of an infection or some other reason, air pressure can’t be equalized in the sinuses; thoracic squeeze (usually associated with deep skin diving, and caused by the fact that skin divers hold their breath as they descend–eventually the pressure causes the lungs to collapse smaller than they’re designed to go), and mask squeeze (caused by the air inside the mask compressing, which lets the outside pressure push the mask painfully against the face), among others.

Another hazard that’s a direct result of pressure is nitrogen narcosis, a.k.a. rapture of the depths. Although the lungs can (and must) breath air that is more dense than surface air when a diver is underwater, that air, being more dense, is also richer. At about 40 metres, where the pressure is five times that of the surface, a diver breathing compressed air is breathing five times as much oxygen as usual–the equivalent of breathing pure oxygen on the surface. Even more troublesome is nitrogen, which makes up most of our air. We don’t even think about nitrogen at the surface, but at 40 metres underwater, where we’re breathing five times as much of it, five times as much of it is also being dissolved into our bloodstream. For different people at different depths, that dissolved nitrogen begins to have a distinct affect on the central nervous system, similar to alcohol intoxication. Divers suffering from rapture of the depths, in other words, are drunk on nitrogen. Worse, they may not realize it: in its early stages, nitrogen narcosis makes divers feel euphoric…but just like alcohol, it also reduces the ability to think straight–dangerous, when you’re in a hostile environment. Experts recommend that recreational divers limit their dives to no more than about 45 metres for this reason.

Nitrogen also causes another famous diving illness, “the bends.” If a diver stays down too long, his body fluids become saturated with compressed nitrogen. This expands as he ascends. As long as he ascends slowly enough, the nitrogen is routed to his lungs by his body and he can get rid of it. A sudden ascent, however, can result in nitrogen bubbles forming in tissues throughout the body, with results ranging from the unpleasant to the fatal. Most sports divers don’t have to worry about the bends, as long as they are aware of the safe time limits for various depths and don’t violate them.

In order for a scuba diver to swim comfortably at any depth, he or she must get as close as possible to neutral buoyancy. Like anything else in the water, a scuba diver is subject to Archimedes’ Principle (the one he was so excited to discover he supposedly jumped out of his bath and ran naked down the street, shouting, “Eureka!”). According to this principle, an object floats if it weighs less than the amount of water it displaces. For example, if a diver weighs 90 kilograms but displaces 91 kilograms (91 litres) of water, he has a positive buoyancy of one kilogram, and will rise. To obtain neutral buoyancy, he’d add a kilogram of weight to his belt. Almost everyone has a small amount of natural positive buoyancy, especially when the lungs are inflated.

All of this may make it sound like diving is fiendishly complicated and hideously dangerous. In fact, it’s not–provided divers are properly trained, keep their heads, and never dive alone. Diving instructors like to say that diving accidents occur ONLY for one of three reasons: faulty judgment, lack of training, or a failure to recognize the limits of the equipment and the diver. The diver’s number-one enemy, they say is panic–and those most susceptible to it are those who are poorly trained. They lose control, lose their breathing regulator, and lose their life.

If you are going south and thinking of trying scuba diving, make sure you either know what you’re doing or that you sign up for a complete set of lessons from a qualified instructor, either here, before you leave, or after you arrive…and then go ahead and take the plunge. The wonders of the underwater world await!

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