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Know It All: Scuba -- Going Down

Published in Geare magazine, Issue #37, 2006

Anyone who knows anything about anything also knows how wrong movies often get it. Guns are routinely mishandled in movies. How often do you see someone jacking the action of a pistol, or pumping a shot gun, simply for dramatic effect? Sometimes they even do it twice. So where's the ejected cartridge?

But perhaps the greatest liberties taken in movies concerns water. Particularly underneath it.

The deep blue sea is an alien environment to humans, and descending into it produces unexpected effects. The first one is the rapid increase in pressure.

Out here on land, we experience pressure from the air around us. It is equivalent to having one kilogram of weight pressing against each square centimetre of your body.

The pressure is caused by the weight of the air above us. Of course, water weighs a lot more than air. The weight of the column of air that extends above each of us for tens of kilometres, is matched by a column of water only ten metres tall.

Or, to put it another way, if you descend a mere ten metres underwater, instead of experiencing one kilogram per square centimetre, you'll be subjected to two kilograms. Go another ten metres down and it becomes three kilograms. And so on.

Within the Mariana Trench north of Papua New Guinea, is a point nearly eleven kilometres deep. There you would experience a pressure of over a thousand kilograms per square centimetre.

There are two strategies for dealing with all this pressure. The most obvious one is to protect yourself from it. Descend in a submarine and the air pressure within its steel shell is maintained at more or less the same level as at the surface. The problem here is that even steel has its limits. The maximum diving depth of the new Virginia class submarines of the US Navy is specified only as greater than 244 metres. The record depth achieved by a military submarine was 1,300 metres, by the Soviet Komsomolets in 1988. To get right to the very bottom, a bathyscape is required, built exclusively for strength. One of these, the Trieste, managed to reach the bottom of the Mariana Trench in 1960. Its steel walls were 127mm thick.

The other way to deal with pressure is to pressurise yourself. Most of you is composed of water and, for practical purposes, doesn't compress. But there is one big hole in you: your lungs. With all that pressure, you wouldn't have to descend very far at all to make breathing impossible.

So, instead, you pump up your lungs to withstand the pressure. That's where SCUBA equipment comes in. SCUBA stands for Self-Contained Underwater Breathing Apparatus and is clever in that it not only supplies air, but supplies it at the same pressure as the environment around you, whatever the pressure happens to be.

The air (22% oxygen, most of the rest nitrogen) in the cylinder strapped to the SCUBA diver's back is compressed to somewhere between 180 and 300 times the air pressure at the surface. If you ever see a movie where a diver attempts to suck air directly from the diving cylinder, consider it accurate only if the diver's head is blown off his neck as soon as the valve is opened.

What makes it possible to breath from the cylinder is the regulator. In essence, this cuts down the pressure in two or more steps, with the final step matching the air pressure to the water pressure around you. You can breath in with no more effort than on the surface because the pressure within your lungs and that on your body is equalised.

Nothing is perfect, though. The higher the pressure of the air, the more that is used with each lung full. So the deeper you dive, the less time you have. And there are limits anyway to how deep you can dive.

Oxygen, life-giving substance that it is, becomes toxic to us when its pressure becomes too high. That occurs at around 65 metres. Nitrogen can become an intoxicant beginning at thirty metres. Being drunk and diving is not a safe mix.

And nitrogen under pressure dissolves in your blood. Spend more than a few minutes at any depth beyond ten metres, and you have to come up very carefully indeed, stopping with every halving of pressure for many minutes to allow the nitrogen to return safely to your lungs. Otherwise nitrogen bubbles form in your blood and you have the bends.

© 2002-2009, Stephen Dawson