Pressure
Have you ever wondered why nobody makes snorkels that are ten, or twenty, or a hundred feet long? The
answer becomes obvious as soon as you try to breathe through a snorkel when your body is more than
two or three feet (~1 meter) beneath the surface of the water. If you have ever tried this, you will know
that it becomes extremely difficult to inhale under those circumstances. That's because the deeper
underwater you go, the greater the
pressure
is. Think of pressure as a force pushing on you from all
directions. At sealevel, we are exposed to about 14.7
pounds per square inch
(psi) of pressure. This
means that each square inch of our bodies has the equivalent force of about 14.7 pounds pressing on it.
The source of this pressure is actually a result of the
weight
of the air in Earth's atmosphere. Like all
gases, the air around us is composed of
molecules
of different gases; in this case, about 21% of these
molecules are oxygen, about 78% are nitrogen, and the rest is composed of assorted trace gases. These
gas molecules have weight, which means that gravity is pulling them toward Earth. As it turns out, if you
took a column of air, one square inch in crosssection, extending from sealevel all the way to the edge of
the atmosphere, all the gas molecules in that column of air would have a combined weight of about 14.7
pounds  which leads to 14.7 psi of pressure at sea level. For convenience, physicists have defined the
"atmosphere" (abbreviated "ATM") as a unit measurement of pressure equal to the pressure caused by
Earth's atmosphere at sea level (14.7 psi).
Like air, water causes pressure by its weight. But of course, water is considerably denser (i.e., heavier for
a given volume) than air. As it turns out, a column of sea water one inch in crosssection would need to be
only about 33 feet (10 meters) tall to weigh 14.7 pounds. Therefore, at a depth of 33 feet (10 meters)
beneath the sea surface, the total ambient pressure is about 29.4 psi, or 2 ATM  1 ATM caused by the
weight of the air in Earth's atmosphere, plus 1 ATM for the weight of 33 feet (10 meters) of seawater. To
avoid confusion, when people discuss pressures underwater, the unit "ATA" (referring to "atmospheres
absolute") is often used to represent the total, "absolute" pressure caused by both the water and the air
above the water.
As is illustrated in the diagram at right, the ambient pressure increases underwater at an almost linear rate
with increasing depth
*
. For every 33 feet (10 meters) of depth in sea water, the ambient pressure
increases by an an additional 14.7 psi (1 atm). At a depth of 99 feet (30 meters), the ambient pressure is 4
ATA  one ATM caused by the Earth's atmosphere, plus 3 ATM for every 33 feet (10 meters) of depth.
Similarly, the ambient pressure 297 feet (90 meters) beneath the surface is 10 ATA. The problem with the
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 Spring '09
 mr.chuck
 Underwater diving, Decompression sickness, Breathing gas

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