SCUBA DIVING EXPLAINED
Physiology and Medical Aspects of Scuba Diving
Lawrence Martin, M.D. Copyright 1997
SECTION D
An Explanation of Pressure and the Laws of Boyle, Charles, Dalton,
and Henry
WHAT IS PRESSURE?
Key to understanding scuba diving is the concept of pressure, and
how it varies with depth. We intuitively understand that pressure
is
some type of force, but how is it actually defined? Pressure is a
force or weight per unit area. All matter, including air, has weight
due to earth's gravity. Accordingly, anything exposed to air is
under pressure - the weight of the atmosphere above it. This
weight of air, due to gravity, is known as atmospheric pressure
(Figure 1).
Figure 1. Earth is surrounded by a layer of atmosphere which is
densest at sea level; the atmosphere becomes thinner - less dense -
with altitude. Figure is not drawn to scale.
From experience just carrying a container of water, we also know
that water is much heavier than air. Since pressure is related to
weight, water pressure must be far greater than air pressure, and of
course it is.
Gravity keeps the atmosphere wedded to the earth. Without
gravity, earth's atmosphere would float away to outer space. Since
gravity diminishes with distance from the center of the earth, air
weighs less at altitude than at sea level. Literally, air at altitude is
"thinner" (meaning less dense) compared to sea level, and air
becomes progressively less dense with increasing altitude (Figure
1).
A cubic foot of air on the summit of Mt. Everest contains only
about a third as many molecules as a cubic foot at sea level, and
hence weighs only about a third as much (Figure 2). (Everything
weighs less at altitude, including people, but we don't notice the
difference except in outer space. You would not feel lighter on the
top of a high mountain. In outer space, miles from earth's center
of gravity, weightlessness is experienced.)
WHAT IS AIR PRESSURE?
Air pressure can be specified in several ways; the most popular
term
used in scuba diving is "pounds per square inch" or "psi." At sea
level
the pressure exerted by the atmosphere is 14.7 psi. "Per square
inch"
refers to the surface area subjected to the weight of the air above
it;
the units could just as well be per "square foot," "square yard" or
"square meter," but then the numbers would be correspondingly
higher.
Stated another way, a column of air one inch square and about 50
miles high (a distance that encompasses virtually all of earth's
atmosphere) weighs just 14.7 pounds (Figure 3). Not a lot of
pounds for something fifty miles high, but not exactly weightless
either. Most of the atmosphere's weight is actually contained in
the first few miles, where gravity exerts its greatest effect. An air
column one-inch square, from sea level and extending 3.4 miles
high (18,000 feet), weighs about half that of the full air column,
or 7.35 pounds. The remainder of the one-inch-square air column,
from an altitude of 3.4 miles to outer space, weighs another 7.35
pounds.
Physiology and Medical Aspects of Scuba Diving
Lawrence Martin, M.D. Copyright 1997
SECTION D
An Explanation of Pressure and the Laws of Boyle, Charles, Dalton,
and Henry
WHAT IS PRESSURE?
Key to understanding scuba diving is the concept of pressure, and
how it varies with depth. We intuitively understand that pressure
is
some type of force, but how is it actually defined? Pressure is a
force or weight per unit area. All matter, including air, has weight
due to earth's gravity. Accordingly, anything exposed to air is
under pressure - the weight of the atmosphere above it. This
weight of air, due to gravity, is known as atmospheric pressure
(Figure 1).
Figure 1. Earth is surrounded by a layer of atmosphere which is
densest at sea level; the atmosphere becomes thinner - less dense -
with altitude. Figure is not drawn to scale.
From experience just carrying a container of water, we also know
that water is much heavier than air. Since pressure is related to
weight, water pressure must be far greater than air pressure, and of
course it is.
Gravity keeps the atmosphere wedded to the earth. Without
gravity, earth's atmosphere would float away to outer space. Since
gravity diminishes with distance from the center of the earth, air
weighs less at altitude than at sea level. Literally, air at altitude is
"thinner" (meaning less dense) compared to sea level, and air
becomes progressively less dense with increasing altitude (Figure
1).
A cubic foot of air on the summit of Mt. Everest contains only
about a third as many molecules as a cubic foot at sea level, and
hence weighs only about a third as much (Figure 2). (Everything
weighs less at altitude, including people, but we don't notice the
difference except in outer space. You would not feel lighter on the
top of a high mountain. In outer space, miles from earth's center
of gravity, weightlessness is experienced.)
WHAT IS AIR PRESSURE?
Air pressure can be specified in several ways; the most popular
term
used in scuba diving is "pounds per square inch" or "psi." At sea
level
the pressure exerted by the atmosphere is 14.7 psi. "Per square
inch"
refers to the surface area subjected to the weight of the air above
it;
the units could just as well be per "square foot," "square yard" or
"square meter," but then the numbers would be correspondingly
higher.
Stated another way, a column of air one inch square and about 50
miles high (a distance that encompasses virtually all of earth's
atmosphere) weighs just 14.7 pounds (Figure 3). Not a lot of
pounds for something fifty miles high, but not exactly weightless
either. Most of the atmosphere's weight is actually contained in
the first few miles, where gravity exerts its greatest effect. An air
column one-inch square, from sea level and extending 3.4 miles
high (18,000 feet), weighs about half that of the full air column,
or 7.35 pounds. The remainder of the one-inch-square air column,
from an altitude of 3.4 miles to outer space, weighs another 7.35
pounds.
Adventure Dominica
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