Cited Passage
An understanding of buoyancy serves the diver in a
number of ways. By using weights, by expanding the air
in a buoyancy compensator, or by increasing the size of a
variable-volume diving suit, a diver can manipulate his
buoyancy to meet operational needs. When working on
the bottom, for example, a slightly negative buoyancy
provides better traction and more stability on the sea
floor. Buoyancy is also an invaluable aid to lifting heavy
items in salvage operations. 2.7 GASES USED IN DIVING
Air breathed on the surface (atmospheric air) is also
the most common gas breathed under water. Gases react
in specific ways to the effects of pressure, volume, and
temperature. 2.7.1 Atmospheric Air
The components of dry atmospheric air are given in
Table 2.5. Depending upon the location and weather
conditions, atmospheric air may also contain industrial
pollutants. The most common pollutant is carbon
monoxide, often present around the exhaust outlets of
internal-combustion engines. Diving safety is jeopardized
if pollutants are not filtered from compressed air prior to
diving. Besides atmospheric air, divers use various mixtures
of oxygen, nitrogen, and helium. In some diving applica-
tions, special mixtures of one or more of the gases may
be blended with oxygen. The physiological effects of
each gas alone, or in combination with other gases, must
be taken into account to insure that no harm is done to
body organs and functions. The so-called “inert” gases
breathed from the atmosphere, or those in gas mixtures
we breathe when diving, serve only to dilute and mix
with oxygen. 2.7.2 Oxygen (O
2
)
Oxygen is the most important of all gases and is one
of the most abundant elements on earth. Fire cannot
burn without oxygen and people cannot survive without
oxygen. Atmospheric air contains approximately 21%
oxygen, which exists freely in a diatomic state (two
atoms paired off to make one molecule). This colorless,
odorless, tasteless, and active gas readily combines with
other elements. From the air we breathe, only oxygen is
actually used by the body. The other 79% of the air
serves to dilute the oxygen. Pure 100% oxygen is often
used for breathing in hospitals, aircraft, and hyperbaric
medical treatment facilities. Sometimes 100% oxygen is
used in shallow diving and in certain phases of diving. For storage in saturation and for deeper diving the per-
centage may be less; in fact, it may be too low to be safe-
ly breathed at sea level. Mixtures low in oxygen require
special labeling and handling to ensure that they are not
breathed unintentionally. Breathing a mixture with no
oxygen will result in unconsciousness, brain damage,
and death within a few minutes. Besides its essential
metabolic role, oxygen is fundamental to decompression. Still, the gas can also be toxic. Breathing pure oxygen
Physics of Diving 2-7
Weight
of
barrel
84 lbs
Seawater
The partially immersed 84 lb. barrel with 332 lbs. attached has displaced 6.5 ft
3
of seawater. 6.5 ft
3
64 lbs/ft
3
= 416 lbs
416 lbs Ð 84 lbs = 332 lbs positive buoyancy
6.5
ft
3
332
lbs. FIGURE 2.5
ArchimedesÕ Principle
Concentration
Percent by Parts per
Component Volume Million (ppm)
Nitrogen 78.084
Oxygen 20.946
Argon 0.934
Carbon Dioxide 0.033
Rare Gases 0.003 30.00*
Neon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18.18*
Helium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.24*
Carbon Monoxide . . . . . . . . . . .
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