Knowledge of the properties and behavior of gases, especially those used for
breathing, is vitally important to divers.
The most common gas used in diving is atmospheric air, the
composition of which is shown in Table 2-2. Any gases found in concentrations
different than those in Table 2-2 or that are not listed in Table 2-2 are considered
contaminants. Depending on weather and location, many industrial pollutants may
be found in air. Carbon monoxide is the most commonly encountered and is often
present around air compressor engine exhaust. Care must be taken to exclude the
pollutants from the divers’ compressed air by appropriate filtering, inlet location,
and compressor maintenance. Water vapor in varying quantities is present in
compressed air and its concentration is important in certain instances.
For most purposes and computations, diving air may be assumed to be composed
of 79 percent nitrogen and 21 percent oxygen. Besides air, varying mixtures of
oxygen, nitrogen, and helium are commonly used in diving. While these gases are
discussed separately, the gases themselves are almost always used in some
mixture. Air is a naturally occurring mixture of most of them. In certain types of
diving applications, special mixtures may be blended using one or more of the
gases with oxygen.
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Table 2-2. Components of Dry Atmospheric Air.
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Oxygen (O2) 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 percent
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 percent of the air serves to dilute the oxygen. Pure 100 percent
oxygen is often used for breathing in hospitals, aircraft, and hyperbaric medical treatment facilities. Sometimes 100 percent oxygen is used in shallow diving operations
and certain phases of mixed-gas diving operations. However, breathing
pure oxygen under pressure may induce the serious problems of oxygen toxicity.
Like oxygen, nitrogen (N2) is diatomic, colorless, odorless, and tasteless,
and is a component of all living organisms. Unlike oxygen, it will not support
life or aid combustion and it does not combine easily with other elements.
Nitrogen in the air is inert in the free state. For diving, nitrogen may be used to
dilute oxygen. Nitrogen is not the only gas that can be used for this purpose and
under some conditions it has severe disadvantages as compared to other gases.
Nitrogen narcosis, a disorder resulting from the anesthetic properties of nitrogen
breathed under pressure, can result in a loss of orientation and judgment by the
diver. For this reason, compressed air, with its high nitrogen content, is not used
below a specified depth in diving operations.
Helium (He) is a colorless, odorless, and tasteless gas, but it is monatomic
(exists as a single atom in its free state). It is totally inert. Helium is a rare
element, found in air only as a trace element of about 5 parts per million (ppm).
Helium coexists with natural gas in certain wells in the southwestern United
States, Canada, and Russia. These wells provide the world’s supply. When used in
diving to dilute oxygen in the breathing mixture, helium does not cause the same
problems associated with nitrogen narcosis, but it does have unique disadvantages.
Among these is the distortion of speech which takes place in a helium atmosphere.
The “Donald Duck” effect is caused by the acoustic properties of helium and it
impairs voice communications in deep diving. Another negative characteristic of
helium is its high thermal conductivity which can cause rapid loss of body and
respiratory heat.
Hydrogen (H2) is diatomic, colorless, odorless, and tasteless, and is so
active that it is rarely found in a free state on earth. It is, however, the most abundant
element in the visible universe. The sun and stars are almost pure hydrogen.
Pure hydrogen is violently explosive when mixed with air in proportions that
include a presence of more than 5.3 percent oxygen. Hydrogen has been used in
diving (replacing nitrogen for the same reasons as helium) but the hazards have
limited this to little more than experimentation.
Neon (Ne) is inert, monatomic, colorless, odorless, and tasteless, and is
found in minute quantities in the atmosphere. It is a heavy gas and does not exhibit
the narcotic properties of nitrogen when used as a breathing medium. Because it
does not cause the speech distortion problem associated with helium and has superior
thermal insulating properties, it has been the subject of some experimental
diving research.
Carbon dioxide (CO2) is colorless, odorless, and tasteless when
found in small percentages in the air. In greater concentrations it has an acid taste
and odor. Carbon dioxide is a natural by-product of animal and human respiration,
and is formed by the oxidation of carbon in food to produce energy. For divers, the
two major concerns with carbon dioxide are control of the quantity in the
breathing supply and removal of the exhaust after breathing. While some carbon
dioxide is essential, unconsciousness can result when it is breathed at increased
partial pressure. In high concentrations the gas can be extremely toxic. In the case
of closed and semiclosed breathing apparatus, the removal of excess carbon
dioxide generated by breathing is essential to safety.
Carbon monoxide (CO) is a colorless, odorless, tasteless, and
poisonous gas whose presence is difficult to detect. Carbon monoxide is formed as
a product of incomplete fuel combustion, and is most commonly found in the
exhaust of internal combustion engines. A diver’s air supply can be contaminated
by carbon monoxide when the compressor intake is placed too close to the
compressor’s engine exhaust. The exhaust gases are sucked in with the air and sent
on to the diver, with potentially disastrous results. Carbon monoxide seriously
interferes with the blood’s ability to carry the oxygen required for the body to
function normally. The affinity of carbon monoxide for hemoglobin is approximately
210 times that of oxygen. Carbon monoxide dissociates from hemoglobin
at a much slower rate than oxygen.
On the surface of the earth the constancy of the atmosphere’s
pressure and composition tend to be accepted without concern. To the
diver, however, the nature of the high pressure or hyperbaric, gaseous environment
assumes great importance. The basic explanation of the behavior of gases
under all variations of temperature and pressure is known as the kinetic theory of
gases.
The kinetic theory of gases states: “The kinetic energy of any gas at a given temperature
is the same as the kinetic energy of any other gas at the same temperature.”
Consequently, the measurable pressures of all gases resulting from kinetic
activity are affected by the same factors.
The kinetic energy of a gas is related to the speed at which the molecules are moving
and the mass of the gas. Speed is a function of temperature and mass is a
function of gas type. At a given temperature, molecules of heavier gases move at a
slower speed than those of lighter gases, but their combination of mass and speed
results in the same kinetic energy level and impact force. The measured impact
force, or pressure, is representative of the kinetic energy of the gas. This is illustrated
in Figure 2-6.
Figure 2-6. Kinetic Energy. The kinetic energy of the molecules inside the container (a) produces a constant
pressure on the internal surfaces. As the container volume is decreased (b), the molecules per unit volume
(density) increase and so does the pressure. As the energy level of the molecules increases from the addition of
thermal energy (heat), so does the pressure (c).
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