CHAPTER 2 Underwater Physics
treatment facilities. Sometimes 100 percent oxygen is used in shallow diving oper-
ations and certain phases of mixed-gas diving operations. However, breathing pure
oxygen under pressure may induce the serious problems of oxygen toxicity.
Nitrogen. Like oxygen, nitrogen (N2) is diatomic, colorless, odorless, and taste-
less, 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. Helium (He) is a colorless, odorless, and tasteless gas, but it is mona-
tomic (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 worlds 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
Table 2-2. Components of Dry Atmospheric Air.
Percent by Volume
Parts per Million (ppm)