LEARNING GOALS
In this chapter, you will learn:
• The composition of air.
• How gases behave.
• The relationship between pressure and
gas volume.
• About the solubility of gases.
• What partial pressure is and how to determine
partial pressure.
• How to determine absolute pressure at depth.
• How to determine the partial pressure of a gas in
a mixture at depth.
Matter has three common forms: solid, liquid, and
gas. Solids have definite volume and shape. Liquids have
definite volume but flow and assume the shape of their
container. Gases have neither shape nor volume. A gas will
uniformly fill the container that holds it. It can be allowed
to expand by increasing the size of the container, or it can
be compressed by forcing it into a smaller container or by
making the container smaller.
A gas is composed of molecules (or atoms) that are
in constant motion. Visualize extremely small, widely
separated individual molecules of a gas in constant,
random motion, bouncing off each other and the sides
of their container in a completely elastic way, and you
will have a rough picture of an “ideal gas.” The behavior
of any real gas only approximates the theoretical ideal,
but at the pressures encountered in scuba diving, the
approximation is very close.
A “gas” can be a mixture of individual component
gases. Air is such a mixture; it is predominately nitrogen
and oxygen. Once combined, the component gases
will be evenly distributed in the gas mixture according
to their proportions. Also, the component gases will not
normally separate; they will stay mixed. When you add
oxygen to air to make nitrox, the oxygen will not separate
out and pool at the bottom of the cylinder. The
component gases will generally be molecules, made up
of more than one atom each composed of one or more
elements (oxygen molecules, carbon dioxide molecules,
etc.), but they may also be individual atoms
(argon, helium, and neon do not form molecules).
Each of the individual components in a gas mixture will
exert its chemical or physical effects independently,
according to its proportion in the mixture.
The pressure of the gas is a manifestation of the
force with which its molecules strike the container. You
can increase the pressure of the gas by compressing it
into a smaller container (there are more molecules in a
given volume) or by increasing its temperature (the
molecules increase their energy and speed up). There is
a proportionality to pressure, temperature, and volume
that will be discussed later in this chapter.
Gases (and solids) will dissolve in liquids. Just as
the amount of a solid, such as salt, that will dissolve is
different for different liquids, such as water and gasoline,
the amount of a gas that can dissolve varies from solvent
to solvent. The amount of gas that can be dissolved in a
liquid is directly proportional to the pressure of the gas
on the liquid. The solubility of a gas also varies with the
temperature: more gas will dissolve at lower temperature.
WHAT’S IN AIR?
Our atmosphere is a mixture of gases. The proportions
of the gases in air, excluding water vapor, are nearly
uniform around the globe. The composition of dry air,
expressed as fractions is:
Oxygen (O2) 0.2095
Nitrogen (N2) 0.7808
Argon (Ar) 0.00934
Carbon dioxide (CO2) ~0.00035 (average)
Others 0.00004
Nitrogen (N 2 ) 78.084%
Oxygen (O ) 20.946%
Argon (Inert)
0.934%
Carbon Dioxide (CO ) 0.035%
and Other Gases 0.004%
2
2
NAUI Nitrox Diver
16 Gases & Gas Mixtures
FIGURE 2-1: COMPOSITION OF DRY AIR