Charles Boyle, an English scientist, was among the
first to experiment with the pressure-volume
relationship of gas. During an experiment when he
compressed a volume of air, he found that the volume
decreased as pressure increased, and by doubling the
force exerted on the air, he could decrease the volume
of the air by half (fig. 3-50).
Temperature is a dominant factor affecting the
physical properties of gases. It is of particular concern in
calculating changes in the state of gases. Therefore, the
experiment must be performed at a constant temperature.
The relationship between pressure and volume is known
as Boyle's law. Boyle's law states when the temperature
of a gas is constant, the volume of an enclosed gas
varies inversely with pressure.
Boyle's law assumes conditions of constant
temperature. In actual situations this is rarely the case.
Temperature changes continually and affects the volume
of a given mass of gas.
Jacques Charles, a French physicist, provided much
of the foundation for modem kinetic theory of gases.
Through experiments, he found that all gases expand and
contract proportionally to the change in absolute
temperature, providing the pressure remains constant.
The relationship between volume and temperature is
known as Charles's law. Charles's law states that the
volume of a gas is proportional to its absolute
temperature if constant pressure is maintained.
Gases serve the same purpose in pneumatic systems
as liquids serve in hydraulic systems. Therefore, many of
the same qualities that are considered when selecting a
liquid for a hydraulic system must be considered when
selecting a gas for a pneumatic system.
Figure 3-50.Gas compressed to half its original volume by a
The ideal fluid medium for a pneumatic system must
be a readily available gas that is nonpoisonous,
chemically stable, free from any acids that can cause
corrosion of system components, and nonflammable. It
should be a gas that will not support combustion of other
Gases that have these desired qualities may not
have the required lubricating power. Therefore,
lubrication of the components must be arranged by
other means. For example, some air compressors are
provided with a lubricating system, some components
are lubricated upon installation or, in some cases,
lubrication is introduced into the air supply line (in-
Two gases meeting these qualities and most
commonly used in pneumatic systems are compressed air
and nitrogen. Since nitrogen is used very little except in
gas-charged accumulators, we will only discuss
Compressed air is a mixture of all gases contained in
the atmosphere. However, in this manual it is referred to
as one of the gases used as a fluid medium for pneumatic
The unlimited supply of air and the ease of
compression make compressed air the most widely
used fluid for pneumatic systems. Although moisture
and solid particles must be removed from the air, it
does not require the extensive distillation or separation
process required in the production of other gases.
Compressed air has most of the desired charac-
teristics of a gas for pneumatic systems. It is
nonpoisonous and nonflammable but does contain
oxygen which supports combustion. The most
undesirable quality of compressed air as a fluid
medium for a pneumatic system is moisture content.
The atmosphere contains varying amounts of moisture
in vapor form. Changes in the temperature of
compressed air will cause condensation of moisture in
the system. This condensed moisture can be very
harmful to the system and may freeze the line and
components during cold weather. Moisture separators
and air dryers are installed in the lines to minimize or
eliminate moisture in systems where moisture would
deteriorate system performance.
An air compressor provides the supply of
compressed air at the required volume and pressure. In