The word pneumatics is a derivative of the Greek word pneuma, which means air, wind, or breath. Pneumatics can be defined as that branch of engineering science that pertains to gaseous pressure and flow. As used in this manual, pneumatics is the portion of fluid power in which compressed air, or other gas, is used to transmit and control power to actuating mechanisms.

This section discusses the basic principles of pneumatics, characteristics of gases, heavy-duty air compressors, and air compressor maintenance. It also discusses the hazards of pneumatics, methods of controlling contamination, and safety precautions associated with compressed gases.

BASIC PRINCIPLES OF PNEUMATICS
Gases differ from liquids in that they have no definite volume; that is, regardless of size or shape of the vessel, a gas will completely fill it. Gases are highly compressible, while liquids are only slightly so. Also, gases are lighter than equal volumes of liquids, making gases less dense than liquids.

Compressibility and Expansion of Gases
Gases can be readily compressed and are assumed to be perfectly elastic. This combination of properties gives gas the ability to yield to a force and return promptly to its original condition when the force is removed. These are the properties of air that is used in pneumatic tires, tennis balls, and other deformable objects whose shapes are maintained by compressed air.

Kinetic Theory of Gases
In an attempt to explain the compressibility of gases, consider the container shown in figure 3-49 as containing a gas. At any given time, some molecules are moving in one direction, some are travelling in other directions, and some may be in a state of rest. The average effect of the molecules bombarding each container wall corresponds to the pressure of the gas. As more gas is pumped into the container, more molecules are available to bombard the walls, thus the pressure in the container increases.

Increasing the speed with which the molecules hit the walls can also increase the gas pressure in a container. If the temperature of the gas is raised, the molecules move faster, causing an increase in pressure. This can be shown by considering the automobile tire. When you take a long drive on a hot day, the pressure in the tires increases and a tire that appeared to be soft in cool morning temperature may appear normal at a higher midday temperature.

Boyle's Law
When the automotive tire is initially inflated, air that normally occupies a specific volume is compressed into a smaller volume inside the tire. This increases the pressure on the inside of the tire.

Figure 3-49.- Molecular bombardment creating pressure.

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.

Charles's Law
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.