The word pneumatics is a derivative of the Greek word pneuma, which means air, wind, or breath. It 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 chapter discusses the origin of pneumatics. It discusses the characteristics of gases and compares them with those of liquids. It also explains factors which affect the properties of gases, identifies and explains the gas laws, and identifies gases commonly used in pneumatics and their pressure ranges. It also discusses hazards of pneumatic gases, methods of controlling contamination, and safety precautions associated with compressed gases.

There is no record of mans first uses of air to do work. Probably the earliest uses were to separate chaff from grain and to move ships. One of the first pneumatic devices was the blow gun used by primitive man. In the latter part of the eighteenth century, heated air was used to carry the first balloon aloft. The heated air, being lighter than the surrounding air, caused the balloon to rise.

Every age of man has witnessed the development of devices which used air to do work. However, man used air to do work long before he understood it.

Many of the principles of hydraulics apply to pneumatics. For example, Pascals law applies to gases as well as liquids. Also, like hydraulics, the development of pneumatics depended on closely fitted parts and the development of gaskets and packings. Since the invention of the air compressor, pneumatics has become a very reliable way to transmit power.

Probably one of the most common uses of pneumatic power is in the operation of pneumatic tools. However, you should understand that pneumatics is also of great importance in large and complex systems such as the controls of vital propulsion and weapon systems.

Recall from chapter 1 that gas is one of the three states of matter. It has characteristics similar to those of liquids in that it has no definite shape but conforms to the shape of its container and readily transmits pressure. Gases differ from liquids in that they have no definite volume. That is, regardless of the size or shape of the containing 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.

Early experiments were conducted concerning the behavior of air and similar gases. These experiments were conducted by scientists such as Boyle and Charles (discussed later in this chapter). The results of their experiments indicated that the gases behavior follows the law known as the ideal-gas law. It states as follows: For a given pressure and the volume occupied, divided by the absolute temperature, is constant. In equation form, it is expressed as follows:

    Equation 11-1

For 1 pound of gas,

    Equation 11-2

The specific volume (v) is expressed in cubic feet per pound.

For any weight of a gas this equation maybe modified as follows:

W = weight of the gas in pounds, V = volume of W pounds of the gas in cubic feet. The volume of 1 pound would then be V/W. If we substitute this for v in equation 11-3, it then becomes

Solving equation 11-4 for pressure,

In chapter 2 we defined density as the mass per unit volume. In equation 11-5,

represents density. (Notice that this is the reverse of the specific volume.) We can now say that pressure is equal to the density of the gas times the gas constant times the absolute temperature of the gas. (The gas constant varies for different gases.) From this equation we can show how density varies with changes in pressure and temperature. Decreasing the volume, with the weight of the gas and the temperature held constant, causes the pressure to increase.

NOTE: During the compression of the gas, the temperature will actually increase; however, the explanation is beyond the scope of this text.

a decrease in volume with the weight held constant will cause density to increase.

As indicated previously, temperature is a dominant factor affecting the physical properties of gases. It is of particular concern in calculating changes in the states of gases.

Three temperature scales are used extensively in gas calculations. They are the Celsius (C), the Fahrenheit (F), and the Kelvin (K) scales. The Celsius (or centigrade) scale is constructed by identifying the freezing and boiling points of water, under standard conditions, as fixed points of 0 and 100, respectively, with 100 equal divisions between. The Fahrenheit scale identifies 32 as the freezing point of water and 212 as the boiling point, and has 180 equal divisions between. The Kelvin scale has its zero point equal to 273C, or 460F.

Absolute zero, one of the fundamental constants of physics, is commonly used in the study of gases. It is usually expressed in terms of the Celsius scale. If the heat energy of a gas sample could be progressively reduced, some temperature should be reached at which the motion of the molecules would cease entirely. If accurately determined, this temperature could then be taken as a natural reference, or as a true absolute zero value.

Experiments with hydrogen indicated that if a gas were cooled to 273.16C (273 for most calculations), all molecular motion would cease and no additional heat could be extracted. Since this is the coldest temperature to which an ideal gas can be cooled, it is considered to be absolute zero. Absolute zero may be expressed as 0K, 273C, or 459.69F (460F for most calculations).

When you work with temperatures, always be sure which system of measurement is being used and how to convert from one to another. The conversion formulas are shown in figure 11-1. For purposes of calculations, the Rankine (R) scale illustrated in figure 11-1 is commonly used to

Figure 11-1.-Comparison of Kelvin, Celsius, Fahrenheit, and Rankine temperature.

convert Fahrenheit to absolute. For Fahrenheit readings above zero, 460 is added. Thus, 72F equals 460 plus 72, or 532 absolute (532R). If the Fahrenheit reading is below zero, it is subtracted from 460. Thus, -40F equals 460 minus 40, or 420 absolute (420R). The Kelvin and Celsius scales are used internationally in scientific measurements; there-fore, some technical manuals may use these scales in directions and operating instructions. The Fahrenheit scale is commonly used in the United States; therefore, it is used in most areas of this manual.

We defined pressure in chapter 2 as force per unit area. Remember, liquids exert pressure on all surfaces with which they come in contact. Gases, because of their ability to completely fill containers, exert pressure on all sides of a container.

In practice, we maybe interested in either of two pressure readings. We may desire either the gauge pressure or the absolute pressure. Absolute pressure is measured from absolute zero pressure rather than from normal or atmospheric pressure (approximately 14.7 psi). Gauge pressure is used on all ordinary gauges, and indicates pressure in excess of atmospheric pressure. Therefore, absolute pressure is equal to atmospheric pressure plus gauge pressure. For example, 100 psi gauge pressure (psig) equals 100 psi plus 14.7 psi or 114.7 psi absolute pressure (psia). Whenever gas laws are applied, absolute pressures Gases are required.

COMPRESSIBILITY AND EXPANSION OF GASES can be readily compressed and are assumed to be perfectly elastic. This combination of properties gives a 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.