PRESSURE

The word "pressure," when used in conjunction with mechanical and

hydromechanical systems, has two different uses.

One is technical;

the other, nontechnical. These two uses can be easily distinguished

from each other by the presence or absence of a number. In technical

use, a number always accompanies the word "pressure."

In

nontechnical use no number is present. These definitions are further

explained in the following paragraphs.

Technical.

The number accompanying pressure conveys specific

information about the significant strength of the force being

applied. The strength of this applied force is expressed as a rate

at which the force is distributed over the area on which it is

acting.

Thus, pounds per square inch (psi) expresses a rate of

pressure just as miles per hour (mph) does of speed. An example of

this is: "The hydraulic system in UH-1 aircraft functions at 1500

psi."

Nontechnical. The word "pressure," when used in the nontechnical

sense simply indicates that an unspecified amount of force is being

applied to an object.

Frequently adjectives such as light, medium,

or heavy are used to remove some of the vagueness concerning the

strength of the applied force.

PRESSURE MEASUREMENT

When used in the technical sense, pressure is defined as the amount

of force per unit area. To have universal, consistent, and definite

meaning, standard units of measurement are used to express pressure.

In the United States, the pound is the unit of measurement used for

force, and the square inch is the unit for area. This is comparable

with the unit of measurement used for speed: the mile is the unit of

measurement for distance, and the hour is the measurement for time.

A pressure measurement is always expressed in terms of both units of

measurement just explained: amount of force and unit area. However,

only one of these units, the amount of force, is variable.

The

square inch is used only in the singular -- never more or less than

one square inch.

A given pressure measurement can be stated in three different ways

and still mean the same thing. Therefore, 50 psi pressure, 50 pounds

pressure, and 50 psi all have identical meanings.