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."
nontechnical use no number is present. These definitions are further
explained in the following paragraphs.
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
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
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.
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.
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.