Pressure can be created by squeezing or pushing on a confined fluid
only if there is a resistance to flow. The two ways to push on a fluid are
by the action of a mechanical pump or by the weight of the fluid. An
example of pressure due to a fluid's weight would be in an ocean's
depths. The water's weight creates the pressure, which increases or
decreases, depending on the depth.
By knowing the weight of a cubic foot of water, you can calculate the
pressure at any depth. Figure 1
-
3 shows a column of water 1 foot
square and 10 feet high, which equates to 10 cubic feet. (One cubic foot
of water weighs 52.4 pounds.) The total weight of water in this column is
624 pounds. The weight at the bottom covers 1,445 square inches (1
square foot). Each square inch of the bottom is subject to 1/144 of the
total weight, or 4.33 pounds. Thus, the pressure at this depth is 4.33 psi.
You can also create an equal pressure of 4.33 psi in a liquid using the
pump and figures shown in Figure 1
-
4.
Before pressure, head was the only way to express pressure
measurement. It was expressed as feet of water. Today, head is still the
vertical distance between two levels in a fluid. In Figure 1
-
3, the head
between the top and bottom of the water is 10 feet, which is equivalent
to 4.33 psi. Therefore, each foot of water is equal to 0.433 psi.