Water Pressure 1

For an understanding of the nature of pressure and its relationship to water depth, consider the pressure exerted on the base of a cubic foot of water at sea level. The weight of a cubic foot of water is 62.4 pounds at 32 degrees Fahrenheit. (One gallon of water weighs 8.33 pounds per gallon). The base may be subdivided into 144 square inches with each subdivision being subjected to a pressure of 0.433 psig. Frequently water pressure is referred to using the terms “pressure head” or just “head,” and is expressed in units of feet of water. One foot of head would be equivalent to the pressure produced at the base of a column of water 1 foot in depth. One foot of head or 1 foot of water is equal to 0.433 psig. One hundred feet of head is equal to 43.3 psig.

Suppose another cubic foot of water were placed directly on top of the first. The pressure on the top surface of the first cube, which was originally atmospheric, or 0 psig. The pressure on the top of the bottom cubic foot of water is not 0.433 psig as a result of the superimposed cubic foot of water. The pressure at the base of the bottom cube was 0.433 psi. Now the pressure at the bottom of both cubic feet of water increases to 0.866 psig, (P= 0.433+0.433) or two times the original pressure. If this process were repeated with a third cubic foot of water, the pressures at the base of each cube would be 0.433 psig, 0.866 psig, and 1.299 psig, respectively. Three times the pressure of one cubic foot of water, which is 0.433 psi. (3 X 0.433 psi =1.299 psi)

It is evident that pressure varies with depth below a free water surface; in general, each foot of elevation change, within a liquid, changes the pressure by an amount equal to the weight -per-unit area of 1 foot of the liquid. The rate of increase for water is 0.433 psi per foot of depth. A column of water 27 3/4” (2.31 feet) tall generates a pressure at the bottom of the column equal to one pound per square inch (psi)