Before looking at the mechanics of how centrifugal pumps lift water, we need to talk a little about atmospheric pressure because it is the physical property that allows a pump to lift water. Just as deep-sea divers experience increasing pressure on their bodies the deeper they go down into the ocean, the weight of the earth’s atmosphere exerts increasing pressure on its occupants the deeper we are below its surface, the ionosphere. At sea level, this downward force exerts a pressure of 14.7 pounds on every square inch of the earth’s surface. In Death Valley, atmospheric pressure is actually higher than 14.7 PSI because it is below sea level. Atmospheric pressure decreases with increasing altitude because there is less atmosphere above you the higher you go.
Atmospheric pressure can be expressed in three terms, PSIA (PSI Absolute), inches of mercury and feet of water (The abbreviated terms “feet of head” or just “feet” refer to feet of water). The three terms are interchangeable using the following conversions factors: To convert PSIA into feet, multiply by 2.31, and to go from PSIA to inches of mercury, multiply by 2.04. At sea level, atmospheric pressure equals 30 inches of mercury, 34 feet of water or 14.7 PSIG. The term you use depends on what you need to do with the information. The only time we are likely to see it expressed in terms of inches of mercury is reporting barometric (atmospheric) pressure. From this point on, we’ll talk in terms of PSIA and feet of head.