The unit of water pressure is the pound per square inch. The pressure exerted by water is due to its weight and is determined by the height of the column of water. For instance, if the pressure exerted by a force pump is 50 pounds per square inch it will balance a column of water about 115 feet high. This pressure, therefore, is equivalent to a head of water 115 feet deep. Head of water at a given point is the vertical distance between that point and the level of the surface of the water. In measuring the depth or static head of water, the vertical distance from the hydrostatic gradient to the point of consideration is always taken regardless of lateral or horizontal distances from the point.

The weight of a column of water one inch square and 12 inches high equals .434 pound. It is just 1/144 the weight of one cubic foot of water which has the same depth of column but 144 times the area. When the height of a column of water is known, its pressure in pounds per square inch can be determined by multiplying the height in feet by *.434, the weight of one foot of water 1 inch square.

* The constant .434 will be found sufficiently accurate for most calculations and when an approximation only is required the constant .4 will suffice.

Example

What is the pressure per square inch at the base of a column of water 200 feet high?

Solution

200X.434=86.8 pounds per square inch.

When the pressure is known the height or head of a column of water can be found by multiplying the pressure in pounds by 2.3, the height of a column of water weighing one pound.

Example

What must be the height of a column of water to exert a pressure of 86.8 pounds per square inch? Solution - 86.8 X 2.3 = 199.64 feet head.

The constants .434 and 2.3 although used in practice are not exactly correct as can be seen by comparing the two foregoing examples.

Heads and corresponding pressures of water in pounds per square inch for every foot in height to 240 feet can be found in Table XXIV.

Pascil's Law Of Pressure

Water confined in a vessel and subjected to a pressure, transmits the pressure with the same intensity in all directions. This law was first discovered by Pascil, and is expressed as follows: "The pressure per unit of area exerted anywhere upon a mass of liquid is transmitted undiminished in all directions, and acts with the same force upon all surfaces, in a direction at right angles to the surfaces."