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310 Chapter 9 Cross-Connection Control
gauge would read zero. Then the absolute pressure would be equal to 14.7 psi (102 kPa),
which is the atmospheric pressure.
The term vacuum indicates that the absolute pressure is less than the atmospheric pres-
sure and that the gauge pressure is negative. A complete or total vacuum would mean a
pressure of 0 psia or 14.7 psig ( 102 kPa gauge). Because it is impossible to produce a
total vacuum, the term vacuum, as used in the text, will mean all degrees of partial vacuum.
In a partial vacuum, the pressure would range from slightly less than 14.7 psia (0 psig) to
slightly greater than 0 psia ( 14.7 psig).
Backsiphonage results in fluid flow in an undesirable or reverse direction. It is caused
by atmospheric pressure exerted on a pollutant liquid such that it forces the pollutant to-
ward a potable water supply system that is under a vacuum. Backflow, although literally
meaning any type of reversed flow, refers to the flow produced by the differential pressure
existing between two systems, both of which are at pressures greater than atmospheric.

9.3.1 Water Pressure
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 (see Fig. 9.11a). The
3
average weight of a cubic foot of water (62.4 lb/ft ) will exert a pressure of 62.4 lb/ft 2
(430 kPa) gauge. The base may be subdivided into 144 square inches with each subdivi-
sion being subjected to a pressure of 0.433 psig (P gauge 3 kPa).
3
3
Suppose another 1 ft (0.0283 m ) of water was placed directly on top of the first (see
Fig. 9.11b). The pressure on the top surface of the first cube, which was originally atmos-
pheric, or 0 psig, would now be 0.433 psig (P gauge 3 kPa) as a result of the superimposed
cubic foot of water. The pressure of the base of the first cube would also be increased by
the same amount of 0.866 psig (P gauge 6 kPa), 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 1.299, 0.866, and 0.433 psig (P gauge 9, 6 and 3 kPa), respectively.

24˝
0.433 psig
62.4#/ft 3
12˝
12˝
12˝

Sea level Sea level
0.866 psig

0.433 psig
(a) (b)

Figure 9.11 Pressures Exerted by (a) 1 ft (0.3048 m) of Water and (b) 2 ft (0.6096 m) of
Water at Sea Level. Conversion factors: 1# 1 lb 0.454 kg; 1 1 in. 25.4 mm;
1 psig 6.94 kPa gauge pressure