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318 Chapter 9 Cross-Connection Control
5. For these reasons, air gaps must be inspected as frequently as mechanical backflow
preventers. They are not exempt from an in-depth cross-connection control pro-
gram requiring periodic inspection of all backflow devices.
Air gaps can be fabricated from commercially available plumbing components or pur-
chased as separate units and integrated into plumbing and piping systems. An example of
the use of an air gap is shown in Fig. 9.18b.

9.4.2 Barometric Loops

The barometric loop consists of a continuous section of supply piping that abruptly rises to
a height of approximately 35 ft (10.6 m) and then returns back down to the originating
level. It is a loop in the piping system that effectively protects against backsiphonage. It
cannot be used to protect against backpressure.
Its operation, in the protection against backsiphonage, is based on the principle that a
water column, at sea level pressure, cannot rise above 33.9 ft (10.3 m) (refer to Fig. 9.12b). In
general, barometric loops are locally fabricated and are 35 ft (10.6 m) high (see Fig. 9.19).

9.4.3 Atmospheric Vacuum Breakers

These devices are among the simplest and least expensive mechanical types of back-
flow preventers and, when installed properly, can provide excellent protection against
backsiphonage. They must not, however, be utilized to protect against backpressure
conditions.
Construction usually consists of a polyethylene float that is free to travel on a shaft and
seal in the uppermost position against the atmosphere with an elastomeric disk. Water flow
lifts the float, which then causes the disk to seal. Water pressure keeps the float in the up-
ward sealed position. Termination of the water supply will cause the disk to drop down,
venting the unit to the atmosphere and thereby opening downstream piping to atmospheric
pressure, thus preventing backsiphonage. Figure 9.20a shows a typical atmospheric breaker.
1
In general, these devices are available in ⁄2-in. through 3-in. (12-mm through 75-mm)
sizes and must be installed vertically, must not have shutoffs downstream, and must be
installed at least 6 in. (152 mm) higher than the final outlet. They cannot be tested once

35´

Figure 9.19 Barometric Loop.
Conversion factor: 1 1 ft
0.3048 m