Which of the following back flow preventers cannot be used in a continuous pressure situation?

An atmospheric vacuum breaker is specifically designed to prevent back siphonage, which occurs when water flows backward due to a drop in pressure. However, it cannot be used in situations where there is continuous pressure on the system. This is because the design of an atmospheric vacuum breaker includes a vent that must be open to the atmosphere in order to function properly. If continuous pressure is applied, it may cause the vacuum breaker to malfunction, leading to potential contamination of the potable water supply.

In contrast, the other types of backflow preventers listed can handle situations with continuous pressure. An air gap creates a physical separation between the water supply and potential contaminants, while a pressure vacuum breaker can maintain its effectiveness under continuous pressure conditions. Similarly, a double check valve is designed to operate under pressure and effectively prevent backflow in both continuous and non-continuous pressure situations. Thus, understanding the operational limitations of an atmospheric vacuum breaker highlights its unsuitability for continuous pressure applications.