Which device/assembly cannot sustain continuous pressure downstream?

The atmospheric vacuum breaker assembly is designed specifically for backflow prevention in applications where a potential backflow could occur due to negative pressure. This device works by allowing air to enter the system when a drop in pressure occurs downstream, preventing a siphon effect that could lead to contamination. However, it is crucial to understand that it cannot sustain continuous pressure downstream, as maintaining constant pressure would prevent its proper functioning.

If there were continuous pressure, the atmospheric vacuum breaker would remain closed, and its primary responsibility—preventing backflow by breaking a vacuum—could be compromised. Because of its operational design, it must be in a position to allow air in freely to operate effectively. This characteristic distinguishes it from the other devices listed, which are capable of maintaining pressure downstream under various conditions.

The reduced pressure principle assembly, double check valve assembly, and pressure vacuum breaker assembly can all maintain continuous pressure downstream, allowing them to handle a range of system pressures without the risk of backflow in the same manner the atmospheric vacuum breaker cannot.