Which back-flow device provides the highest level of protection against back-flow?

The air gap is considered the device that provides the highest level of protection against back-flow due to its design and operational principle. An air gap is a physical separation between the end of a water supply outlet and the point where it enters a receiving vessel or drain. This separation prevents any contaminated water from flowing back into the potable water supply.

In scenarios where back-flow could occur due to changes in pressure or other factors, the air gap ensures that even if there were a siphoning or back-pressure situation, the physical distance prevents contaminants from entering the clean water system. This completely eliminates the possibility of back-flow since there is no connection that would allow the two water supplies to mix.

In contrast, while other devices such as the reduced pressure principle assembly and double check assembly provide good protection against back-flow, they still rely on mechanical components that can potentially fail or require maintenance. The vacuum breaker is effective in preventing backsiphonage but does not offer the same level of reliability as the air gap when it comes to dealing with back-pressure scenarios.

By understanding the air gap's role and effectiveness, one can appreciate why it is deemed the highest level of protection among back-flow prevention devices.