A what type of backflow preventer provides the best mechanical protection against backflow?

The best mechanical protection against backflow is provided by a Reduced Pressure Principle backflow preventer. This type of device is specifically designed to prevent backflow caused by both back pressure and back siphonage, which is essential for protecting potable water supplies from contamination.

The Reduced Pressure Principle backflow preventer operates by maintaining a pressure differential, keeping the pressure downstream lower than the pressure upstream in the water system. This design includes two check valves and a pressure relief valve, which together create a barrier that effectively mitigates the risk of contaminants flowing back into the safe water supply.

In contrast, the other options, while effective to varying degrees for specific applications, do not provide the same level of protection. A Double Check Valve prevents backflow but may not be sufficient in high hazard situations as it does not address back siphonage effectively. An Air Gap is a physical separation that can prevent backflow, but it requires a specific installation that may not be feasible in all scenarios. A Check Valve functions well for preventing backflow in certain conditions but lacks the comprehensive protective features of the Reduced Pressure Principle backflow preventer, especially in more complex plumbing systems or where there's a higher risk of contamination.