(1) Air Gap. The unobstructed vertical distance through the free atmosphere between the lowest opening of any pipe or faucet supplying potable water to a tank, plumbing fixture or other device and the flood level rim of the tank, plumbing fixture or other device. An approved air gap shall be at least double the diameter of the supply pipe or faucet and in no case less than one inch.

(2) Reduced Pressure Principle Assembly (Hereinafter RP). An assembly containing two independently acting approved check valves together with a hydraulically operating, mechanically independent pressure differential relief valve located between the check valves, and at the same time below the first check valve. The assembly shall include properly located test cocks and tightly closing shut-off valves at each end of assembly.

(3) Double Check Valve Assembly (Hereinafter DC). An assembly composed of two independently acting, approved check valves, including tightly closing shut-off valves located at each end of the assembly and fitted with a properly located test cock.

(4) Pressure Vacuum Breaker Assembly (Hereinafter PVB). An assembly containing an independently operating, internally loaded check valve and an independently operating, loaded air inlet valve located on the discharge side of the check valve. The assembly shall be equipped with properly located test cocks and tightly closing shut-off valves located at each end of the assembly.