The key characteristics for an air gap is protection from back flow and back siphonage. Back flow occurs when there is a clog or blockage downstream in the drain line which with more water entering causes the nonpotable water to back up fully engulfing the air gap unit or at least reach in the back siphonage critical level (C/L) of the air gap unit. Back siphonage happens when there is suction on the potable water line and air gap inlet that sucks non-potable water port back into the potable (drinking) water line. The ideal air gap would provide protection from back flow and back siphonage.