The devices covered by this standard are those which have functional capabilities for preventing both back siphonage and back pressure, and which can operate under continuous or intermittent pressure conditions. These devices have two (2) independently operating check valves separated by an intermediate chamber with a means for automatically venting it to the atmosphere and can be installed in the horizontal, vertical up or vertical down orientations. The check valves are force loaded to a normally closed position and the venting means is force loaded to a normally open position.