Super-Pro® SP0821-30 3"S White Swing PVC Check Valve with 0.5 lb Spring. The “Smart” Check® provides you with reliable back-flow prevention without the inline restriction of a conventional check valve. With the use of high powered magnets, the “Smart” Check valve remains in the closed position to 2 PSI, opening fully when 2 lb. line pressure is exceeded. The “Smart” Check has a Spring-Loaded gate mechanism allowing the valve to be installed in any orientation.