The Watts 88005611 APEX (BA112) 1 1/2" BigAl High Flow Variable Level Diaphragm Activated Reservoir/Water Tank Valve efficiently monitors and controls fluid levels to prevent excess pressure or volume in your tank. The pilot center of the diaphragm-activated valve includes a steel pin that works as a self-cleaning mechanism, a feature unique to Apex valves. UV-stable and fully serviceable for long-term use. From the manufacturer: BigAL is ideal for use wherever water is stored for supply and can serve as a top-up valve for rainwater harvesting. It operates in a full-on/full-off position, using single or variable-level weighted floats to maintain minimum and maximum water levels. The valve stays closed when both weights are submerged at the maximum level, then opens again once the bottom weight is exposed. Closes softly to prevent water hammer, the banging noise that can occur when water quickly stops or changes direction.