An excellent value on UL-recognized flow sensing switches that can turn on or off a heater, chiller, alarm, pump, etc., up to 2 HP (@ 230V). The ST9 is factory adjusted to close the circuit at 14 GPM and open it at 6 GPM, but it is also field adjustable over a wider range. At 40 GPM, it causes only 6" of H2O pressure loss. A flapper (piston for ST12) moves with water flow, actuating a magnetic switch on the outside of the pipe. Water cannot reach the electrical section. Operates by sensing water flow only and is unaffected by pressure. Plumbing connection is PVC slip sockets (2" Spig for ST11). ST11 and ST12 have a selectable N.O./N.C. setting. Rated to 50 psi, 125VAC to 25 A for a 1 HP motor or 250VAC to 25 A for a 2 HP motor. Switch uses a single pole, single throw (one wire in, one wire out) with 1/2 FNPT conduit connections on both sides. One plastic plug (shown in black) is only included in the ST9 model. Switches measure 6" x 6". One-year warranty. Made in USA.