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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 ST9is factory adjusted to close the circuit at 14 gpm and open it at 6 gpm, but is also field adjustable over a wider range. At 40 gpm, it causes only 6” of H2O pressure loss. A “flapper” (piston in 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 (unaffected by pressure). Plumbing is PVC slip sockets. ST11and ST12have a selectable N.O./N.C. setting. Rated to 50 psi, 125VAC to 25 A (1-hp motor) or 250VAC to 25 A (2-hp motor). Switch is 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 included. Switches measure 6” x 6”. One-year warranty. Made in USA.