US Patent 7445163 Suck back valve

A suck back valve comprises an ON/OFF valve, a suck back mechanism which includes a second piston connected to a second diaphragm to make displacement integrally and a second spring member fastened to the second piston, a load sensor which detects a load applied by a resilient force of the second spring member corresponding to a pressure of a pilot pressure to be supplied, and a second control unit which detects loads before and after displacement of the second diaphragm by using the load sensor to calculate a displacement amount of the second diaphragm based on a difference between the detected loads.