The HWFA3 piston flow switch contains a magnetic piston with a precise spring support inside that pushes the piston backward, and the magnetic piston triggers an external micro switch or spring tube. Optional pointer flowmeter head to display instantaneous flow rate. Optional digital flowmeter head to display instantaneous flow. Output analog or high switching model. The inner part is all-metal structure, optional pointer flow switch, with high reliability and repeatability, complete isolation between mechanical part and electronic part, high voltage, normally open output, large contact capacity, temperature and pressure compensation.