A flow measuring device includes a housing having an inlet, an outlet and a passage extending between the inlet and outlet, a shaft disposed within the housing and rotatable about a central axis and a flow detector mounted on the shaft and disposed partially within the passage, such that fluid flow through the passage rotates the detector about the axis. A magnet is mounted on the shaft and spaced axially from the detector and a sensor disposed within the housing and configured to sense rotation of the magnet as the shaft angularly displaces about the central axis so as to detect angular displacement of the flow detector. Preferably, the sensor includes a switch adjustable from open and closed states when either one of the poles of the magnet is proximal to the switch. A processor is connected with the switch and determines flow volume and/or flow rate through the passage.