Abstract: A flow sensor to detect changes in the mass flow per unit time of pneumatically conveyed particles within a system. The flow sensor or detector includes a conduit segment formed of a hard material so that a wall of the segment will vibrate due to the impacting of the particles thereagainst as they flow through the conduit segment. There is associated with the segment a vibration excitable piezoelectric transducer which has a resonant frequency and which at the resonant frequency generates an electrical signal or high voltage as compared to that generated by the transducer at all other frequencies, above and below the resonant frequency. Any deviation in the high generated voltage is utilized to detect a change in the kinetic energy of the particles flowing through the conduit segment.