Electromagnetic flow meters apply Faraday's law of electromagnetic induction: a conductive fluid moving through a perpendicular magnetic field generates a voltage across two electrodes proportional to the average fluid velocity, independent of density or viscosity, but requiring a minimum fluid conductivity. Ultrasonic flow meters use acoustic energy transmitted through the fluid via piezoelectric crystals, in two variants — Doppler-shift meters, which measure the frequency shift caused by reflection off entrained particles/bubbles, and transit-time meters, which measure the time (or equivalent frequency) difference between upstream and downstream acoustic propagation. A related non-obstructive technique, the hot-wire anemometer, infers flow velocity from convective heat transfer between a heated resistance wire and the flowing fluid. All belong to non-differential-pressure, non-invasive or minimally invasive flow-sensing theory within industrial instrumentation.
Electromagnetic flow meters apply Faraday's law of electromagnetic induction: a conductive fluid moving through a perpendicular magnetic field generates a voltage across two electrodes proportional t…