Electromagnetic Flow Meters are based on FARADAY'S LAW INDUCTION. These meters are also called as Magflow or Electromagnetic Flow Meters. A magnetic field is applied to the metering tube, which results in a potential difference proportional to the flow velocity perpendicular to the flux lines.
Ultrasonic flow meter are based on Transit Time Technology. The transmitter processes the signals from the wave & its reflections determine the flow rate. Transit time meters transmit & receive ultrasonic waves in both the directions within the pipe.