Ultrasonic
 

The meter transmits sound waves both up stream and down stream.  Due to the resistance of the flow, the upstream sound pattern will require a greater interval of time to reach its receiver.  The larger the difference in these times, the higher the velocity and the larger the amount of liquid moving through the pipe.  This change in time can be mathematically related to the rate of flow.  The ultrasonic meter can be used with other devices to help measure flow by finding depth and velocity.