S1 Ultrasonic Water Meters, as the name suggests, are meters that use sound waves to measure flow. These meters are attached around the inside or outside of a pipe, and when activated, transmit ultrasonic waves from one side of the meter to the other. These transmitted waves are then collected and measured to determine the flow. http://www.shmeter.com/ultrasonic-water-meter/s1-ultrasonic-water-meter/