After four years, the earliest flow meters have appeared in industrial applications. At present, there are many manufacturing companies were designing different types of clamp-on flow meters to measure the liquid flow within a pipe. These meters use high-frequency sensors by penetrating throughout the pipe wall as well as the liquid by using Doppler otherwise transit time propagation method. So that fluid velocity and flow rate can be determined.http://www.jyflowmeter.com/ultrasonic-flow-meter/