Clamp-on ultrasonic flow meters are flexible instruments that provide a number of benefits. When used properly, they can save a significant amount of time and money, and they can often replace several devices. Ultrasound is used to calculate the velocity of a fluid in an ultrasonic flow meter, and is used in a number of fluid applications. Water and other liquids fit ultrasonic flowmeters well. Clamp-on ultrasonic flow meters have high precision at low and high flows, save time by eliminating the need for pipe cutting or process shutdown, and are unaffected by external noise.

How do ultrasonic flow meters work?

The clamp on ultrasonic flow meter uses sound waves that are higher in frequency than the human hearing range (typically 0.5, 1, or 4 MHz). Wetted (insertion) transducers, which make direct contact with the liquid, or external (clamp-on) transducers, which deliver the ultrasound through the pipe wall, are used to transmit the ultrasound signal through a stream of flowing liquid. Clamp-on ultrasonic flow meters reduce installation and maintenance costs by allowing users to calculate the volumetric flow rate of a fluid in a pipe without needing to penetrate the pipe.

Two ultrasonic transducers that serve as both transmitter and receiver are used in a standard transit-time ultrasonic liquid flow meter. The ultrasonic flow meter measures the time it takes for sound to pass between the two transducers in both directions by alternately transmitting and receiving a burst of ultrasound between the two transducers. The velocity of the liquid in the pipe is directly proportional to the difference in transit length (time) measured. The ultrasound in this application is transmitted from the first transducer and travels through the pipe wall, through the liquid, then bounces off the pipe’s back wall, then travels through the pipe wall, and is picked up by the second transducer.

The procedure is then repeated in reverse, with the ultrasound being transmitted by the second transducer. The Time is the difference in time between the times of flight up and down. The Time would be zero if the liquid in the pipe was not flowing.

You must translate the raw Time into the velocity of the liquid in the pipe in order to determine the liquid velocity. Knowing the speed of sound of the pipe and the liquid allows the angle of the ultrasound direction to be measured. To convert the ultrasound path into a straight line in the pipe, this angle is used in a popular trigonometry equation. The velocity of the liquid in the pipe will be this.

As with any velocity-based flow meter, once the velocity is determined, it’s simply a matter of multiplying it by the cross-sectional area of the pipe to get a flow rate.