An ultrasonic flowmeter is one of the types of velocity type flowmeters. It is also known as non-intrusive Doppler flow meter. It is a volumetric flow meter which needs particulates or bubbles in the flow. The working principle is based upon the Frequency shift, or Doppler Effect of an ultrasonic signal when it is reflected by suspended particles or gas bubbles i.e. discontinuities in motion.
The discontinuities reflect the ultrasonic wave with a somewhat different frequency which is directly proportional to the rate of flow of the fluid. At least 100 parts per million (PPM) of 100 micron or larger suspended particles or bubbles must be contained by the liquid as per current technology.