Ultrasonic flowmeters have been used in industry for over 20 years for the volumetric flowmetering of liquids, gases and – more recently – steam. Basically, the distinction is made between two methods: The Doppler process In order to work, this method needs reflectors in the process product. Due to malfunctioning, incorrect application and, as a result, a feeling of uncertainty, it gave ultrasonic flow measurement a bad name in the 1970s. It is now used only in a few special applications. The transit-time differential method This method has been working successfully in industrial applications for more than 20 years. It is accurate and reliable. Besides measuring the volumetric flow rate, the method provides information on the type of liquid product based on the sound velocity that is measured in parallel.
KROHNE (2000) Fundamentals of Ultrasonic Flow Measurement