There are three types of Ultrasonic Flowmeters. The principle of operation of each type is dependent upon the speed at which high frequency sound propagates.

The first meter type measures the travel time of two sound waves. One wave travels the same direction as the flow and the other beam travels against the flow. The measured difference of the time taken to travel upstream verses downstream is a function of the process' velocity and direction. The transducer pairs can be mounted outside the pipe or inserted through the pipe (especially helpful in some difficult to measure applications). With the advent of digital signal processing, it has become possible to apply digital signal coding to the transmitted signal. This can eliminate many of the problems associated with noise and variations in liquid chemistry. This method of ultrasonic flow measurement requires, however, that the process is either clean (no suspended solids or gas bubbles in liquids) or has no entrained liquids if a gas, and is in a closed and full piping system. Multiple path gas ultrasonic meters are able to achieve high degrees of uncertainty allowing them to be used in custody transfer applications.

Transit Time Ultasonic flowmeter is an example of traveling time type ultrasonic flowmeter.

The second type of ultrasonic meter is based on the Doppler-shift or the phenomenon in which the wavelength of an approaching sound source is shorter than the wavelength of that same source as it is moving away. The transducer emits a sonic beam into the process and entrained particles or bubbles reflect the beam back to the transducer. The measured difference in the wavelengths (transmitted verses reflected) is proportional to the process' velocity. These meters are used on closed full pipes and the transducer may be mounted either on the outside of the pipe or inserted (hot tapped) into the pipe. Further explanation of Doppler Ultrasonic flowmeter can be found in a separate page.

The third type of Ultrasonic Flowmeter operates on the same principle as the second, however, this meter is usually used exclusively on open channel flows or partially filled pipes. A transducer is mounted on the floor of the channel and ultra sound waves are beamed up-stream against the flow. The wave length of the reflected wave is compared to the emitted wave and the Doppler shift noted and translated into a velocity. Mounted integrally in the transducer is a pressure transducer to continually monitor the depth of the channel. The cross-sectional area of the channel or flowing conduit is programmed into the electronics and from all this information an accurate flow measurement can be made.