There are three types of ultrasonic flow meters. The principle of operation of each type is dependent upon the speed at which high frequency sound propagates. To learn more about the advantages of using ultrasonic flow meters click here.
Transit Time Ultrasonic Flow Meters
Transit time ultrasonic flowmeters measure the difference of the transit time of ultrasonic pulses propagating in and against the flow direction. The sound waves are generated by piezo-type sensing transducers. One wave travels the same direction as the flow and the other sound beam travels against the flow. The measured difference of the time taken to travel upstream versus downstream is a function of the process’ velocity and direction. This time difference is a measure for the average velocity of the fluid along the path of the ultrasonic beam.
By using the absolute transit times both the averaged fluid velocity and the speed of sound can be calculated. Using the two transit times 
And 
where 
is the average velocity of the fluid along the sound path and 
is the speed of sound.
The transducer pairs can be mounted outside the pipe or inserted through the pipe (such as non-uniform pipe material like concrete, or where pipe insulation is too thick, wall thickness cannot be measured due to corrosion of inside pipe walls, etc.). 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 density. The transit time ultrasonic flow meter requires that the process is either clean (few 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.
SmartMeasurement’s AlsonicDSP family of ultrasonic flow meters uses Digital Signal Processing (DSP) techniques along with a patented “fine time measurement technology” where sound waves are beamed at picoseconds time resolution enabling accurate, drift-free flow measurement in liquids that contain a second phase of entrained solids or gas bubbles. The flow transmitters of the AlsonicDSP family enable “Cross Correlation” of ideal signals stored in a database to erase noise, and make a three-dimensional cross section of the velocity distribution flow profile through the pipe. AlsonicDSP technology also allows the use of “FFT (Fast Fourier Transform)” to make the two signals the same frequencies, greatly increases its signal-to-noise ratio for accurate, drift-free flow measurement in liquids. The AlsonicDSP family of ultrasonic flowmeters can measure fluids of up to 30% of suspended solids or gases bubbles in liquids.
Doppler-Shift
The second type of ultrasonic flowmeter 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 ultrasonic flow 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.
The third type of ultrasonic flowmeter operates on the same principle as above. However, this meter is usually used exclusively on open channel flowmeter applications or partially filled pipes. A transducer is mounted on the floor of the channel and ultrasonic 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.