Ultrasonic flowmeters measure the traveling times (transit time models) or the frequency shifts(Doppler models) of ultrasonic waves in a pre-configured acoustic field that the flow is passing through to determine the flow velocity.
Ultrasonic flowmeters can be categorized into two types based on the installation method: clamped-on and inline.
The clamped-on type is located outside of the pipe and there are no wetted parts. It can easily be installed on existing piping systems without worrying about corrosion problems. Clamped-on designs also increase the portablility of the flowmeter.
The inline type, on the other hand, requires fitting flanges or wafers for installation. However, it usually offers better accuracy and its calibration procedures are more straightforward.