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.

The transit time ultrasonic flowmeters measure variations of traveling time of ultrasonic waves while the Doppler ultrasonic flowmeters measure frequency shifts in a pre-configured acoustic field. See Transit Time Ultrasonic Flowmeter and Doppler Ultrasonic Flowmeter for more details.

Common specifications for commercially available ultrasonic flowmeterss are listed below:

	Fluid Phase:	Score Phase Condition  Gas   Clean     Dirty   Liquid   Clean     Corrosive      Dirty     Open Channel   Liquid   Viscous  : Recommended : Limited applicability
	Line Size:	Inline model: 10 ~ 1200 mm (0.4 ~ 48 inch) Clamped-on model: 75 mm (3 in) and up
	Turndown Ratio:	100: 1

•	Pros:
	-	No obstruction in the flow path, no pressure drop
	-	No moving parts, low maintenance cost
	-	Multi-path models have higher accuracy for wider ranges of Reynolds number
	-	Can be used to measure corrosive or slurry fluid flow
•	Cons:
	-	Higher initial set up cost