Two different types of ultrasonic flowmeter exist which employ distinct technologies,
one based on Doppler shift and the other on transit time. In the past, the existence of
these alternative technologies has not always been readily understood, and has resulted
in ultrasonic technology being rejected entirely when one of these two forms has been
found to be unsatisfactory in a particular application. This is unfortunate, because
the two technologies have distinct characteristics and areas of application, and many
situations exist where one form is very suitable and the other not suitable. To reject
both, having only tried out one, is therefore a serious mistake.
Particular care has to be taken to ensure a stable flow profile in ultrasonic flowmeter
applications. It is usual to increase the normal specification of the minimum length
of straight pipe-run prior to the point of measurement, expressed as a number of pipe
diameters, from a figure of 10 up to 20 or in some cases even 50 diameters. Analysis of
the reasons for poor performance in many instances of ultrasonic flowmeter application
has shown failure to meet this stable flow-profile requirement to be a significant factor.