The ultrasonic technique of volume flow rate measurement is, like the magnetic
flowmeter, a non-invasive method. It is not restricted to conductive fluids, however, andis particularly useful for measuring the flow of corrosive fluids and slurries. Besides its
high reliability and low maintenance requirements, a further advantage of an ultrasonic
flowmeter over a magnetic flowmeter is that the instrument can be clamped externally
onto existing pipework rather than being inserted as an integral part of the flow line
As
the procedure of breaking into a pipeline to insert a flowmeter can be as expensive as the
cost of the flowmeter itself, the ultrasonic flowmeter has enormous cost advantages. Its
clamp-on mode of operation has significant safety advantages in avoiding the possibility
of personnel installing flowmeters coming into contact with hazardous fluids such as
poisonous, radioactive, flammable or explosive ones. Also, any contamination of the
fluid being measured (e.g. food substances and drugs) is avoided. Ultrasonic meters
are still less common than differential pressure or electromagnetic flowmeters, though
usage continues to expand year by year.
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.