Two types of ultrasonic flowmeters are in general use for closed pipe flow measurement.
In terms of the interaction between ultrasound and the moving fluid, the
first type may be termed transmission, and the second, scattering. Other categorizations
may be found in the literature [Refs. 5, 6, 15, 28, 44, 50, 52, 53, 60, 61, 80-
86, 99-102]. See also www.uspto.gov to search for U.S. patents by key words, inventors
or other characteristics.

Principles of Operation

Transmission includes ultrasonic waves of short duration (pulses) as well as cw
(continuous waves); sometimes both are found in one instrument, analogous to
coarse and fine resolution in a microscope [Ref. 35]. Transmission flowmeters
(e.g., Figure 20-1), also called transit time, ?t, contrapropagation, or counterpropagation,
like other flowmeters, are most accurate when they properly take into
account the variation of flow over the cross section. This usually dictates measurements
of flow along multipaths where the paths are not just two or more tilted
diameters but typically involve off-diameter paths such as midradius or quadrature
paths. On the other hand, adding too many off-diameter paths can disturb the flow.
In small conduits, area-averaging is achieved by beams that interrogate 100% of
the cross section, or interrogate along preferred paths. The goal might be
expressed as achieving the required performance (which is often but not necessarily
accuracy) at minimum or reasonable cost without introducing undesired side
effects. Because more paths can improve accuracy, but add cost, it is necessary to
achieve an engineering compromise with respect to the number of paths. Cost
includes purchase price + installation cost + cost of ownership.

Scattering also uses pulses and cw. The earliest ultrasonic scatter-based flowmeters
were called Doppler flowmeters because they measured flow in terms of
the shift in the frequency of cw ultrasound scattered off inhomogeneities in the
fluid (e.g., red corpuscles in blood; undissolved solids in slurries). Later Doppler
flowmeters used pulses to enable them to range gate and thereby take into account
the flow at different distances from the wall [Refs. 12, 69, 134, 135]. Some use
two frequencies [Refs. 77(b), 104].

Other scatter-based interactions are commercially available. One of these is
analogous to Doppler and can be thought of as Doppler's time-domain equivalent.
It is sometimes called speckle-tracking or stroboscopic scattering. Irrespective of
its name, it is based on timing echoes from successive interrogations of an ensemble
of scatterers as they move toward or away from the source of ultrasound [Refs.
11, 31].

A second alternative is tag, where the naturally occurring eddies or inhomogeneities
scatter ultrasound beamed across two axially-displaced paths [Refs. 9, 20,
34, 43, 54, 56, 98, 126, 136].