The cells have flow channels of square, rectangular or circular cross sections, ranging from 1.27 to 30 cm across by 10 to 200 cm long. The interrogating ultrasonic waves, preferably transmitted upstream and downstream over a common path as simultaneously as practical, are introduced parallel, oblique or perpendicular to the flow axis. Transducers used to generate and detect these waves are sometimes mounted external to the pipe or cell. In other cases, recesses or penetrations are machined for geometric control of the sound beam. Here, internally-flush-mounted shims or screens are sometimes used, to minimize undesirable turbulence that would otherwise be generated at the transducer interruptions of the flow channel.