Following on the theme of requiring bubbles or particles of sufficient size, another limitation of
Doppler ultrasonic flowmeters is their inability to measure flow rates of liquids that are too clean
and too homogeneous. In such applications, the sound-wave reflections will be too weak to reliably
measure. Such is also the case when the solid particles have a speed of sound too close to the
that of the liquid, since reflection happens only when a sound wave encounters a material with a
markedly different speed of sound. In flow measurement applications where we cannot obtain strong
sound-wave reflections, Doppler-type ultrasonic flowmeters are useless