One potential problem with the transit-time flowmeter is being able to measure the true average
fluid velocity when the flow profile changes with Reynolds number. If just one ultrasonic “beam” is
used to probe the fluid velocity, the path this beam takes will likely see a different velocity profile
as the flow rate changes (and the Reynolds number changes along with it). Recall the difference
in fluid velocity profiles between low Reynolds number flows (left) and high Reynolds number flows
(right):