Which statement is true about ultrasonic flow meters?

• A. Ultrasonic flow meters require particles in the fluid to reflect signals back to the flow sensor.
• B. They do not require particles in the fluid to reflect signals back to the flow sensor.
• C. They measure mass.
• D. They measure density.

Answer: (A)

Ultrasonic flow meters determine flow by using sound waves through the fluid. In the Doppler-type design, the ultrasound is reflected off particles or bubbles in the fluid. The motion of those scatterers causes a Doppler frequency shift in the reflected signal, and that shift is what the meter uses to compute the flow velocity. Because this measurement relies on signals bouncing off particles to create the detectable shift, reflections from particles are essential for this type of meter. There are other ultrasonic approaches, like transit-time meters, that don’t depend on particle reflections and instead compare travel times with and against the flow, but the statement matches the Doppler-based method. Ultrasonic meters don’t directly measure mass or density; they infer flow from velocity (and, with cross-sectional area, volume flow, while mass flow would require density).