A sonic meter that measures water velocity works by sending out sound pulses which way?

• A. Alternately toward the flow, then away in a diagonal direction across the page
• B. In a straight line downstream of flow
• C. Only toward the flow
• D. Random directions

Answer: (A)

Measuring water velocity with a sonic meter relies on comparing how fast sound travels with the flow versus against it. By sending pulses toward the flow and then in the opposite direction (often along a diagonal across the cross-section), the instrument captures two travel times. Sound moving downstream is effectively aided by the water, so it arrives sooner; moving upstream is slowed by the flow, so it arrives later. The difference between these travel times directly relates to the velocity along that path. Using directions in two orientations helps resolve the velocity more accurately across the cross-section and reduces errors from nonuniform flow. That’s why alternating toward the flow and away in another direction is the correct approach.