Which statement about velocity-type flow meters is true?

• A. They require no straight pipe length
• B. They rely on measuring static pressure
• C. They can operate with any flow regime without restrictions
• D. They assume laminar flow and specify minimum straight-pipe distance on either side

Answer: (D)

Velocity-type flow meters rely on a predictable velocity field to interpret the measurement accurately. To achieve that, the flow needs to be fully developed and free from disturbances caused by fittings, bends, or nearby valves. That’s why these meters specify a minimum straight-pipe length upstream and downstream: it lets the flow stabilize into a steady, laminar-like profile (or at least a stable, well-defined velocity distribution) so the meter can correctly relate the measured signal to the actual flow rate. If the flow is turbulent or disturbed, the velocity distribution becomes erratic, leading to biased readings and reduced accuracy.

Why the other statements don’t fit: these meters do not rely on static pressure measurements; that’s a hallmark of differential-pressure devices rather than velocity-type meters. They also don’t work without regard to the flow regime—disturbances or non-fully-developed flow can compromise accuracy, and many meters specify limits or conditions tied to the flow regime. Finally, insisting they can operate with any flow regime ignores how disturbances and turbulence affect velocity measurements, which is why a straight-pipe requirement and flow-development assumption are essential.