A ___ measures the rate of water flowing through the tube as the velocity increases and the pressure decreases through the tube.

• A. Orifice plate
• B. Pitot tube
• C. Venturi tube
• D. Rotameter

Answer: (C)

The concept here is using a constriction in a pipe to convert velocity changes into a measurable pressure difference, then relate that difference to flow rate. When fluid passes through a Venturi tube, the throat narrows the cross-sectional area, so the fluid speeds up to maintain the same overall flow. According to Bernoulli’s principle, this speed increase comes with a drop in static pressure. By measuring the pressure difference between the upstream section and the throat, you can calculate the flow rate, because the velocity at the throat is tied to the flow rate through continuity, and the pressure difference provides the needed information to solve for that velocity.

Venturi meters are designed to do this with minimal energy loss, using a smooth contraction to keep flow stable and the relationship between ΔP and Q reliable. The other devices operate differently: an orifice plate also uses a constriction to create a pressure drop for measuring flow, but it introduces more energy loss and is typically less precise. A Pitot tube measures velocity rather than directly giving a flow rate in a piping system. A rotameter provides a direct reading of flow rate based on a float’s position in a tapered tube, not on velocity-induced pressure changes.