In flow measurement, how does a venturi meter determine flow rate?

• A. By estimating flow from water level height
• B. By counting the number of valve openings per minute
• C. By measuring pressure drop caused by a constriction
• D. By using a magnetic flow sensor

Answer: (C)

The main idea is that a venturi meter uses a constriction to create a measurable pressure drop that relates to how much fluid is flowing. As the fluid passes the narrow throat, its velocity increases and the static pressure drops. This pressure difference between the upstream section and the throat can be related to the flow rate through the pipe by the continuity of flow and Bernoulli’s principle, with a discharge coefficient to account for real-world losses. By measuring that pressure drop with sensors, you can compute the flow rate—larger ΔP means more flow, following a relationship that roughly scales with the square root of the pressure drop once geometry and fluid density are included. Other options don’t use the constriction-driven pressure change: water level height isn’t how a venturi gauges flow, counting valve openings isn’t measuring flow, and a magnetic flow sensor uses a different principle entirely.