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Rotameter flow physics — FRCA Primary MCQ

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ModeratePhysics and Clinical MeasurementRotameter flow physicsFRCA Primary

An oxygen flowmeter bobbin is read at the top while the rotameter is vertical. The bobbin rises as flow increases. Which physical effect predominates at low flows around the bobbin?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: A — Gas density determines flow because flow is turbulent

At low flows in a rotameter, flow is predominantly laminar and viscosity is the main gas property determining flow. At high flows, turbulence increases and density becomes more important. The pressure drop across the bobbin remains approximately constant while the annular orifice area changes. The pearl is that rotameters are gas-specific and must remain upright.

Reference: RCoA Primary FRCA Syllabus 2026; Davis and Kenny Basic Physics and Measurement in Anaesthesia