Rotameter Working Principle — FRCA Primary MCQ
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Correct answer: A — The pressure drop across the annular orifice equals the weight of the bobbin
In a rotameter gas flows upward through a tapered tube lifting the bobbin. It rises until the annular gap produces a pressure drop equal to the bobbin weight divided by its cross-sectional area. At low flows: laminar / viscosity-dependent. At high flows: turbulent / density-dependent.
Reference: Al-Shaikh Stacey 2019 Essentials of Anaesthetic Equipment