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Rotameter Principle — FRCA Primary MCQ

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HardPhysics & Clinical MeasurementRotameter PrincipleFRCA Primary

In a vertically mounted variable-orifice flowmeter, gas flows upwards through a tapered tube and the float has reached a stationary indication. Which statement most accurately describes the mechanism determining the float's equilibrium height?

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

Reveal the answer and explanation

Correct answer: B — The upward hydrodynamic force and buoyancy balance the weight of the float

Explanation lettering: B = shown as A · D = shown as B · A = shown as D

D is correct. A stationary float has zero acceleration, so its downward weight is balanced by the upward hydrodynamic force produced by gas flow around the float, together with the smaller buoyancy force. As flow increases, the float initially rises; the tapered tube then provides a larger annular flow area until a new force equilibrium is reached. In simplified treatments, the hydrodynamic force may be represented as pressure difference multiplied by an effective area. Pressure drop itself cannot equal weight because pressure and force have different dimensions. Charles's and Boyle's laws do not determine the equilibrium height. Bernoulli effects contribute to the pressure distribution but do not replace Newton's force balance. The device is therefore not explained solely by the Venturi effect.

Reference: Royal College of Anaesthetists. Example Questions and Explanations: Primary FRCA Single Best Answer Questions, Physics and Clinical Measurement, Question 13. Accessed 19 August 2026. https://rcoa.ac.uk/media/50901