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Compressed oxygen cylinder contents — FRCA Primary MCQ

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ModeratePhysics and Clinical MeasurementCompressed oxygen cylinder contentsFRCA Primary

During transport, an oxygen cylinder pressure gauge falls from 137 bar to 68 bar while flow is constant. The cylinder contains compressed oxygen rather than nitrous oxide. Which statement best describes the remaining contents?

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

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Correct answer: E — Approximately half of the original oxygen remains

For compressed gases above their critical temperature, cylinder pressure is approximately proportional to the amount of gas remaining, so half the pressure indicates about half the contents. Nitrous oxide behaves differently because it is stored partly as liquid and pressure stays relatively constant until liquid is exhausted. Colour coding identifies the gas but not its remaining amount. This distinction is important during transfers.

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