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Nitrous oxide cylinder contents — FRCA Primary MCQ

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ModeratePhysics and Clinical MeasurementNitrous oxide cylinder contentsFRCA Primary

A nitrous oxide cylinder attached to an anaesthetic machine shows a pressure near 44 bar for much of use, then falls rapidly. What is the most likely explanation?

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

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Correct answer: C — Liquid nitrous oxide remains in the cylinder

Nitrous oxide is stored as liquid and vapour at room temperature, so cylinder pressure reflects vapour pressure while liquid remains. Once the liquid is exhausted, pressure falls rapidly. Oxygen does not behave this way because it is stored as compressed gas. Cylinder weighing is a better estimate of nitrous oxide content than pressure.

Reference: Dorsch and Dorsch Understanding Anesthesia Equipment; Primary FRCA physics texts