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N2O Cylinder Contents Weighing — FRCA Primary MCQ

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ModeratePhysics & Clinical MeasurementN2O Cylinder Contents WeighingFRCA Primary

A full size E nitrous oxide cylinder at room temperature cannot have its contents estimated by pressure gauge because:

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

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Correct answer: A — N2O exists as liquid and gas in the cylinder so pressure remains constant until all liquid has evaporated

N2O is stored below its critical temperature (36.5°C at room temperature) so it exists as both liquid and gas. The gauge pressure remains constant at the SVP (~4400 kPa at 20°C) until all liquid has evaporated. The only reliable way to estimate remaining N2O is by weighing the cylinder. Once all liquid is gone pressure falls linearly with use.

Reference: Al-Shaikh Stacey 2019 Essentials of Anaesthetic Equipment