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N2O Pressure Constant Then Falls — FRCA Primary MCQ

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ModeratePhysics & Clinical MeasurementN2O Pressure Constant Then FallsFRCA Primary

The pressure gauge on a nitrous oxide cylinder reads 4400 kPa at room temperature. As gas is used the pressure will:

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Correct answer: B — Remain constant at 4400 kPa until all liquid has evaporated then fall rapidly

N2O exists as liquid + gas in the cylinder at room temperature (below critical temperature 36.5°C). The pressure remains constant at the SVP (~4400 kPa at 20°C) as long as liquid is present — gas evaporates to replace withdrawn gas maintaining equilibrium. Once all liquid has evaporated (~75% of contents used) the pressure falls linearly to zero. The cylinder must be weighed to determine remaining contents.

Reference: Al-Shaikh Stacey 2019 Essentials of Anaesthetic Equipment