skip to main content

Poynting Effect Entonox — FRCA Primary MCQ

Instant feedback + full explanation. One question, done properly.

HardPhysics & Clinical MeasurementPoynting Effect EntonoxFRCA Primary

The Poynting effect (overpressure effect) explains why Entonox remains gaseous at room temperature despite N2O having a critical temperature of 36.5°C. The mechanism is:

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

Reveal the answer and explanation

Correct answer: B — The presence of oxygen under pressure lowers the critical temperature of N2O allowing both gases to coexist in the gaseous phase at cylinder pressure

In an Entonox cylinder the 50:50 O2/N2O mixture is stored at ~137 bar. The Poynting effect: the presence of a gas (O2) at high pressure dissolved in a liquid (N2O) changes the liquid's properties lowering its critical temperature to approximately −7°C (the pseudocritical temperature). Above −7°C both gases remain in the gaseous phase at cylinder pressure.

Reference: Al-Shaikh Stacey 2019 Essentials of Anaesthetic Equipment