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Vaporiser Cooling Effect — FRCA Primary MCQ

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HardPhysics & Clinical MeasurementVaporiser Cooling EffectFRCA Primary

An unheated variable-bypass vaporiser without temperature compensation is operated continuously at a high fresh-gas flow and a high dial setting. The liquid anaesthetic temperature progressively falls and measured vapour output decreases, although ambient pressure, carrier-gas composition and inlet pressure remain unchanged. Which mechanism best explains the reduction in output?

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

Reveal the answer and explanation

Correct answer: C — Vaporisation removes latent heat, and the lower liquid temperature reduces its saturated vapour pressure.

Explanation lettering: D = shown as A · C = shown as B · B = shown as C · A = shown as D

B is correct. Vaporisation is endothermic: molecules escaping from the liquid require latent heat, which is drawn from the anaesthetic and vaporiser body. Sustained rapid vaporisation therefore cools the liquid. Saturated vapour pressure falls with temperature, so gas leaving the vaporising chamber contains less anaesthetic and the delivered concentration falls unless compensation alters the bypass-to-chamber flow ratio. Heat conduction from the vaporiser body opposes rather than causes this fall. The pumping effect results from intermittent back-pressure in older vaporisers and can increase output; it is excluded by the stable inlet pressure. Altitude and carrier-gas effects cannot explain the observation because ambient pressure and carrier-gas composition are unchanged.

Reference: Chakravarti S, Basu S. Modern anaesthesia vapourisers. Indian Journal of Anaesthesia. 2013;57(5):464–471. https://pubmed.ncbi.nlm.nih.gov/24249879/