2017B Question 02

Using a labelled diagram, describe how a mechanical (non-cassette) variable-bypass vapouriser achieves the concentration set on the dial. Describe the mechanisms that compensate for temperature and downstream pressure changes.

Examiner Report

45 % of candidates achieved a pass in this question.

Most candidates drew an acceptable schematic diagram of a vaporiser and its major components. Temperature compensation provided by the bimetallic strip was well understood. Some candidates were able to calculate splitting ratios from first principles. Candidates could have obtained additional marks by discussing the extrinsic factors that impede the ability of the vaporiser to achieve the dialled concentration.

Many candidates struggled to explain vaporiser function succinctly. At times, it was unclear whether candidates were referring to ambient temperature or vaporiser temperature. Many candidates made erroneous statements contradicting the basic laws of thermodynamics. The function of the heat sink was frequently misunderstood.

Model Answer


  • Variable bypass vapouriser: Diagram, how it works, why it’s needed
  • Temp compensation: What and why
  • Pressure compensation: What and why

Variable Bypass Vapouriser

Factor Detail
Vapouriser stream

- Gas passes through volatile liquid; evaporation till SVP reached

- Ensure saturation: Wicks, baffles, formerly bubbling gas through liquid

- SVP sevoflurane ~22%, lethal if not diluted

Bypass stream - Does not pass through volatile liquid
Control of splitting ratio

- User-controlled dial alters resistance in vapouriser channel, corresponds to desired % or partial pressure

- e.g. Splitting ratio 12:1 → ~2% sevoflurane

- A bimetallic strip controls resistance to flow through the bypass channel

- Note device is less precise at very high >15L/min or very low ≤0.5L/min FGF rate

Need for controls - SVP of many drugs is deadly, e.g. Desflurane 660mmHg

Temperature Compensation

Property Detail

- Latent heat of vapourisation → ↓ Temp → ↓ SVP → ↓ Volatile partial pressure

- (Gay-Lussac’s law: ↓ Temp → ↓ SVP, non-linear relationship)

Bi-metallic strip

- Controls resistance to flow through bypass channel

- Two metal strips with different coefficients of thermal expansion

- ↓ Temp in device → ↑ Resistance to flow in bypass channel → Restored partial pressure volatile

Heat sink (copper)

- Provides latent heat of vapourisation

- Has high SHC: i.e. supply lots of heat → Minimal ↓ Temp

- Has high thermal conductivity → ↓ Minimal lag

Pressure Compensation

Property Detail

Pumping effect:

- ↑ Downstream pressure → Gas forced back into vapouriser ± fresh gas supply → Resaturation

- ↑↑ Volatile concentration → CVS depression, death


- Check valve between vapouriser and circuit (↓ back flow)

- Reasonable length of tubing between vapouriser and circle

- Inspiratory valve in circuit (↓ backpressure during expiration)

- Pressure-limiting valves in circuit (APL valve if spont vent, APL bypass if PPV)

Last updated 2021-08-23

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