Piezoelectric crystal resonance

Principle

Piezoelectric effect - when an electric potential is applied across a crystal of quartz, it contracts slightly
--> crystal can be made to oscillate at its resonant frequency by a suitable alternating voltage

This resonant frequency is altered when anaesthetic agents dissolve in the silicone oil coating the crystal

Change in frequency is proportional to concentration of volatile agent present.

Setup

2 quartz crystals: one is coated in silicone oil and the other is uncoated.

Disadvantages

• Does not detect the type of volatile agents
• Requires removal of water vapour
• Cannot measure CO2, O2, N2O