This instrument is based on the principle of

This is based on Henry's law which states that when a volatile chemical (ethanol) is dissolved in a liquid (blood) and is brought to equilibrium with air (alveolar breath), there is a fixed ratio between the concentration of the volatile compound (ethanol) in air (alveolar breath) and its concentration in the liquid (blood), and the ratio is constant at a given temperature. Correlation between the breath and blood level, and the ratio is generally 2100 : 1.