What is the principle of breath analysers to detect drunken-driving?

Breath analyser is a device used by police to test drivers suspected of being drunk. This device use the reduction-oxidation reaction. A sample of the driver's breath is drawn into the breath analyser, where it is treated with an acidic solution of potassium dichromate. The alcohol (ethanol) in the breath is converted to acetic acid. In this reaction the ethanol is oxidised to acetic acid and the chromium in the orange-yellow dichromate ion is reduced to green chromic ion. The alcohol level in the blood is determined readily by measuring the degree of the color change from orange-yellow to green, read from a calibrated meter in the breath analyser.