When alcohol comes in contact with the sensor, it generates a small amount of electricity. The electricity is measured and converted into the breathalyser reading.
For those with a technical interest, when the user exhales into the breathalyzer, any ethanol present in their breath is oxidized to acetic acid at the anode: CH3CH2OH(g) + H2O(l) →CH3CO2H(l) + 4H+(aq) + 4e- At the cathode, atmospheric oxygen is reduced: O2(g) + 4H+(aq) + 4e- →2H2O(l). The overall reaction is the oxidation of ethanol to acetic acid and water: CH3CH2OH(l) + O2(g) →CH3COOH(l) + H2O(l). The electrical current produced by this reaction is measured, processed, and displayed as an approximation of overall blood alcohol content by the breathalyzer.
UK Certified Breathalyzers assume that the subject being tested has a 2300-to-1 partition ratio in converting alcohol measured in the breath to estimates of alcohol in the blood. This measure is in direct proportion to the amount of grams of alcohol to every 1 ml of blood. However, this assumed partition ratio varies from 1300:1 to 3100:1 or wider among individuals and within a given individual over time. Assuming a true blood-alcohol concentration of .07%, for example, a person with a partition ratio of 1500:1 would have a breath test reading of .10% over the legal limit. Most individuals do, in fact, have a 2300-to-1 partition ratio in accordance with William Henry’s law, which states that when the water solution of a volatile compound is brought into equilibrium with air, there is a fixed ratio between the concentration of the compound in air and its concentration in water but it is important to appreciate that this ratio is constant at a given temperature; very few “personal” breathalyzers incorporate a temperature check in their software/hardware solutions. Breath leaves the mouth at a temperature of 34 degrees Celsius. To ensure that variables such as fever and hypothermia could not be pointed out to influence the results in a way that was harmful to the accused, most instruments are calibrated at a ratio of 2300:1, underestimating by 9 percent. In order for a person running a fever to significantly overestimate, he would have to have a fever that would likely see the subject in the hospital rather than driving in the first place. Thus, a machine using a 2300-to-1 ratio could actually overestimate the BAC. As much as 14% of the population has a partition ratio above 2300, thus causing the machine to under-report the BAC.