Breathalyzers are the most common judges of a suspected impaired driver’s blood alcohol content (BAC). How they get results is a science unto itself.
According to BACtrack, a breathalyzer manufacturer based in San Francisco, breathalyzers use one of three systems: semiconductor oxides, fuel cells, and infrared spectrometry. Fuel cells are widely fielded by law enforcers on the road due to their high accuracy and portability (even though the results can be somewhat less reliable). Infrared spectrometry, on the other hand, is normally used at police stations or labs.
A semiconductor oxide sensor generates readings by detecting the amount of ethanol present in a person's breath. Metal oxides like silica, copper oxide, and tungsten trioxide are highly sensitive but affordable enough for personal use. You can buy one and carry it with you on your night out to keep you informed of your BAC.
Fuel cells are more accurate than metal oxides as they can't be fooled by non-alcoholic sources, unlike semiconductors. A fuel-cell breathalyzer normally comes with a pair of electrodes that oxidize alcohol and produce an electric current. BAC readings, in this case, depend on the output the electrodes produce.
Due to their size and weight, infrared spectrometry breathalyzers usually stay at the station or labs. This is the most accurate way of determining BAC from a person’s breath.. By observing how molecules absorb infrared light, they can identify which of the molecules belong to ethanol.