When police stop a suspected drunk driver, standard protocol is to have the driver complete a set of field sobriety tests and take a breath test to determine their blood alcohol content. Each time a person with alcohol in their system breathes out, they expel a small amount of vaporized alcohol. Breathalyzer machines are calculated to detect the infrared radiation, or IR, of ethanol, the chemical in alcoholic beverages that causes intoxication. Different organic compounds have different IR wavelengths.
Most Breathalyzer machines require a person to blow into the machine for several seconds. This is so the air that enters the machine comes from the deepest part of the lungs. Otherwise, the machine could pick up mouth alcohol that is on a person's breath right after they have taken a drink. Most machines will not register unless a person has blown a sufficient sample of air into the machine.
After a sufficient sample is inside the chamber in the machine, an IR beam is blown through it. An IR detector measures the amount of ethanol in the sample and calculates the person's BAC based on the amount.
A person who has been arrested for drunk driving charges can refuse to take a breath test, but in most states, that will result in suspension of a person's driving privileges. It is legal to refuse to submit to field sobriety tests on the side of the road. It may be a good idea for individuals who have physical conditions that could cause them to fail the tests to refuse.
An attorney may be able to help a person facing drunk driving charges stay out of jail or even work to get their case dismissed. The outcome of a DWI case is highly dependent on the facts involved as DWI cases often turn on technical requirements, especially in cases where a suspected drunk driver had few obvious signs of impairment.