How does a breathalyzer work?Adey Hill:

The breathalyzer or intoxilyzer is a modified IR spectrometer.

Before I get into that, let's talk about alcohol and how it gets into the blood and breath: Alcohol is a general name for a group of organic compounds. Ethanol, the alcohol we consume in alcoholic beverages, is a relatively small molecule. As such it is easily absorbed through the stomach into the blood. Alcohol is volatile and your body is warm. Each time you breathe, a small amount of the alcohol in your blood vaporizes and is passed into the alveoli in your lungs and passed out of your body. The more alcohol you have consumed, the greater the amount that passes out.

Organic compounds absorb infrared radiation (IR) at different wavelengths and have an IR signature. The infrared (IR) spectrometer in the intoxilyzers is calibrated so that it is at the wavelength that ethanol absorbs it.

When you blow into an intoxilyzer, the breath you expire passes into a sample chamber and if you have been drinking then so will some of the alcohol that has passed from your blood to your breath. In the case of the Lion Intoxilyzer 6000s produced by Lion Labs, the machine starts sampling the breath immediately as it starts to enter the chamber and does so 37 times per second. The machine continues to monitor this until you blow a consecutive reading for three seconds. This is so that it is taking the reading from the deep lung air, where the greatest concentration of alcohol is. Once this is achieved the machine will register that a satisfactory sample has been taken. It will then purge itself and move to the next stage (either a second sample or a calibration check).

The breath in the sample chamber is surveyed by a beam of infrared radiation (there is an IR detector behind the chamber). The machine knows the amount of IR that was fired at the chamber and the detector calculates how much has been absorbed. Some clever math works out how many micrograms of alcohol are present in 100 ml of breath. This reading is then displayed.

Some other factoids about the machines: breathalyzers can detect other substances such as methanol, isopropyl alcohol, or acetone. If these are present in sufficient quantities, the machine will register them as an interfering substance and your sample will not be deemed satisfactory.

The breath tube (which you blow into via a single use mouthpiece) is preheated to a specific temperature to ensure that the samples conform to Boyle's law and are consistent and accurate every time.

The machines can detect mouth alcohol. If you have recently consumed alcohol or used an alcohol-based mouthwash, the machine can detect it. The intoxilyzer starts sampling as you start to blow, so it will detect a high concentration of alcohol present at the start of the process, followed by a downward slope (on a graph); this spike tells the machine that there was more alcohol present at the beginning of the sampling process than at the end, and that this must be due to the presence of mouth alcohol.

You cannot cheat or defeat the intoxilyzers by trying to blow down the sides of the mouthpiece, putting your tongue over it, or putting some kind of catalyst (like a copper coin) in your mouth first. The only way to beat it is to not drink and drive!

This post originally appeared on Quora. Click here to view.