How a Breathalyzer Works

Copyright 2006 Lance Knowlton
Technically, Breathalyzer is a proprietary name for piece of testing equipment that determines blood alcohol concentration (BAC). There are two other types of devices that measure BAC, Intoxilyzer and Alcosensor, and they work differently than the Breathalyzer. This article will explain how each one works.
All three devices require a person to exhale into them. That is because they all work on the same principle of testing, which is that alcohol shows up in the breath because it is absorbed into the bloodstream, not digested, where its chemical composition remains unchanged. As the blood flows through the lungs, the alcohol in it, which is volatile, evaporates into the lungs.
There is a mathematical relationship between the amount of alcohol in the lungs and the amount of alcohol in the blood, and it can be manipulated to determine how much alcohol is in the blood without actually drawing any. All BAC testing equipment measures alcohol in the breath and uses this formula to calculate how much alcohol is in the blood. Accordingly, all the devices have a mouthpiece through which the test subject blows air and a sample chamber to hold the air. Beyond that, each device is different.
The Breathalyzer detects alcohol by monitoring a chemical reaction that produces a color change. Besides the mouthpiece and sample chamber, it consists of two glass vials to contain the chemical reaction and a system of photocells connected to a meter. The air is bubbled through one vial containing a chemical mixture and into another vial. From that vial, the air is passed over the photocells so the meter can measure the color change and calculate the BAC.
Alcosensor works by detecting the occurrence of a chemical reaction of alcohol in a fuel cell. The fuel cell has two platinum electrodes with a porous acid-electrolyte material layered between them. As the exhaled air flows past the electrodes, the platinum oxidizes any alcohol in the air to produce acetic acid, protons and electrons. The electrons flow through a wire connecting the first electrode and an electrical-current meter and the second electrode, which converts the other particles into water. The more alcohol oxidized, the greater the electrical current. A microprocessor measures the electrical current and calculates the BAC.
The Intoxilyzer detects alcohol by infrared spectroscopy, which identifies molecules based on the way they absorb light. Each molecule absorbs light differently so alcohol molecules can be identified, counted and fed into the formula by computer. In the Intoxilyzer, a lamp generates a multiple wavelength infrared beam. The beam passes through the sample chamber and is focused by a lens onto a spinning filter wheel. The wheel contains band filters for the specific wavelength of alcohol. The light passing through the filter is detected by a photocell, where it is converted into an electrical pulse. Each pulse is relayed to a small computer which uses them to calculate the BAC.
Each device is a sophisticated piece of testing equipment that must be calibrated before each use. Police officers are often trained on how to use and calibrate them, and DUI defense attorneys often get the results of the tests thrown out based on improper use or calibration.