There are three major types of breath alcohol testing devices, and they're based on different principles:
- Breathalyzer - Uses a chemical reaction involving alcohol that produces a color change
- Intoxilyzer - Detects alcohol by infrared (IR) spectroscopy
- Alcosensor III or IV - Detects a chemical reaction of alcohol in a fuel cell
Regardless of the type, each device has a mouthpiece, a tube through which the suspect blows air, and a sample chamber where the air goes. The rest of the device varies with the type.
The Breathalyzer device contains:
- A system to sample the breath of the suspect
- Two glass vials containing the chemical reaction mixture
- A system of photocells connected to a meter to measure the color change associated with the chemical reaction
To measure alcohol, a suspect breathes into the device. The breath sample is bubbled in one vial through a mixture of sulfuric acid, potassium dichromate, silver nitrate and water. The principle of the measurement is based on the following chemical reaction:
In this reaction:
- The sulfuric acid removes the alcohol from the air into a liquid solution.
- The alcohol reacts with potassium dichromate to produce: chromium sulfate potassium sulfate acetic acid water
The silver nitrate is a catalyst, a substance that makes a reaction go faster without participating in it. The sulfuric acid, in addition to removing the alcohol from the air, also might provide the acidic condition needed for this reaction.
During this reaction, the reddish-orange dichromate ion changes color to the green chromium ion when it reacts with the alcohol; the degree of the color change is directly related to the level of alcohol in the expelled air. To determine the amount of alcohol in that air, the reacted mixture is compared to a vial of unreacted mixture in the photocell system, which produces an electric current that causes the needle in the meter to move from its resting place. The operator then rotates a knob to bring the needle back to the resting place and reads the level of alcohol from the knob -- the more the operator must turn the knob to return it to rest, the greater the level of alcohol.