How does a breath alcohol machine work?

When a person consumes alcohol, it enters his or her bloodstream through the digestive system. When this alcohol-carrying blood reaches the lungs, a portion of the alcohol evaporates in a process known as gaseous exchange. This evaporated alcohol is then exhaled during the normal breathing process and can be detected by a breath alcohol machine. Based on a solubility principle known as the Henry's law constant, the percentage of alcohol that is present in the breath corresponds to the concentration of alcohol in the blood.

Alcohol breath machines are designed to detect the level of alcohol present in a test subject's exhaled breath, and then calculate a blood alcohol concentration based on that value. The resulting number is often expressed as a percentage called the “blood alcohol content" (BAC).

Alcohol breath testing may be performed at various alcohol testing sites using a preliminary screening device, a self-administered device, or a more sophisticated evidential breath alcohol machine. Preliminary screening devices are usually portable and report immediate results to the test administrator. These machines are used for law enforcement and workplace alcohol screenings. Self-administrated breath alcohol machines are often used by law enforcement agencies to monitor alcohol consumption remotely. Evidentiary breath alcohol machines, as their name implies, are used to provide breath alcohol test results that can be used as legal proof of the test subject's blood alcohol content. Because the human body is not a controlled environment, the breath-to-blood ratio for calculating BAC based on breath analysis relies on an average calculated from a range of actual results. Additionally, external and physiological factors may affect an individual’s breath-to-blood ratio. A breath alcohol test result is often used as proof of blood alcohol concentration but should not be considered an exact equivalent of a blood test.

Types of alcohol testing methods

Blood alcohol machines measure the amount of ethanol alcohol in a test subject's breath against guidelines for prohibited alcohol concentrations using three primary methods. Most devices employ only one of these methods, although evidentiary blood alcohol machines may use more than one method.

One of the most common technologies used for preliminary alcohol breath machines is fuel cell technology. In these devices, the combination of alcohol with a chemical catalyst in the device creates an electrical charge that is then measured to determine the level of alcohol in the specimen.

Semiconductor sensors use an electrical current that changes in intensity when exposed to alcohol or other substances. Semiconductor sensors are considered less accurate than fuel cell devices because other volatile gasses may alter the baseline current, resulting in a false positive reading.

Machines that rely on infrared (IR) spectroscopy, such as the Intoxilyzer, detect alcohol in the breath by measuring the alcohol molecules’ absorption of light. An ethanol molecule will produce a distinctive wavelength pattern based its reaction to infrared light and the amount of IR light absorbed is used to calculate the amount of ethanol present in the sample.

A fourth method of detection is used in chemical oxidation/photometry devices such as the original Breathalyzer machine. These alcohol detectors use chemical reactions between alcohol and a solution in the testing device. In this test, the test subject's breath is passed through a vial containing a chemical solution that changes color upon exposure to alcohol. The alcohol concentration is measured by the amount of color change caused by the reaction.

Finally, Microelectro-Mechanical Systems (MEMS) technology is a more recent but growing entry into the breath alcohol detection field. MEMS-based detection machines use millions of tiny silicon sensors or motes to detect the presence of alcohol molecules in the breath. MEMS machines are highly sensitive and should only be used in areas where the ambient air is free from contaminants such as smoke.

How are samples collected for an alcohol breath test?

The method of collection for breath alcohol machines also varies. For some devices, a test subject will be directed to breathe into a sterile mouthpiece or straw-like device. For these machines, the person's breath is either passed directly into the instrument or an inflatable balloon attached to the device. Passive alcohol sensing (PAS) devices allow the test subject to exhale near the machine without the use of a mouthpiece. The PAS collects the breath vapor for testing either through the use of a fan or mechanical action. Some blood alcohol machines automatically collect a specimen when the test subject exhales properly while collection for other machines is controlled by the person administering the test. In cases where the test subject has recently consumed alcohol, a breath alcohol test may be delayed for a 15-minute interval to ensure that any residual alcohol in the mouth's lining has dissipated.