How does a breathalyzer measure blood alcohol content

How does a breathalyzer measure blood alcohol content

On Behalf of | Dec 30, 2021 | Drunk Driving

One the most popular tools for enforcing Maryland’s drunk driving law is the breathalyzer, a device that can measure the amount of alcohol in a person’s blood stream – known as blood alcohol content or BAC – by taking a sample of the person’s breath. If a person refuses to provide a breath or blood sample after a request by a police officer, that person’s driver’s license can be immediately revoked for up to nine months. If a person has had more than one DWI conviction, the penalties for failing a breathalyzer test can be much as a two year suspension.

The basics

Given the severity of the penalties associated with a breathalyzer test, every motorist should have a working knowledge of how such devices perform their function.

When a person drinks alcohol, the alcohol is rapidly absorbed by the blood. Once in the blood, the alcohol moves to the lungs where some of it is expelled in the person’s breath. As the blood goes through the lungs, some of the alcohol moves across the air sacs in the lungs (aveoli). Scientists have measured the ratio of blood in the aveolar air to blood in the blood stream. The ratio is approximately 2,100 to 1, that is, 2,100 milliliters of aveolar air will contain the same amount of alcohol as 1 ml of blood. A breathalyzer uses this relationship to determine the percentage of blood in the blood stream.

Measuring the percentage of alcohol

Police use three different kinds of breathalyzers to determine the percentage of alcohol in the blood. One type uses a chemical reaction that produces a color change in the suspect’s breath sample as it is affected by a mixture of sulfuric acid, potassium dichromate, silver nitrate and water. The degree of color change depends upon and will reveal the percentage of alcohol in the suspect’s blood. A second device, the intoxilyzer, uses a spinning discs with colors lenses that measure the breath’s reaction to alcohol. A third type uses a small fuel cell to measure the alcohol in the subject’s breath.

As can readily be inferred from the complexity of the breath sampling technique, errors by the sampling technician are common and can be used by an experienced trial attorney to impeach the results and reduce if not eliminate their evidentiary value.