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Just How Does the old Breathalyzer Work?

Posted by Kevin Trombold DUI Defense Attorney | May 18, 2015 | 0 Comments


We all know and understand that alcohol can have a negative impact on your basic motor skills, affecting depth perception, judgment, and movement. All of these make driving a car more risky, both for the driver and for others on the road. In response to this increased danger, the government has made it illegal to drive drunk, and passed laws to penalize those that are caught driving under the influence (DUI).

A problem: Some people become drunk quicker than others, and everyone shows different signs of drunkenness. How do you effectively determine who's too drunk to drive?

Scientifically, alcohol is neither completely digested by the body, nor changed in the bloodstream – as your blood flows through your body, it picks up any alcohol that you've had, especially through the stomach and small intestine, and carries it around. The amount of alcohol in your blood is called your blood alcohol content (BAC), and is measured in milliliters of alcohol per 100 milliliters of blood. Determining how much alcohol is in your blood during a traffic stop, though, is difficult, as it would involve taking a blood sample.

There is, however, another way of determining how much alcohol is in your blood. When your blood flows through your lungs, some of the alcohol stays there. This happens at a steady rate – the same amount of alcohol is in 2,100 milliliters of breath as in 1 milliliter of blood, but varies greatly throughout the day and between people. Therefore, when you exhale, the amount of alcohol on your breath shows how much is in your blood. Using this fact, some police officers rely on an old breath testing machine called the Breathalyzer to determine your BAC.

A  breathalyzer is made up of 3 parts: A mouthpiece, a sample chamber, and a system of photocells that reads the results.


The mouthpiece is the tube that a driver has to blow into. The tube takes the breath to the sample chamber. Because the mouthpiece goes into the driver's mouth, it is sealed – no other air can get into the sample chamber.

In a typical breathalyzer, several chemical reactions take place in the sample chamber. First, the breath sample gets combined with sulfuric acid. This acid removed the alcohol from the air sample, and puts it into a liquid solution. This liquid alcohol solution then reacts with potassium dichromate to create a new chemical compound: chromium sulfate potassium sulfate acetic acid water. Importantly, before this reaction, the potassium dichromate had a reddish-orange color. After reacting with the alcohol, however, the new compound, chromium sulfate potassium sulfate acetic acid water, changes to a greenish color. A system of photocells measures this color change. The greater the color change, the higher the BAC.

While the chemical reactions are based on solid science, there are still lots of things that can cause variation and error with a breathalyzer measurement. An attorney that has experience with DUI defense knows all of them. Kevin Trombold has been defending people against DUI charges for nearly 20 years, and knows what to look for to challenge a DUI charge based on a breathalyzer test. Call his law office at 206-971-0067.

About the Author

Kevin Trombold DUI Defense Attorney

Highly rated by former clients, who praise his warm, knowledgeable courtside manner and his fierce determination to reduce or eliminate charges. An accomplished speaker, author, and leader in DUI defense statewide Kevin is well respected by judges, prosecutors, and other attorneys across the State of Washington for his expertise, integrity, and knowledge in the complicated forensic science area of impaired driving allegations.


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