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How Breath Tests Work

Posted on in Criminal Law

DUI conviction, breath tests, McHenry County DUI attorney Breath testing devices play a critical role in DUI convictions and enforcement for the modern police officer. Small hand-held devices can provide indications of intoxication, and larger, more sophisticated ones can provide BAC levels that prosecutors can use as evidence in court.

Yet for all the people who worry about being forced to take a breath test during a traffic stop, most people do not understand how the devices work. Even more importantly, few people realize how inaccurate they can actually be, and what sorts of factors can throw off a breath test.

The Different Types of Breath Tests

There are three major types of breath tests, which operate on similar principles. Each measures the alcohol content in a person's lungs, which is itself a function of the amount of alcohol in a person's blood. However, they all measure differently.

The first type of test is an optical test that determines the BAC level based on a color change. As the person breathes into the machine, the ethanol on his or her breath reacts with the chemicals in the device. Before the reaction, the chemicals are a red-orange color, but the presence of ethanol turns them green. The machine then reads the amount of the color change and calculates the person's BAC.

The next type of breath test uses infrared (IR) light to check a person's BAC. Each type of molecule absorbs different wavelengths of IR light. This type of breath test generates many different wavelengths and checks which ones are absorbed and which ones are reflected. Based on the absorption pattern, the machine can then calculate BAC.

The final type of machine uses a fuel cell to determine BAC. Ethanol stores energy that can be used to power a fuel cell. A person breathes into the cell, and the ethanol generates an electric current. The more electricity, the higher the BAC that machine calculates.

What May Affect Breath Test Results

Despite the science behind chemical breath testing, the machines still make mistakes. There are a variety of different factors that can throw off a breath test. For instance, diabetics may not get accurate breath test results because their condition increases the amount of acetone in their blood. Acetone reacts much like ethanol for the purposes of the test, so it can skew the reading.

Additionally, the machines make assumptions about people's physical characteristics when they run the calculations for BAC. A person with a different lung capacity or partition coefficient (the ratio of alcohol in the blood to alcohol in the lungs) can get an inaccurate reading from the test.

Those are just a few examples of the several factors that can affect a breath test reading. If you have recently been charged with a DUI and want to learn more about your options for defending yourself, please contact an experienced McHenry County DUI attorney today.

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