DUI defense attorney

DUI in Tennessee: Understanding How a Breath Test Works

Despite monumental efforts aimed at getting people to stop driving while under the influence DUI, someone dies about every 40 minutes in a drunk driving accident in the United States. To address this ongoing problem, states have enacted harsher DUI penalties and law enforcement agencies regularly ramp up enforcement efforts. If you were recently arrested for DUI in Tennessee, you probably took a chemical breath test. Because that test will likely be used as evidence against you by the prosecution, it is important to understand the test. With that in mind, a Murfreesboro criminal defense lawyer at Bennett | Michael | HornsbyDUI defense attorney explains how a chemical breath test works and what the results mean for your Tennessee DUI case.

A Brief History of Driving Under the Influence

Before the advent of the user-friendly “breathalyzer” machine, states laws typically made it illegal to operate a vehicle while “under the influence” of alcohol or drugs. Prosecutors ran into problems during the prosecution of DUI cases because the term “under the influence” is somewhat vague. Not only was it difficult to define the term but it was equally difficult to prove, beyond a reasonable doubt, that a defendant was under the influence at the time he/she was operating a vehicle. The breathalyzer, and other similar machines, made it possible to quickly, easily, and reliably (arguably) test for the presence of alcohol in a suspect’s system, leading to a change in the law in many states that made it illegal to operate a vehicle with a Blood Alcohol Concentration (BAC) above a specified percentage.

How Does a Breath Test Measure Alcohol?

While it is still illegal to operate a vehicle while under the influence in Tennessee, Tennessee law also prohibits the operation of a motor vehicle if “the alcohol concentration in the person’s blood or breath is eight-hundredths of one percent (.08 %) or more.” To convict a defendant of violating this section of the law, the State needs to prove that the defendant’s BAC was above the limit. To accomplish this, a chemical breath test machine is typically used. 

The overall purpose of a breath test machine is to measure the level of alcohol present in a suspect’s blood. A blood test will provide an accurate measure of the amount of alcohol in a person’s blood; however, a blood test is much more invasive, costly, and time-consuming. Because the concentration of alcohol in the lungs relates to the concentration present in the blood, a breath test machine can accomplish essentially the same thing by testing the breath exhaled from a suspect. 

Using a partition ratio, it is possible to determine the BAC from the air a person exhales. The ratio of breath alcohol to blood alcohol is roughly 2,100:1, meaning that roughly 2,100 milliliters (ml) of breath will contain the same amount of alcohol as 1 ml of blood. Using the partition ratio, a breathalyzer can calculate a person’s BAC using a complex chemical reaction. The alcohol vapor in a person’s breath reacts with an orange solution known as potassium dichromate. When alcohol is present, this solution turns green. This color change creates an electrical current, which the breathalyzer can convert into a value to determine the BAC. If the machine detects a BAC over 0.08 percent, that result can be used against a defendant at trial to prove that the defendant was operating a vehicle while under the influence.

Is a Breath Test Machine Accurate?

If you are the defendant in a DUI case, you undoubtedly want to know if you have a defense when your chemical test result was over 0.08 percent. The short answer is “possibly.” Breath test machines are not infallible. Moreover, there are numerous factors that can directly impact the accuracy of the results, including:

  • Failure to properly calibrate the machine.
  • Operator error.
  • Equipment defects.
  • Medications the suspect was taking.
  • Medical conditions.

Furthermore, there are factors that could impact the meaning of the test results. Just because your test result was over 0.08 percent does not always mean that your BAC level was over 0.08 percent at the time you were operating a vehicle

Never assume that a conviction is a foregone conclusion based solely on the results of a breath test. Instead, consult with an experienced criminal defense attorney to discuss possible defense strategies that could prevent a conviction.

Contact a Murfreesboro Criminal Defense Lawyer 

If you have been charged with a DUI in Tennessee, consult with an experienced Murfreesboro criminal defense lawyer at Bennett | Michael | Hornsby as soon as possible. Contact the team today by calling 615-898-1560 to schedule your free  appointment.


Dinah Michael