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Why and how are breath tests used in Drunk Driving Case?

Breath tests are complicated and have been the subject of many scholarly articles. The information here simplified and meant for informative purposes. If you have further questions, please contact me, Gregory A. Miller, a Fort Wayne Criminal Defense Lawyer.

Years ago, when the State of Indiana first began enforcing Drunk Driving (DUI/OWI) laws, the Prosecutor had to prove that a driver was guilty, beyond a reasonable doubt, of driving a motor vehicle while intoxicated. Proving intoxication (that someone was impaired or drunk) was difficult, because some officers are not properly trained or if the case isn't properly prepared by the prosecutor. Breath tests were initially created to help a police officer in determining whether or not someone had been drinking alcohol.

Most legislators wanted to make it easier for prosecutors to get convictions in drunk driving cases. Therefore, many states enacted laws that that made it a crime to drive a vehicle with a Blood Alcohol Content (BAC) over a certain limit. In Indiana, this limit was initially .10. However, the current limit is .08 BAC. These laws are known as "Per Se" laws. Breath tests now are used to try and prove that a driver had a specific Blood Alcohol Content.

The first breath test machine was created in Indiana. It was known as the Drunkometer. Other similar machines were created. All with the same goal, to determine if someone had consumed alcohol. These older machines were neither reliable nor as scientific as some of today's breath testing machines. Additionally, they needed great care and maintenance. Through the years, scientists have searched for newer and better ways to conduct breath tests.

Currently, the majority of breath testing is completed using infrared light. The State of Indiana now uses the BAC Datamaster for breath testing. The BAC DataMaster is manufactured by National Patent Analytical Systems in Mansfield, Ohio and is used in more than 15 states.

The BAC Datamaster is an infrared light based machine. This type of testing is based upon the belief that infrared light can and will be absorbed by an alcohol molecule. There is a chamber inside the BAC DataMaster through which infrared light is directed. The infrared light passes through the chamber and hits a sensor/detector at the other end of the chamber. This sensor/detector is calibrated to expect a certain amount of light. When a person blows breath into the BAC Datamaster, the breath sample, along with the infrared light will pass through the internal chamber. The sensor/detector is programmed to notice if some light is missing. The BAC Datamaster then makes a calculation, based on a computer program, to determine how much less light travelled through the chamber. This calculation is then used by the machine to calculate the amount of alcohol, by weight in grams, in the person's breath.

This calculation is complicated and not wholly reliable. The machine assumes that all people have the same body temperature. Common sense tells us that everyone is different. Body temperature can affect BAC test results. So, by presuming that each person providing a breath test sample has the same body temperature, the BAC Datamaster results are potentially unreliable. Additionally, the BAC DataMaster is programmed to assume that each person giving a breath sample has the same volume of water in their breath/body. Greater water content dilutes alcohol. Therefore, this programming assumption in the BAC Datamaster is also flawed and could lead to potentially inaccurate results.

Many scientists, legislators and prosecutors believe that infrared testing is a good way to test for breath alcohol. But as discussed above, there is a great potential for error. If you have questions about Drunk Driving (OWI/DUI), breath tests, or other criminal law matters, please contact me, Gregory A. Miller at (260) 833-7249.