Breath test machines are accurate with a margin of error of around .01%. In a case where the BAC is barely over the legal limit, it is difficult for a prosecutor to prove beyond a reasonable doubt that a driver was operating a vehicle while over the legal limit.
Following proper procedure is critical to get an accurate breath test result. Breathalyzer machines measure the percentage of alcohol from the air in a person’s lungs. Alcohol in your mouth (from recent consumption or regurgitation) can throw off the breath test results. To avoid this, a police officer monitors a driver for 20 minutes prior to administering the breath test.