When drivers are stopped by police in New Jersey on suspicion of DUI, they may be asked to submit to a Breathalyzer test. A Breathalyzer is a device that is designed to detect the presence and amount of alcohol in the human body.
Breath test machines detect alcohol by collecting a sample of a person's exhaled breath and then examining it using a scientific process called infrared spectroscopic analysis. Essentially, this means that a breath test machine studies how much light is absorbed by particles of vaporized alcohol within the exhaled air. This process works because different amounts of alcohol vapor will absorb different amounts of light. This information is then translated into a blood alcohol content number.
The results of breath alcohol tests are routinely admitted as evidence in DUI cases. Although these tests are not the most accurate way to test the level of alcohol in a person's system, they are considered reliable enough to be used in the process of charging a driver with DUI. However, some people have challenged the use of these tests as evidence. A New Jersey court considered scientific evidence that indicated that some breath test machines could give inaccurate BAC measurements based on the temperature of the machine itself and the body temperature of the test subject in a case in 1988.
Even though Breathalyzer machines are a common component of DUI prosecution, these devices are not foolproof. Breath test devices require regular calibration in order to give accurate readings. A defense attorney may be able to use a device's calibration history records as evidence to challenge a DUI charge. If it can be shown that a breath test device had been improperly calibrated, the prosecution may not be able to use that evidence against the defendant.
Source: FindLaw, "BAC Test FAQs", November 18, 2014