Everyone knows that when someone is arrested on suspicion of driving under the influence, he is given a breath test. Most people don't really understand why those tests are given or how they work. Breath test devices are developed and sold by competitive companies. These companies fight hard to keep their trade secrets secret. For this reason, it is sometimes hard to know exactly how (or if) the devices work without seeing the source code used by the computer chips inside the machines. Generally, however, we do know how the machines operate.
Oregon uses the CMI Intoxelizer Alcohol Analyzer Model 8000. It is a very impressive name. The last version was the 5000, and it is unclear why CMI skipped to 8000 for this version. Other popular competitors are the DataMaster and the Alcosensor.
Since I practice DUI law in Oregon I will focus on the intoxelizer. The intoxelizer uses inferred spectrometry to measure the changes in vibration rates when molecules are exposed to inferred light. Although this sounds complicated, it is really pretty straightforward. When someone blows into an intoxelizer, the breath is taken into a chamber. Inside that chamber, the breath has inferred light shone upon it. A sensor then measures the vibration rates of the molecules in the breath. Because different molecules have different vibration rates, CMI claims they can identify the concentration of alcohol in the breath.
One problem with this method of alcohol detection is that other molecules have similar vibration rates. Therefore, the intoxelizer could create false positives if someone has high levels of other chemicals such as acetone in their breath. This is common among people with diabetes.
Another problem with breath testing in general is that it assumes a correlation between breath and blood. This problem is present even if breath testing is accurate for breath alcohol. Once the breath is measured, a computer program spits out a number. That number is supposed to be the blood alcohol concentration. To generate that number the computer assumes that the person who blew is an average person. The computer assumes that the person's lung capacity and cardio vascular system work at the some functionality as the average person.
These assumptions make testing easy for the breath testing device manufacturers and the governments who buy the devices. It is far easier to celebrate a machine to an average person then to the actual individual being tested. The assumptions do introduce an added layer of doubt into the accuracy of the machines however.
