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How are breath test analyzers supposed to work?

Back in October, we presented a post in which we talked about the questionable value of the numbers that may be collected as evidence in drunk driving cases. As any driver in Tennessee surely is aware, if police are able to show that test results indicate a blood alcohol content of 0.08 percent or more, the likelihood of conviction improves.

But as we noted in that previous post, getting to a usable number depends on two important elements being met. The test has to have been performed correctly and the device used to do the testing has to have been properly calibrated. If either of those two factors is in question, so is the reliability of the evidence.

There are several different types of breath test machines on the market today. They generally use one of two processes to assess blood alcohol content. Older systems might use a "wet chemical" analysis to compare subject's output to an existing sample. Newer ones use infrared spectroscopic analysis of a subject's exhaled breath vapor.

The scientific principle behind the infrared machines is that they assess the light wave absorption of the vapor. The presence of alcohol alters the reading from what would be considered normal and a computer translates the information into a BAC level.

As far as how reliable they are, many courts have deemed them acceptably accurate as a source of evidence. But over the years, independent research has shown that there can be significant anomalies between breath tests and blood draw tests. In fact, there are cases in which breath test evidence has been ruled inadmissible because faulty machine design, body differences of subjects, and improper machine calibration can throw off results.

Not every breath test is faulty, but data from such tests presented as evidence to support a drunk driving charge deserve to be challenged in a way the court will consider acceptable. It is the burden of the prosecution to show that its evidence is accurate.

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