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Joseph Coupal - Wednesday, March 24, 2010
Many times in a DUI trial, the most significant evidence against a defendant is the result of the breath test administered at the police station. Every breath test operator in Massachusetts must be certified to operate the Alcotest 7110 (the only breath testing device approved in this state for DUI prosecutions). But when it comes to convincing jurors about how this device arrives at accurate numbers, many of these same officers have difficulty. At the conclusion of the breath test, the Alcotest produces a printout showing a blood alcohol percentage. But this machine does not even test blood. Instead, it tests only the breath of the person blowing into the machine. There is a relationship between the amount of alcohol in a person’s breath (a gas) and the amount in a person’s blood (a liquid). This breath-to-blood relationship, arising out of a principle known as Henry’s Law, is assumed to be 1 to 2100 by the breath test maching. But this so-called “partition ratio” is only an average that the breathalyzer applies when converting a breath alcohol result to a blood alcohol result to use in court. The actual ratio varies among different people, and even varies within the same person at different times. Scientists have determined that that the normal range of the partition ratio varies in people from 1 to 1100 to 1 to 3000. Why does this matter? A person with a partition ratio of 1 to 1500 and an actual blood alcohol content of .07 percent would generate an artificially high breath test reading of .10 on a properly functioning breathalyzer, since the machine is applying a ratio of 1 to 2100. And since the police have no way of testing an individual’s actual partition ratio, they must come to court and acknowledge that the partition ratio that the breathalyzer machine assumes is just an average of the population, which could result in an artificially high reading. Given the many other shortcomings of these devices, properly explaining this assumption can go a long way in generating reasonable doubt in the minds of jurors.

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