As a Massachusetts OUI attorney, an issue that will arise in many cases will be the accuracy of the breathalyzer machine that displays a driver’s blood alcohol content. One of the main reasons this test is unreliable is because of the partition ratio which is used to convert alcohol on the breath into blood alcohol content. The big problem with this however is that a breathalyzer machine will use a standard partition ratio for every person who blows into the machine. However, the partition ratio may be different for every person which may cause the machine to read a much higher blood alcohol content then is actually present. The Arizona Supreme Court just weighed in on the admissibility of evidence of this partition ratio that is used to make the conversion.
The case of Arizona v. Cooperman had the defendant driver being charged with two drunk driving charges. The first charge was driving while impaired to the slightest degree. The other charge was a per se drunk driving charge meaning the defendant was driving with a blood alcohol of over .08. With this per se charge, all that is needed is to show that the defendant was driving within two hours of blowing the .08. In the impaired driving charge, there is no presumption of intoxication, but the breath test can be used as relevant evidence along with other factors to prove intoxication.
In this case, the Arizona court was ruling on the admissibility of partition ratio evidence and whether it was relevant. The court upheld the evidence and stated the evidence for the partition ratio should be allowed. The state argued that the prosecution was only using the breath test on the second charge of the per se violation and no the impaired driving charge so it was therefore irrelevant. The defendant wanted to introduce this evidence to prove he was not impaired even though the prosecution was only using the breathalyzer test to prove the per se violation. The court allowed the evidence stating that there is a strong correlation between blood alcohol and impairment. The evidence of the partition ratio is relevant as it can show the defendant had a lower blood alcohol content and therefore was not impaired. This evidence is still relevant even though the state was only using the blood alcohol content for the per se violation. The partition ratio could not be used as evidence for the per se violation because the per se violation is based solely on alcohol on the breath and the partition ratio will not come into play.
When facing an OUI charge, partition ratio evidence can be an important defense for a defendant. Scientific evidence has proven that there is a large range of partition ratios among people so the test is flawed by giving a generic ratio to every person. These tests have been proven that due to these ratios, the blood alcohol content can be off by significantly. This becomes an extremely important issue when somebody registers close to a .08 as they could actually have very little alcohol in their system. This Arizona case supports that this partition ratio is extremely relevant in proving impairment.