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Partition Ratio: More Reason to Question the Breath Test

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James W Harwood
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We all know the situation: A person blows into a breath testing device, the thing emits a few clicks and beeps, a ticket chugs out saying 0.10, and another DUI statistic goes into the books.  What many don’t know is that the machine did not measure the one thing that matters in a driving under the influence case:  Blood alcohol. 

According to Vehicle Code section 23152(b), it is unlawful for a person to drive with 0.08 percent or more blood alcohol.  Breath testing devices, such as the Alco-Sensor IV, measure the amount of alcohol in a test subject’s breath, not blood.  The computer inside of the machine assumes a particular ratio between the amount of alcohol in the sample of breath and the subject’s blood.  This is commonly known as blood/breath partition ratio.  However, for reasons discussed below, this ratio can actually vary from person-to-person and from time-to-time for a particular person. 

The Alco-Sensor IV assumes in every breath test a partition ratio of 2100:1.  This means that it assumes all humans have 2100 times more alcohol in their blood than in their breath.  Statutory law in California unfortunately buys in to this assumption by saying, “For purposes of this article…percent, by weight, of alcohol in a person’s blood is based upon grams of alcohol per 100 milliliters of blood or grams of alcohol per 210 liters of breath.”   This essentially eliminates individual partition ratio relevancy as it relates to the (b) count.

However, many scientific studies have shown that actual partition ratio can vary widely between individuals, and even over periods of times.  One study from New Zealand found partition ratios in different people ranging from 878:1 to 3770:1.  As for a particular person, factors such as how long it has been since that person drank his last drink and how much that person has in his stomach can change his partition ratio at any given moment. 

While assuming 2100:1 partition ratio may not matter at very high alcohol levels, it can be profoundly misleading at blood alcohol levels closer to 0.08.  For example, suppose a test subject blows 0.10 with an assumed 2100:1 partition ratio.  If that person’s individual partition ratio at the time of testing was actually 1500:1, then his true blood alcohol level was 0.071.  This example shows how potentially unfair assuming a particular partition ratio can be. 

Even though the legislature took away much of the relevance of partition ratio in 23152(b) DUI cases by setting it at 2100:1, it can still be effectively argued in other ways.  When defending 23152(a) charges, partition ratio evidence can and should be used to argue against the assumption that a person who blew 0.08 or more was under the influence, since the statutorily assumed 2100:1 should not apply to that charge.

Partition ratio evidence should also be used by the defense when the prosecution asserts that the defendant is a liar, because his breath test results do not match the drinking pattern he gave to the arresting officer.  A certain degree discrepancy may be explained by partition ratio variability.  The defendant’s drinking pattern may become plausible if another partition ratio is plugged into the equation. 

Category: General


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