The main problem with the ER/IR II (the breath testing machine currently used in Virginia) is that, like all breath testing machines, it has to make certain assumptions about people in general. The problem with any assumption is that all people are not alike. The primary example of why making any assumption in breath testing is problematic is best illustrated by partition ratios.
The ER/IR II machine makes a major assumption regarding the ratio between the concentration of alcohol present in a person's blood and the alcohol present in the person's lung air. This ratio is called a partition ration. The machine assumes that each person has a partition ratio of 1:2100. In other words, the machine assumes that there is 1 molecule of alcohol in a person's breath for every 2100 molecules of alcohol in the blood. This is critical because a breath test's fundamental purpose is to measure BLOOD alcohol content, not breath alcohol content. The machine simply uses the breath as a means by which to determine the amount of alcohol in one's blood.
The problem with the assumed partition ration of 1:2100 is that not every person has a partition ratio of 1:2100. The idea that every person has the same partition ratio has been rejected by many scientists. Rather, the partition ratio varies from person to person by 60% or more from the 1:2100 ratio. Read More.
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