Much has been said about breathalyzers in the past, particularly with regard to their unreliability. Just to take one example, breathalyzers typically work by multiplying the alcohol that is present in the breath of the DUI suspect 2100 times in order to derive the blood alcohol concentration or BAC. Now the question that this brings up is why “2100″? This is where the problem lies. The computer in the breathalyzer is programmed with the assumption that the ratio of alcohol units in the blood of a DUI suspect versus alcohol units in his or her breath is 2100 to one, in a measurement that is more commonly known as the partition ratio. The problem is that this ratio is only the average at best, with real world ratios varying widely from as low as 900 to 1 to as high as 3500 to 1. Now obviously, the differences in individual ratios mean that the BAC will be radically different as well. If the DUI suspect’s BAC is actually .06%–which means that he or she is well below the legal definition of DUI–and the partition ratio is 1300 to 1, the breathalyzer reading will come out .10%, which IS legally DUI.

Now obviously that kind of result is skewed to say the least. The suspect in this case is guilty of nothing more than not conforming to the average blood alcohol ratio. When faced with this type of scenario, jurors typically hand out a not guilty verdict. Unfortunately many prosecutors objected to this and subsequently, the state of California–as well as many others–revised the drunk driving law to include a clause to the effect that the percent of alcohol in the blood would be based upon the number of grams of alcohol in 210 liters of breath, regardless of what the actual amount in the blood is. What this in effect means is that the legal basis for determining the blood alcohol level in DUI suspects now disregarded scientific fact, and ALWAYS assumed that it is 2100 times the amount that is present in your breath.

This has naturally polarized the legal system, with conservative judges on the one hand supporting this new law and others being staunchly against it. Today, the mention of partition ratios has been basically banned from courtrooms across the country, and there has been a resulting rise in convictions for drunk driving.