Problems in Measuring a DUI Breath Sample

by Jamison Koehler on July 24, 2013

When my daughter was still a toddler, I was strapping her into the child safety seat of an overheated car when she began to blow small breaths. It took me a moment to figure out what she was doing:  Having learned that she could cool down hot liquid by blowing on it, she was using the same principle to try to cool off the sun-baked air in the car.

One of my daughter’s breaths in relation to the total amount of air in the car is roughly equivalent to the relationship between the amount of breath that is actually tested by the Intoximeter EC-IR II and the volume of air that sets the standard.  Any error in measuring the amount of ethanol present in that small amount will be magnified many times while doing the calculations necessary to arrive at the standard.

To be more precise:  The amount of ethanol present in a DUI breath sample is measured in terms of grams per 210 liters.

Two hundred and ten liters is a whole lot of breath. As Dick McGarry likes to put it, imagine the size of a two liter Pepsi bottle.  Then multiple that volume by 105 to arrive at the amount of air present in 210 liters.  Other people liken it to the amount of air present in a large oil drum.

The Intoximeter EC/IR II – the breath test machine used in both DC and most jurisdictions in Virginia — requires a breath sample of at least 1500 cubic centimeters (or 1.5 liters) before it can provide a result.  Of this amount, McGarry says, the machine measures only 2 cubic centimeters (or 0.002 liters).

You need to multiply 0.002 by 105,000 to equal 210.  This means that any error in the measurement of ethanol in the 2 cubic centimeter breath sample will be magnified 105,000 times.

McGarry also uses the analogy of a railroad car full of grain.  Can you take a small sample of that grain, test it and then be confident you know the contents of that railroad car?

2 Comments on “Problems in Measuring a DUI Breath Sample

  1. Dear Jamison,

    One could say that the breathalyzer test can accurately say your sober but not necessarily be accurate in determining how drunk a person is. That’s why in Italy a person subject to a breathalyzer test can request a counter-analysis to be made at a hospital on a blood sample, which will be very accurate. Is this the case in the U.S.A. as well?


  2. Hello Fred.

    Most jurisdictions in the U.S. use breath tests far more often than blood tests. This means that instead of measuring blood alcohol directly as with a blood test, they use a sample of a person’s deep lung air as a surrogate measure. Breath tests are based on certain assumptions about a person’s physiology.

    Breath tests are quicker and cheaper to administer. They are not invasive. And, because the breath sample is lost after the measurement, there is no way for the defense to re-test.

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