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Over 50,000 people in North Carolina were charged with Driving While Impaired (DWI) or a related offense in the fiscal year 2015/2016.  Many of them were charged (and most later convicted) based largely on a reading from the Intox EC/IR II breathalyzer instrument manufactured by Intoximeters, Inc.  In many of those cases, the BAC reading registered at or near the threshold of North Carolina’s legal limit of .08 grams of alcohol per 210L of breath.  But how accurate is that measurement?

The short answer is: not very, especially when the numbers are close.  (Not to mention the fact that many individuals are simply not impaired at a .08 in the first place, even when alcohol concentration is accurately measured, but I’ll save that for another day.)

Let’s talk about how this breathalyzer machine works — this old white box resembling a mid-1980s PC that decides the fate of so many North Carolinians.  If you ask the “certified chemical analyst” who administers the test — in most cases, the arresting officer — more times than not, the answer is something to the effect of “Oh I couldn’t tell you how it works.”  Chemical analyst certification requires the knowledge of which buttons to push, not much more.

But how does the instrument convert a sample of the accused’s breath into an assertion of the grams of alcohol per 210 LITERS of breath, proclaimed by prosecutors across the land as proof beyond a reasonable doubt of the guilt of the accused?  It certainly isn’t by collection or analysis 210 liters of breath.  The average human exhales about half a liter of air per breath.  And yet, based on a single breath, the EC/IR II purports to report the alcohol concentration of 210 liters of breath — or approximately 420 breaths.  In other words, the instrument extrapolates an estimation of the alcohol concentration of a substantial quantity of breath based on a minute sample.  The instrument — and the court system, in faithful reliance on it — takes a giant leap between the lips of the accused and that reported number.

First, let’s just look at the numbers.  Here’s a test ticket from the preventive maintenance records of an EC/IR II at the Durham County Jail:

This BAC test ticket is more or less identical to what you’d receive if you were arrested, with the exception of those .00s you see next to the lines that read SUB TEST, which is the “subject” sample (which are air blanks at the time preventive maintenance is performed).  One of the most interesting pieces of information on this test ticket is the value on the line that reads ACCY CHK, which on the above ticket came out at .08.  That’s a control sample, produced in a lab, and is engineered to be exactly .080 grams of alcohol per 210 liters of gas.  So how do you explain this test ticket from the EC/IR II from a BAT Mobile (mobile unit used at checkpoints) in Wake County?

Note that the value on the ACCY CHK line on this instrument came at a .07 — under the legal limit in North Carolina.  Two instruments read the control samples, both manufactured in a laboratory to have precisely .080 grams of alcohol per 210 liters of gas, with two different values.  One that would support a criminal conviction, and another that would not.

But the instrument has what’s known as a tolerance range, which is why both of these instruments passed their respective calibration tests.  And the tolerance range is not just the .01 difference you see when you compare these two samples that only report out to two decimal places, but in fact the instrument tolerates a range of a full .02.  With respect to that control sample, the instrument will tolerate any value between .070 and .089.  It simply doesn’t report the third decimal place.  So your .08 could be a .06, or your .09 could be a .07.

That’s only scratching the surface of our courts’ deeply problematic reliance on this wildly inaccurate machine to determine the fate of individuals accused of driving while impaired.  More in posts to follow.


DWI/DUI lawyer Ben Hiltzheimer is a criminal defense attorney in Durham, North Carolina, who represents individuals charged with DWIs and the full spectrum of misdemeanors and felonies. Contact us for a free, confidential consultation and case evaluation at (919) 899-9404.

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Ben is an experienced trial lawyer who earned his law degree from The George Washington University in Washington, D.C. He was trained in trial practice at the nation’s preeminent Public Defender agency, the federally funded Public Defender Service for the District of Columbia, described by United States Attorney General Eric Holder as “the best public defender office in the country.”

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