Breath Tests in DUI Cases


If you have been charged with a DUI in Virginia, you most likely were taken to the police station and required to give a breath sample. Virginia uses the EC/IR 11 machine to measure blood alcohol content (BAC) in an individual.

How Does the Breath Test Machine Work in DUI Cases?

When a person blows into the EC/IR 11 the machine collects the breath sample in a cylinder where it is exposed to infrared light. Alcohol vapor in the breath will absorb some of that light. The machine then determines how much light is absorbed; and then by using a standard formula, calculates the blood alcohol content (BAC) of the sample’s provider.

What is the Margin of Error?

The EC/IR 11 is supposed to be tested for accuracy every 6 months. The testing is performed by having the machine take a reading from a solution where the alcohol quantity is a known amount, 0.08. If the machine registers a reading between 0.072 and 0.088 (plus or minus 10%) then the machine is deemed “accurate”.

When someone is charged with a DUI an provides a breath sample, the machine will generate a certificate indicating the results. the result is “truncated” to the nearest 100th. Thus, any sample between 0.070 and 0.079 will be reported officially as a BAC of 0.07. The legal threshold in Virginia for a DUI charge is a BAC of 0.08. Therefore, an individual may have an actual BAC of 0.074 (truncated to 0.07, below the legal limit), but the machine, given the built in acceptable margin of error, may provide a reading of 0.081 (truncated to 0.08, legally under the influence).

Individual Physiological Effects on Breath Test Results

Using Boyle’s Law regarding the relationship between liquids (blood) and gases (breath), the EC/IR 11 breath test machine works from the assumption that all individuals have a blood to breath ratio of 2100:1. Every 2100 parts of alcohol in someone’s blood will equate to 1 part of alcohol in their breath.

That ratio is presumed to be the average ratio in all human bodies. However, it is not uncommon for an individual to have a lower blood/breath ratio. Scientists have documented individuals with a ratio as low as 1100/1. If an individual with a lower ratio were to provide a breath sample, the machine will register a BAC in excess of the legal limit when their actual BAC may be much lower.

In addition, a person’s body temperature at the time of testing can affect the result. If the individual has a fever then the temperature of his body and blood are higher than normal. When liquids (bloods) are heated, they produce more gases (alcohol). Thus, that particular individual will register a higher BAC than normal.

For over 20 years, The Law Office of James J. McCoart, III, has vigorously represented those accused of Driving Under the Influence. If you, or your family member, are interested in a no-cost consultation regarding a DUI charge in Virginia, then contact The Law Office of James J. McCoart, III, by phone (703) 369-2734, or email.