One of the many things that DWI lawyers present to juries in breath test cases is that the software that actually calculates the number that it reports is kept secret.  The police technical supervisor will claim that the reason it is secret is that it is “proprietary,” but good DWI lawyers know how to bring that out as a trial issue.  This software contains what is called the “source code” of the intoxilyzer.

Since this can be an issue in DWI trials, we have excerpted the following article from DWI Journal:  Law & Science, Vol. 21, No. 6; “Source Code Defenses:  Taking the ‘Byte’ Out of Breath Testing,” by Patrick T. Barone, Esq. for your review:

Source code basically refers to the computer program that runs the breath testing equipment. “Source code” may be defined as, “code written by a programmer in a high-level language and readable by people but not computers.” Source code must be converted to object code or machine language before a computer can read or execute the program. What makes breath test devices largely unique among measuring devices is that after the amount of alcohol in the breath sample is measured, a variety of mathematical calculations must be performed to make the numbers relevant to the human system. In other words, to make the raw number relevant to the question at hand, does the motorist have an unlawful bodily alcohol level?

The questions posed by source code litigation are essentially four-fold. First, are the mathematical equations employed by the particular breath testing equipment properly formulated, calculated and applied, second, is the code acceptable to the relevant scientific community, third, has the code been subjected to an appropriate level of scientific scrutiny both inside and outside of the breath testing equipment itself, and fourth, have any changes to the code been made after the device was approved for use by law enforcement? The answers to these questions govern whether or not it can be persuasively argued that the test results should be excluded on the basis that the state cannot guarantee that they are reliable enough to sustain a criminal conviction. A complete understanding of this issue however will require a more vivid understanding of the science behind breath testing.

The Science of Breath Alcohol Testing

The first thing that a breath testing device must be able to do properly is assure that an appropriate breath sample has been collected from the motorist. For example, breath testing equipment measures the flow rate of the breath sample as it is delivered by the motorist, and in order for the sample to be accepted, certain preprogrammed parameters must be met. While these parameters vary from manufacturer to manufacturer, it remains a critical calculation because a fundamental theory of breath testing for forensic purposes is that only deep lung air be analyzed.

A related calculation is based on a breath/ blood conversion ration, and in the forensic setting this conversion is necessary because breath alcohol is only relevant as it relates to a comparative blood alcohol level. Thus, the breath testing equipment must covert the breath alcohol measurement to a comparative blood alcohol measurement. This conversion is required because breath alcohol does not impair or intoxicate, and it is only after the alcohol has been distributed throughout the body via the blood that the alcohol can cause the brain to be affected. This conversion is based on “Henry’s Law”, which states that in a closed system, the amount of alcohol in a liquid will reach equilibrium with amount of alcohol in the air above the liquid. When applied to breath testing, the liquid is of course blood, and the air is the breath. The closed system is the lungs. Henry’s law says that for drunk driving purposes a particular ratio(at a constant pressure and temperature) can be assigned to this system, and the ratio commonly accepted for law enforcement purposes is 2100/1. So, according to the theory, for every one molecule of alcohol in the breath, there are 2100 molecules of alcohol in the blood. Thus, in order for the breath test result to have relevance in drunk driving law enforcement, the source code used in the breath testing equipment must incorporate mathematical functions that will allow it to apply Henry’s Law, along with the 2100/1 ratio.

Another calculation of great significance is loosely termed “slope detection.” The purpose of slope detection is to allow the breath testing equipment to discern the presence of mouth alcohol because this would invalidate the test. The operator would know mouth alcohol has been detected because the equipment would produce an error message or status code that reads “invalid sample.” Slope detectors are also used to help assure that alveolar (deep lung) air actually composes the breath that is being tested. This is essential because in order for Henry’s Law to have any applicability, deep lung air must be measured because the deep lung tissue most closely approximates a closed system. Slope detection is an essential safeguard over the breath test results, and like Henry’s Law, it is totally dependant on properly formulated source code.

Looking again at source code the question is either or not the math used to make these conversions and calculations were properly formulated by equipment manufacturer when they developed the particular breath test machine used, and also, whether or not this math was properly programmed into the source code. The only way to know this would be to have an independent computer expert look at the code, and this is often the very goal and purpose of source code litigation. If these calculations are in any way invalid, that is, they don’t properly calculate the breath alcohol number in a way that is forensically acceptable, then the breath test results should not be admitted into evidence during drunk driving prosecutions. So, if inspections of the source code revealed and definitively determined that problems existed this would of course have a dramatic and deleterious impact on drunk driving prosecutions.

Compliance Testing Issues

One might assume that as the equipment was being developed, each manufacturer would have done a great deal of internal testing during the production of their equipment, which presumably would have included compliance testing the source code in some way. The only way to confirm that the internal testing has integrity however, would be to independently test the code, either outside the company or outside the breath testing “box.” It is within the realm of possibilities that the code is so riddled with errors that the validity or accuracy of breath test results could not be guaranteed, or that the code is so badly written that it could not carry out its intended functions. Independent testing is required to either prove or disprove these conjectures.

In scientific circles, this type of independent testing is called peer review. Peer review (also known as “refereeing”) is a screening process used prior to the publication of manuscripts in scientific journals and in the awarding of funding for research. Peer review is used by publishers and funding agencies to select and to screen submissions for accuracy. In fact, peer review is a fundamental for publications that wish to assure that authors meet the standards of their discipline. Publications and awards that have not undergone peer review are often regarded with suspicion by scholars and professionals. The purpose of peer review is to help increase the likelihood that substantive flaws in methodology will be detected.

It appears that the no breath testing source code has ever undergone any type of true peer review. In an ideal situation the peer review process would have been conducted before the source code for the breath testing equipment was “approved” for state or federal use.

A significant aspect to the source code litigation has been to obtain the source code so that it can be submitted to this type of testing by the relevant scientific community. Until such independent testing occurs in an unequivocal way, source code litigation will likely continue.