Breath Testing

It isn’t too surprising that the general public trusts the breath machine. Afterall, if we can’t trust a “forensic” piece of equipment like the breath machine, can we really trust the justice system at all? I am surprised, however, by the large number of attorneys who steadfastly believe that breath test cases cannot be won. I don’t know whether their cynicism stems from a lack of knowledge on the machine or an idealistic view of the criminal justice system. What I do know, though, is that they are wrong! With the right set of facts and a knowledgeable attorney, the breath machine is vulnerable to attack.
Like any measuring device, the breath machine is subject to a wide range of problems that affect its reliability. To better understand the breath machine’s vulnerabilities, it is helpful to understand its origins and how it works.

The Origins of the Breath Machine

In Missouri, the vast majority of police departments are using first generation Datamaster breath machines. These machines are made by a company called National Patent Analytic Systems (“National Patent”) out of Mansfield, Ohio and operate on technology from the 1970s! It wouldn’t make any sense to use a Commodore 64 to run your business, because we have far superior and reliable computing systems. Similarly, you wouldn’t use a typewriter to type an important paper or presentation. Why? Because it isn’t as fast or reliable as a modern computer. And yet, despite significant advances in technology and methods, the State of Missouri continues to rely upon the oldest of models of the oldest machine.

Sadly, the same machine that made its debut in the early 1980s is still being presented as fact in courtrooms throughout the State of Missouri. Worse yet, not one significant change has been made to the machine since it was introduced.

How does a breath machine work?

There are two main types of breath testing technology being used in the United States: fuel cell and infrared spectrometry. Fuel cell devices are largely (although not entirely) limited to handheld devices used by police officers in the field. These devices are known as portable breath testers or “PBTs.” In Missouri, a PBT result is not admissible for the number (i.e. the BAC) on the machine. Since fuel cell devices are notoriously unreliable and susceptible to a number of reliability concerns, Missouri has chosen to exclude the number from evidence. Many judges (correctly so) exclude the PBT from a criminal case altogether. Given the limited evidentiary use of fuel cell devices in Missouri, I am going to limit my discussion to the table top devices. These devices operate on infrared spectrometry.

The Datamaster consists of a mouth tube (where the person blows), a long cylinder called a sample chamber (where the sample passes through), a light source (infrared), filters (which are supposed to discriminate between ethyl alcohol and other substances) and a detector (which converts the output into a useable BAC). When a person blows into the mouth tube, his or her breath travels through the tube and into the sample chamber. Once inside the sample chamber, the person’s breath and, more importantly, the particles inside the breath, are pushed through the tube and out the other side of the machine.

Meanwhile, an infrared light is shot through the sample chamber. At the end of the sample chamber are varying numbers of filters (i.e. some machines have two filters, some three). The filters’ purpose is to detect certain wavelengths of infrared light. Simply put, a person’s BAC is a function of the difference in the quantity of infrared light shot into the machine and the quantity of infrared light detected at the end of the sample chamber. Simple…right?! Well, hidden within this simple explanation are a number of limitations.

Limitations of the Breath Machine

Like all measuring devices, the breath machine is subject to error. On this page, we have included many of the most common limitations of the breath machine.

How does the machine know what alcohol looks like?

The breath machine’s inability to distinguish alcohol from other similar substances creates what is known among attorneys as the “specificity defense.”
Like all substances, ethyl alcohol has an infrared “fingerprint.” That is, it operates on certain frequencies of the infrared spectrum. The theory is, that if one knew that fingerprint, he or she could identify ethyl alcohol based on the infrared light that it absorbs. To help us understand the machine’s limitations when it comes to alcohol’s “infrared fingerprint,” let’s think about human fingerprints. We were trained as children to believe that each human fingerprint is unique. That is, the curves and lines on your fingers are unique to you. How are fingerprints “matched?” Well, a “trained” examiner looks at a half dozen or more points on a fingerprint and statistically determines the probability of a similar combination on another human. Some examiners use dozens of points before declaring a fingerprint a “match.” With alcohol, we know it absorbs infrared light exclusively at 3.00, 3.39, 7.25, 9.18, 9.50 and 11.5 microns. That is, if we were checking for all six infrared points, we could be certain that what we are seeing is alcohol. Instead, in Missouri, we check only two points.
What other substances absorb at those two frequencies, you might wonder? As examples, substances found in paint thinners, certain paints, naturally in our bodies (to some degree) and substances produced as the result of some diseases (i.e. diabetes) can affect the machine’s ability to detect alcohol. Think about it. If the machine is just checking for particles coming through it on two infrared wavelengths, any substance operating on those same wavelengths will be added to your BAC. The end result is an inflated BAC (i.e. an unreliable result). Why, you might ask, don’t breath machines check for all wavelengths? You won’t be surprised by the answer. Cost and bureaucracy! The government’s chief concern, in many cases, is obtaining convictions. Getting the right person, on the other hand, is secondary.

How does the machine convert a breath sample to a blood alcohol content?

How does the machine take a sample of breath (i.e. breath alcohol result or “BrAC”) and spit out a blood alcohol content (i.e. “BAC”)? The answer might surprise you. When you blow into the breath machine, the detectors (discussed previously under “How does the breath machine work?”) determine the amount of particles in your breath sample that resemble alcohol (see “Specificity” for a discussion on the machine’s detection limitations) and then multiply that number times 2100 to determine your BAC. This number is commonly referred to in the legal and scientific community as a “partition ratio.” This conversion from breath to blood is based on a scientific principle called Henry’s Law. Henry’s Law states that, at any given temperature, the concentration of a volatile substance (like ethanol) in the air above a fluid is proportional to the concentration of the volatile substance in the fluid. While good in theory, Henry’s Law makes a whole lot of assumptions.

The most obvious assumption that Henry’s Law makes is a constant and known air temperature. To account for this, the Datamaster is programmed to assume that the human breath is entering the machine at 34 degrees Celsius. If only this assumption were true. Recent studies have shown that the average human breath temperature is just north of 35 degrees Celsius. You may think, who cares? What difference does one degree make? That one degree results in an overstatement of BAC by approximately 7%. So, if a person has a reported BAC of .085, but his breath temperature is 35 degrees, then his true BAC is .079! As you might imagine, this can make a huge difference in the outcome of a case. In the most recent studies, some subjects had breath temperatures in excess of 36 degrees. For that person, the Datamaster would overstate his or her BAC by over 14%!
When every assumption is taken into account, studies have found that a breath sample should be multiplied by anywhere from 1,100 to 3,000 to arrive at a true BAC. That’s quite a range! To help us understand the impact this number can have, let’s look at an example.

A person has a reported BAC of .120 on a Datamaster breath machine. Consider the difference in the different partition ratios:

Partition Ratio True BAC
1,100 .063
2,100 .120
3,000 .172

So, in fact, all we really know for sure is that this person’s true BAC is somewhere between .063 and .172!

Rising Alcohol

Under Missouri law, your license can be suspended and you are guilty of driving with an excessive blood alcohol content if your BAC is in excess of .08 at the time you are driving. The key phrase there is, “at the time you were driving.” With a basic understanding of alcohol physiology, this defense makes sense. As you consume alcohol, your body absorbs the alcohol through your stomach and small intestine, and your BAC rises. On average, for every drink you consume, your BAC will rise by .020. It takes (on average) 50 minutes for your body to reach that full .020, though, because it takes time for you to fully absorb the alcohol into your bloodstream. Once in your bloodstream, your kidneys and liver begin the process of eliminating the alcohol. To that end, your BAC will drop approximately .015 per hour. Not surprisingly, this process has a name. It is called the blood alcohol curve. That is, when plotted on a graph, the rise, peak and fall of your BAC looks like a bell curve.
With that knowledge, let’s talk about the defense. In most cases, after a driver is pulled over, it can take a considerable amount of time (i.e. an hour or more) before the police officer asks the driver to blow. In the end, all the officer can say with any degree of certainty is that the driver’s BAC was over the legal limit at the time he/she blew into the machine. The ultimate question, however, is what was the driver’s BAC at the time of driving (much earlier)? In scientific terms, where on the BAC curve was the driver at the time of driving.
As a simple (and outrageous, yet useful) example, assume a man consumes 10 shots just a few minutes before leaving the bar. Assuming he takes them back to back, at the moment he takes the last shot, his BAC is 0.00. Since none of the alcohol has yet absorbed, none of the alcohol is in his blood. This man is pulled over five minutes later leaving the parking lot of the bar (an unfortunately common occurrence). At the moment he is pulled over, his body is in the early stages of alcohol absorption. Although it varies by a lot of factors, this man’s BAC is well under the legal limit. By the time he gets to the station, however, let’s assume his BAC is .25 (over three times the “legal limit”). Under these circumstances, despite the breath test result, this man is NOT GUILTY of DWI.