August 29, 2006
Breath Analyzers as Black Boxes
Breath analyzers designed to test drivers’ level of intoxication are black boxes. Breath goes in and an estimated blood alcohol concentration (BAC) reading comes out. Typically, states require periodic testing of these instruments to assess their accuracy. The testing involves using a sample with a known alcohol concentration to assess whether the instrument is properly calibrated. Defendants from time to time attempt to force the state to do even more to certify every step of the operation. In State v. Ensey, 881 A.2d 81 (N.J. 2005), the defendant moved to suppress the results of a breath test because the state failed to provide a witness who could testify concerning the preparation of the mixture used to calibrate the breath analyzer used in this case. The court rejected the defense argument that this was required to lay an adequate foundation for the admission of the test results.
A more interesting development is a flurry of decisions in Florida on the question of whether the state must turn over to the defense the software used by the breath analyzer to calculate an individual’s alcohol content. Breath analyzers must make at least three separate calculation in order to provide a BAC. See Patrick Barone, Commentary: Unlocking the Mystery Behind Breath Testing: The Right of Access to Source Codes. 4/ 17/ 06 Mich. Law. Wkly, 2006 WLNR 9002476.
First, the machine must measure the level of alcohol in the breath. This is done by measuring the resistance of an infrared light beam as it passes through a sample chamber containing the individual’s breath. The theory behind this methodology is that ethanol absorbs infrared light and, therefore, the loss of energy in the beam may be attributed to the level of alcohol in the sample.
Second, the breath alcohol level (BrAC) must be used to estimate BAC. This is done using Henry’s Law: At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid. In this context the liquid is blood and the gas is breath in the closed system of the lungs. The typically accepted ratio used in law enforcement is 2,100 to 1. For every molecule of alcohol in the breath (BrAC) there are 2,100 molecules in the blood (BAC). This, of course, is the ratio under ideal conditions and in the past there has been a fair amount of litigation over the appropriate constant to apply.
The third calculation relates to the proper application of Henry’s law. The law applies to gas in equilibrium with the liquid. This is the case only with respect to alveolar (deep lung) air. In an attempt to obtain alveolar air, officers are routinely asked the suspect to blow hard into the machine. Under normal conditions, when there is no alcohol present except that in the lungs, as one blows into the machine the breath alcohol level rises and then levels off as alveolar air passes through the machine. That is, the highest BrAC is found in alveolar air. If one does not blow hard enough alveolar air may never reach the machine, but if the only alcohol present is that evaporating into the lungs from the blood the result is a reading that underestimates one’s true BAC. Problems arise, however, if there is other alcohol present. If one were to rinse one’s mouth out with a glass of bourbon right before taking a test some of this mouth alcohol would evaporate when taking the test and thus the machine would return a reading that overestimates BAC.
In order to avoid this difficulty, many states require that an officer observe the defendant for 15 or so minutes prior to administering the breath test to be certain the individual has not put any alcohol into his mouth or done anything else such as belch that would affect the reading. Failure to observe the individual during this period occasionaly leads to the exclusion of the subsequent test. See State v. Korsakov. 34 S.W.3d 535 (Tenn. Crim. App. 2000).
The equipment itself is designed to detect this problem and to return an “invalid sample” message when some other source of alcohol seems to be present. This is accomplished through “slope detection.” The machine attempts to detect a rising “slope” of greater and greater BrAC as the individual blows into the machine and when it does not detect a slope it should return the “invalid sample” message. When calibrating an instrument, the state should test to see if the slope detector is working properly. However, there is some question as to whether this slope detection feature always works correctly. See Michael Hlastala, Wayne Lamm and James Nesci, The Slope Dectector Does not Always Detect the Presence of Mouth Alcohol, 20-MAR Champions 57 (March 2006).
In a number of recent Florida cases, defense attorney’s have asked for the computer code by which the breath analyzers make these calculations. They argue that without access to the code there is no way to know if the results of the machine would pass muster under a Daubert or Frye reliability analysis. Several judges have found that defendants are entitled to this code. See Paul Quinlan, Surprise Court Ruling threatens to Nullify Results of DUI Tests, 11/4/05 Sarasota Herald Trib. A1, 2005 WLNR 17892443. The manufacturer of the equipment used in Florida, the Intoxilyzer, has refused to produce the code which it says is a trade secret.
This litigation raises a pair interesting questions concerning all equipment used in forensic testing. First, how far back may the parties go in an attempt to deconstruct the scientific knowledge proffered by an opponent? Second, to what extent should the results of an instrument be admitted if it passes a reliability testing protocol even when we are not certain exactly why it is able to pass the protocol?
August 29, 2006 | Permalink
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