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Preliminary Breath Tests in DUI Cases


  • Term “preliminary breath test” (PBT), sometimes called “alcohol screening device” (ASD) or “passive alcohol sensor” (PAS), covers a variety of different instruments increasingly being used by police officers in the field to obtain initial indications of blood-alcohol levels.
  • Some of these devices are used primarily to detect the presence of alcohol in the subject, or to roughly determine a “pass” or “fail” level (or, in some instances, an intermediary “warn” level). If the instrument indicates that alcohol is present in the subject, or it reflects a possibly high blood-alcohol level by using reading “fail,” the officer can use this information in deciding whether to detain the individual for further field sobriety tests or even to arrest him for more accurate blood, breath, or urine testing. Other PBTs, however, indicate specific levels blood-alcohol concentration.
  • Two types of PBTs, both designed to measure alcohol on the breath: fuel cell devices and Taguchi gas sensor
  • The fuel cell device measures ethanol by electrochemically oxidizing it, or “burning it up,” in a fuel cell. This oxidation generates a small amount of electrical current, which is then measured over a specific period of time. The more ethanol there is in the breath, the greater the oxidation and the more current is generated, resulting in a higher blood-alcohol reading.
  • The primary problem with fuel cell devices is their lack of specificity: The devices will detect a large number of chemical compounds, indiscriminately “reading” them as ethanol. Jones &Goldberg, “Evaluation of Breath Alcohol Instruments I: In Vitro Experiments with Alcolmeter Pocket Model,” researchers found that an “Alcolmeter Pocket Model” reacted positively to ethanol__ as well as to acetaldehyde, methanol, isopropanol, and n-propranolol.
  • fuel cell PBTs are subject to a number of additional flaws. As with any breath-testing device, the possibility of “mouth alcohol” always exist__ and requires an observation period of 15 to 20 minutes to ensure that there has been no belching, burping, or regurgitation.
  • Fuel cell PBTs are also subject to a buildup of oxide film, or “poisoning,” on the electrode, causing false readings.
  • Very sensitive to operating temperature. Counsel should always determine whether the officer checked (and recorded) the device's operating temperature at the time of the test__ or if the device even has a temperature gauge.
  • The usage logs (not to mention the calibration/maintenance records) of the PBT used on the client should be obtained in the course of discovery, and the time of any previous test determined. If the PBT was used with another suspect a few minutes before (as may be common at a sobriety checkpoint), there is a very real possibility that the client's reading has been contaminated. The fuel cell device requires time for the reaction products are still present, they will be oxidized in the client's test, creating current flow that will increase the blood-alcohol reading.
  • The Alcohol Sensor III - Researchers concluded that the device, utilizing an electrochemical fuel cell, was definitely not sufficiently accurate for use in evidentiary testing and, in fact, should be used with caution as a preliminary testing device.


  • Taguchi gas sensors are small porous stannic oxide semiconductor elements. Alcohol in the breath is attracted to the sensor, increasing its electrical conductivity; the more alcohol, the more electricity flowing and the higher the reading.
  • Two types of ASDs using Taguchi sensors: active and passive. The active devices involve capturing a breath sample from the suspect with the use of a mouthpiece; passive units do not.
  • Both types have the same problem with specificity as the fuel cell devices. Specifically, Taguchi units will detect (and register as ethanol) methanol, acetic acid, ethylene glycol, paraldehyde, acetaldehyde, isopropanol, and other compounds. A second problem common to both active and passive Taguchi devices is that sensitivity varies according to the temperature in the environment and in the unit. In addition to these problems, passive devices will register alcohol in the air, such as from the breath of passenger in the suspect's car. The cumulative effect of all these problems in the passive device is reflected in a scientific study in which field trials showed that only one-fourth of all subjects who tested positive on the devices had, in fact, blood-alcohol concentrations of .10 percent or higher; another one-fourth had levels below .05percent.
  • Life-Loc PBA 3000, typical of the passive Taguchi sensor, is built into the head of a modified police flashlight. The flashlight is held approximately five to seven-and-a-half inches from the subject's mouth and an electronically controlled pump draws in a breath sample. An ultrasonic device then corrects the reading for the distance from the sensor to the subject.The sensors can be set so that the messages are triggered by given BACs, for example, “pass” can be set from .00 to .29, “warning” from .03 to .09, and “fail” for any reading higher than .10.
  • Inherent problems. First, it may be reading vapors from sources other than the subject's breath. Second, the device is only sampling exhaled breath, and at a distance; any kind of accuracy requires that the breath consist of alveolar__ that is, deep lung air.
  • Actual sobriety checkpoint in Washington, D.C. The results: 20 percent of those detained after using the IIHS sensor subsequently proved to have levels below the legal limit (below .10 percent), while 41 percent of those detained with the Honda units proved to have unacceptably low levels.
  • That state is apparently making plans to use handheld breath-testing devices as evidential breath test instruments. In other words, steps have been taken to phase out current PBT devices as well as the evidential machines currently found in police stations and replace them with a single new handheld unit, that accomplishes the functions of both.
  • California Department of Justice to the California Association of Toxicologists Newsletter and printed in its February 1999 edition:

The Alcotest 7410 Plus hand-held breath analyzer with “Smart Cal” and PC software, is the result.

This California version of the Draeger 7410, the “7410 Plus,” varies from the standard model primarily in using a special software, “Smart Cal.” With the capacity to give the tests in the field, of course, retrograde extrapolation will cease to be a significant problem.

First, it appears that the new device is not currently designed to incorporate a mouth alcohol detector. Second, there is also, apparently, only a very primitive Radio Frequency Interference (RFI) detector planned for the device.

[A] The Alco-Sensor

  • most commonly used preliminary breath test (PBT) instruments appear to be Intoximeter's Alco-Sensor model IV (by the makers of the Inbox EC/IR® II.)
  • The accuracy of the Alco-Sensor has been tested in the laboratory. The device, utilizing an electrochemical fuel cell, was definitely not sufficiently accurate for use in evidentiary testing and, in fact, should be used with caution as a preliminary testing device.
  • It uses a fuel cell, consisting of five layers of material compressed into a wafer and soaked with an electrolyte, to analyze alcohol in the breath. An electrical piston pump within the device draws one cubic centimeter of breath into the fuel cell for analysis. The captured breath vapor is exposed to the active surface of the fuel cell and, through oxidation, any alcohol releases electrons that create an electronic flow along a conductor and out of the fuel cell. The device can be used every 15 seconds if no alcohol is encountered; a two-minute wait is required if there is a positive reading.
  • According to the Intoximeters website,

                  [t]he instrument utilizes an electrochemical sensor (fuel cell) to quantify alcohol in the provided sample. The fuel cell is a porous disk with a thin layer of platinum black on both faces. The disk is saturated with an electrolyte and mounted in a valve assembly that is designed to draw a fixed volume sample across the surface of the fuel cell. When a subject's breath sample is drawn across the top surface of the cell, any alcohol that comes in contact with the cell's surface is broken down chemically. For each molecule of alcohol that is broken down a given number of electrons are freed during the process. The number of electrons generated is in proportion to the amount of alcohol in the sample. By measuring the number of electrons (the resulting electrical signal) a digital BrAC (Breath Alcohol Concentration) is calculated and reported.

Possible Problem Conditions for the Alco-Sensor

  • Some possible sources of error noted by the manufacturer include: (1) The manual warns that “a recent drink of an alcoholic beverage or regurgitation could introduce ‘mouth alcohol' to the breath, causing an exaggerated reading. A 15-minute waiting period prior to testing will insure the elimination of ‘mouth alcohol.'” (2) Any cigarette smoke blown into the device “will permanently damage the fuel cell.” (3) “Sufficient time after each test must be allowed for all traces of alcohol on the cell surface to be eliminated…. Even when exposed to breath samples with high alcohol levels, a cell should clear within 2 minutes.” (4) New mouthpieces must be inserted with each new test, or alcohol from the previous user can elevate the reading. (5) Monthly calibration checks are recommended.

[B] Defense Use of Favorable PBT Evidence

                  In most jurisdictions, evidence of preliminary breath test results are not admissible in trial.

                  Consider, however, the reasoning in an Arkansas case, Patrick v. State. The appellate court accepted the defendant's reasoning that since the PBT results were considered sufficiently reliable to establish probable cause to arrest him, they are sufficiently reliable to be admitted as exculpatory evidence.

                  This is consistent with the decision in Rock v. Arkansas, where the U.S. Supreme Court held that evidence of hypnosis offered by the defendant was admissible, even though it would not be admissible if offered by the prosecution. The Court reasoned that prohibiting such evidence effectively precluded the defendant from presenting a defense. In order words, an accused's constitutional right to present evidence in his defense rises to a higher level than the prosecution's “right” to present evidence against him, and, thus, the standards for admissibility should reflect this.

                  Counsel should consider filing a motion in limine to obtain an advance ruling as to the admissibility of the intended evidence.

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