The BAC DataMaster: Margin of Error and Interfering Substances

HOW THE MACHINE WORKS[1]

The BAC DataMaster is an evidential breath testing machine which works on the principles of infrared spectrophotometry.  Many vapors, including ethanol in the breath, absorb[2] infrared energy.  The machine captures a breath sample in its testing chamber and sends a beam of infrared energy through the sample.[3] When infrared energy passes through the ethanol molecule[4], the molecule begins to vibrate at its Hydro-Carbon bond and in the process uses up some, or all, of the energy.[5] Repeated scientific experimentation has allowed chemists to conclude that ethanol vibrates at – to varying degrees – are 3.4, 5 and 9.4 microns.  The machine employs a detector to detect vibrations in the known ethanol micron ranges.

The machine works to filter out frequencies that are not wanted.  The method used by the BAC DataMaster isolates the wavelengths of energy specific to ethanol via a process called “Optical Filtering.”  The Optical Filters are compounds of various oxides that are deposited on substrata made of silicon, calcium fluoride or sapphire.  The oxides are responsible for blocking the frequencies that are not wanted, i.e. frequencies that would cause an unwanted response on the detector.  The MARGIN OF ERROR refers to the fact that low levels (.001 to .003 alcohol equivalent) of potential interferant could be present in each subject sample which are not “filtered” and represent possible error.[6] Fusco and Dombroski at 25 state:

…[T]he DataMaster is designed to detect interfering compounds.  Very low levels (.001 to .003) are ignored simply because this level is very close to the background electronics noise level of the instrument.  When ignored the levels would be additive, if present.  If the detected levels exceeds .003 but is lower than .010, it is automatically deducted from any reading.  Above .010 results are reported separately or the test is aborted, depending on the software.

Here the BAC DataMaster did not abort the test.  Instead, it reported the tests and stated the level of interferant present in each test (.025 and .023 respectively).  What this means is that during the test the “optical filters” were unable to filter .025 on one or both of its filters.

When the machine detects an interference it is supposed to automatically score the test by checking to see what the other “optical filter” detected.  One “optical filter” acts to automatically “cover” for the other.  So, if the first filter reads at a .125 with an interfering substance and the second filter reads at a .100 with no interfering substance the machine automatically reduces the .125 to a .100.  This process takes place in the machine’s software.[7] The ticket does not, and cannot, determine which filter is being corrected for nor can the machine determine what the interfering substance is.


[1] Counsel has been certified on the BAC DataMaster by attending a three-day course of study at the N.P.A.S. facility in Mansfield, Ohio.  The scientific principles set forth herein are taken from course materials prepared by David M. Radomski and John D. Fusco of N.P.A.S. and are set forth in “Basic Science of Evidential Breath Alcohol Testing.”

[2] Absorption is the process of using up infrared energy.  The energy, a form of heat, is used up in exciting the molecular bonds that hold the atoms together.

[3] Defense arguments about where the BAC DataMaster is plugged in arise from the necessity that the beam of infrared energy be consistent.  The lamp or wire must be controlled with a “highly stabilized, current-regulated voltate to minimize drift”  Here “drift” means misreading the energy.

[4] The source of the infrared energy is a wire that is heated electrically.  The new Intoxilyzer 8000’s use a lamp.

[5] Scientific Truth #1: Different chemical bonds respond to different frequencies of infrared energy.  Scientific Truth #2: The makeup and complexity of each molecule determines the absorbance capability of that compound.

[6] Ohio has adopted a MARGIN OF ERROR of +/- .005 when it tests its solution for purposes of calibration.  Prosecutors may argue for the .003 MARGIN OF ERROR described above, but simple fairness would require that the MARGIN OF ERROR the State uses for itself should apply to criminal defendants.  Some Prosecutors may argue that the State adopted a MARGIN OF ERROR of +/- .005 not for the machine but for the solution.  Again, simple fairness would dictate that if the State allows a “know chemically tested solution” to vary up to .005 it should allow a volatile human the same range of complexity.

[7] The way the machine works in dealing with an interfering substance is set forth in Fusco & Radomski, Basic Science of Evidential Breath Alcohol Testing.

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