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Uncertainty and other variances in measured values on the BAC Datamaster and Datamaster CDM

Posted by Nathan Webb | Jul 10, 2014 | 0 Comments

I'm no scientist, let's get that out of the way right off the bat, but I do have a basic understanding of breath testing principles utilized in garnering results with a BAC Datamaster and Datamaster CDM.  Both of these "instruments' (or as I say, machines) are approved for the quantitative measurement of alcohol in a person's breath.  See WAC 448-16-020.  Essentially, the State Toxicologist has approved these machines for use in testing person's breath alcohol concentration when arrested for an alcohol related driving offense in Washington State.

Breath testing for evidentiary purposes in Washington consists of an operator collecting two separate samples of breath independently from an arrested subject.  The operator (usually the arresting law enforcement officer) enters some basic information into the machine, such as date of birth of the subject, etc. then just waits 15 minutes or more and has someone blow into the tube attached.  Each time a DUI subject blows the operator is to ensure a new mouthpiece is used.  After the samples are collected, if everything went right, meaning there were no errors for an invalid sample, samples outside of 10% of the mean, ambient air, radio frequency interference, etc., it will produce a ticket showing two subject samples, that is the two numbers related to the persons breath alcohol concentration.  The State of Washington's legislature has made it very simple for prosecutor's to admit these samples (two numbers on the ticket) into evidence and present to a jury.  Literally an elementary student could ask the breath test technician the necessary questions for admissibility, I'm not kidding.

Once the officer testifies about the "pushing of buttons" and holding a tube for a person to blow and the technician testifies the printout looks right, then the numbers are admitted.  The prosecutor hardly ever asks questions regarding the uncertainty of the measured values, the presumption of a standard partition ratio and the presumption of a standard temperature reading on a simulator solution.  What is all that about?  Well let's discuss all of these below.

Once the prosecution ends its' direct of the technician, this is where a trained DUI defense attorney needs to pounce.  As an experienced Seattle DUI attorney I have cross examined breath test technicians on many many occasions when challenging the results of a breath test document. (I have even been asked to help train these technicians and toxicologist on how to testify). The prosecutor's job is done once they get the number in front of the jury, but the DUI defense attorney's is just beginning.  What do I mean by pounce, well you need to hammer home to the jury that the alleged numbers are really a probability fraught with uncertainty and the technician will have no choice but to admit the numbers are not perfect.  Any measured value has uncertainty, as I understand it, that is how science works!  No scientifically produced value is perfect and each measured "subject sample" could be much lower or much higher given uncertainty.  Why didn't the prosecutor ask the technician about that and disclose it to the jury?  Well, of course they want that precious numbered to be free from any speculation.  That is why it is absolutely necessary to discuss that with the technician on cross and to make sure and point out in summation that the prosecutor essentially hid that information from them.

Once you discuss uncertainty, make sure and then discuss what a partition ratio is with the technician. The jury needs to know everything about how these measured values are based upon presumptions.  A partition ratio is, in layman's terms, an assumed value that it utilized to compare blood to breath.  The dumbed down version is this, the ratio assumes that 2100 mL of breath contains the same amount of alcohol as 1 mL of blood.  However, just by doing some minimal research on partition ratio's will reveal numerous treatises wherein scientists, medical students, doctors, etc. have concluded the 2100:1 value is incorrect and can have vast ranges, meaning that someone's measured breath test on a Datamaster presumes this 2100:1 value but if the partition ratio is actually lower or higher, as has been concluded by these treatises, the actual breath test result is vastly skewed.  Make sure you bring this out of technician and discuss the revelation to the jury.  Again, a great point to make in summation is "why didn't the prosecutor inform you of this presumption when they presented their case?"  It shows that the prosecutor doesn't want the jury to speculate on things that are clearly speculative!

One additional argument I commonly make is that the presumed temperature of the simulator is also based upon skewed conclusions.  The simulator solution is a mixture of ethyl alcohol and some type of distilled water to simulate a known value, e.g. .04, .08. 10 or .15.  The toxicology lab prepares a simulator solution to be approximately .08 for use in the datamaster.  The mixture is tested by several toxicologists and certified for use. The measured value also has an uncertainty and that should also be exposed to the jury, especially if the simulator solution is closer to the .072 to .088 ranges (that is the allowable range of a simulator solution for use with the datamaster).  The mixture, for use in the datamaster, must be heated to a known value and the common value is 34 degrees Celsius plus or minus .2.  In between a subject's sample, the simulator solution runs into the Datamaster for an "external" check to make sure the Datamaster is reading a .08 mixture (although if the reading is anywhere between .0-72 and .088 it is acceptable?  Really? Anyway, that is an argument for another day).  Why is that 34 degrees value important?  Well that is the presumed temperature of a person's mouth when exhaling.  Again, there are many treatises and experts who agree that 34 degrees is not the correct average temperature of every person, and technician's and toxicologists who testify in DUI trials must agree, there is too much proof to the contrary and science simply doesn't support we are all the same.  Here is where it is necessary for your DUI attorney to expose the speculative nature of breath testing.  If someone's mouth temperature is lower or higher, invariably the breath test measured on the datamaster will be either lower or higher.  Again, in summation it is essential your DUI attorney points out this non-disclosure of information to the jury.

At the end of the day, a trained DUI defense attorney can show that these alleged measured values are fraught with speculation, yet when determining the guilt or innocence of a citizen the government has no problem with presumptions.  There are numerous other situations in which a trained Washington State DUI attorney can expose speculative breath tests results to a jury.

If you find yourself in need of a renowned, trial tested DUI Defense attorney, look no further than the Webb Law Firm.  Call today (425) 398-4323

About the Author

Nathan Webb

Nathan "Nate" Webb has been repeatedly recognized by Washington Law and Politics Magazine as a Super Lawyer Rising Star, named one of Seattle's Top Attorneys by Seattle Met Magazine and is rated Superb (perfect 10 out of 10) by


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