Pennsylvania DUI Breath Testing, Reasonable Uncertainty, and The Standard of Error

Pennsylvania Driving under the influence (DUI) charges require that the District Attorney (prosecution) prove, beyond a reasonable doubt, that a person operated a car or a motor vehicle while impaired (Section 3802). While the prosecution can meet its burden of proof without establishing a specific blood alcohol content (BAC) (Section 3802(a)(1) General Impairment), the DA will usually attempt to introduce the results of either a blood or breathalyzer test into evidence in order to convict a person of a more serious DUI charge. These more serious DUI charges (Section 3802(b); (Section 3802(c)) require that the prosecution show, beyond a reasonable doubt, that the defendant’s BAC was over .08 which is the legal limit in Pennsylvania and many other jurisdictions like New Jersey. I have written previous articles on the concept of reasonable doubt. I have also made a number of videos on DUI defense that I encourage you to watch.

Proving a case beyond reasonable doubt, however, doesn’t mean proving it to a scientific certainty. So the prosecution isn’t required to present evidence (test results) that provide no room for error or uncertainty. While Pennsylvania does allow some uncertainty within a measurement for Blood Alcohol Content (BAC), it’s important that your criminal defense attorney look at the amount of that deviation. Deviation and standards of error are important parts of your DUI defense. Your DUI lawyer can’t overlook this issue or worse, ignore it!

In Pennsylvania, the results of Breathalyzer tests are inadmissible if the deviation is beyond certain limits when it comes to a person’s breath samples as well as the calibration results of the actual device. A person’s breath sample is deemed invalid in Pennsylvania if the difference between the two actual samples is .02 or more, when the device is read to the second decimal place or .020 when the device is read to the third decimal place. With regard to the actual Breathalyzer’s calibration, the average deviation between the simulated solution (the constant) and the reading during the simulated test can’t exceed more than .05.

The simulated solution is run five (5) times during the calibration process of the device. The average of those five readings is what the court will use to determine if the average deviation exceeds the permissible standard of error of .05. In addition to this standard of error, the simulated tests can’t give results less than .09 % or greater than .10% when the breath test device is read to the second decimal place or give results less than .090% or greater than .109% when the breath test device is read to the third decimal place.

Uncertainty and standard deviation are important concepts in your DUI case in addition to other issues like whether you were observed 20 minutes prior to the administration of the Breathalyzer. Your attorney should also argue possible 4th amendment violations if police stopped your car without reasonable suspicion or probable cause. If you have more questions about DUI, I encourage you to read my book, 5 Ways to Fight & Win Your Pennsylvania DUI Case.

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