A breathalyzer relies on scientific assumptions and constants.

Henry’s Law states that when a liquid containing a volatile substance, like alcohol, makes contact with air in a closed container (Breathalyzer) the amount of alcohol in the air and the liquid are static (consistent or unchanged). Breath testing devices measure the amount of alcohol found in a person’s breath and multiplies it by a pre-determined co-efficient to arrive at a person’s estimated BAC. Breath testing, therefore, relies on scientific assumptions and constants.

Since a breathalyzer measures the amount of alcohol in a person’s body through their breath it is important to determine if any other medical conditions could negatively effect breath testing. In addition, it’s important to determine if a medical condition could cause a person to appear intoxicated when, in fact, he was just suffering from the effects of the illness.

Criminal defense attorneys who fight DUI cases in Pennsylvania often focus on what occurred prior to their client’s arrest. While this tactic is appropriate and sometimes effective, the legal analysis should never stop there. Focusing on what occurs before the arrest forms the basis for Motions to Suppress Evidence which is a defense well established in the Pennsylvania and United States Constitutions. Stopping your defense analysis at this point, however, substantially reduces the possibility that you will actually win your DUI case. It is often much harder to win a Pre-trial motion like a Motions to Suppress evidence than to win your case at trial.

Always remember that the burden of proof at a pre-trial motion is “by the preponderance of the evidence” while the burden of proof at a criminal trial is “guilt beyond a reasonable doubt.” The preponderance standard means that prosecution only has convince a judge that its argument is slightly more persuasive than the defense’s argument. At trial, however, the defense only has to create reasonable doubt! The Pennsylvania Standard Jury Instructions define reasonable doubt as the type of doubt that would cause a person to pause before making a decision of extreme importance. You therefore often have a much better chance of winning your case at trial than you do at a pre-trial motion.

A proper DUI defense is a 2 pronged attack—Pre-Trial and Trial! If you win at trial after the first prong, great, but if you don’t, your attorney must focus these 5 numbers if your DUI case involves a breathalyzer test.  Here are the 5 areas 

  1. The 2 Hour Rule – The prosecution must establish beyond a reasonable doubt that you drove or were otherwise in actual physical control of your car within 2 hours of imbibing (drinking) enough alcohol to impair your ability to operate a car safely on a Pennsylvania road or highway. Your attorney must focus on this point especially in cases where police found you outside of the car when they arrived at the scene.


  1. The 20 Minute Observation Period – After you are arrested, the police must observe you for 20 minutes prior to administering a breathalyzer test. During this 20 minute period the police must ensure that you haven’t done anything that would taint the results of the breath test results. The breathalyzer calculates Blood Alcohol Content (BAC) by converting the amount of alcohol in your breath sample (Henry’s Law.) Anything that corrupts that breath sample such as burping, vomiting, and even smoking could corrupt that sample. If the police officer is not able to testify that he was in a position to observe all of these possibilities, the court will find the test result invalid or unreliable.


  1. .08 BAC or higher – While there is section of the DUI statute that doesn’t require the prosecution to establish a specific BAC level beyond a reasonable doubt, the more serious DUI charges always require it. The prosecution must establish that your BAC is above .08 to get a conviction. In addition to this number, your attorney must understand that DUI is a “tiered” offense and so the higher the BAC, the more serious the consequences (license suspension and jail time).


  1. Breathalyzer Simulator Solution of less than .09% or greater than .10% –   Prior to and during the actual breath testing, the machine’s simulator solution must provide a specific reading. If the simulator test gives a result less than .09% or greater than .10% when the breath test device is read to the second decimal place, or if the simulator test gives a result less than .090% or greater than .109% when the breath test device is read to the third decimal place, the court should consider the results unreliable.


  1. Standard Deviation of .02 or more – The police officer will obtain 2 breath samples from you. If the difference between the results of the two actual alcohol breath tests is .02 or more, for machines read to the second decimal place, or .020 or more for machines read to the third decimal place, the court should consider the results.

If you have more questions about DUI defense, watch my videosread my book, or call our office.