We Get Results
  1. Home
  2.  | 
  3. DUI
  4.  | How does a breathalyzer measure blood alcohol count?

How does a breathalyzer measure blood alcohol count?

One of the most common tools used by police in Phoenix and elsewhere in Arizona to enforce the state’s laws against drunken driving is the “breathalyzer,” a small hand-held electronic device that can measure a person’s blood alcohol content (BAC) in a few seconds. Breathalyzers are not, however, infallible. A breathalyzer that has been improperly calibrated cannot provide a reliable measure of BAC. Also, if the police officer administering the test has not been properly trained, the readings provided by the breathalyzer may not be admissible in evidence. The average motorist need not be a chemist to understand how a breathalyzer should be used, but the basis steps may provide essential knowledge about whether a breathalyzer has been used properly.

The basics

The breathalyzer contains a blow pipe into which the motorist is asked to exhale. The breathalyzer has an internal chamber filled with potassium dichromate which turns green when mixed with alcohol. The breathalyzer also contains a chamber filled with potassium dichromate that has not reacted with alcohol. A photocell in the breathalyzer measures the electrical current produced when the alcohol in the subject’s exhaled breath mixes with the potassium dichromate and provides a digital read-out that tells the officer the subject’s BAC. The more alcohol that has been dissolved into the subject’s breath, the higher the reading that will be provided.

In calculating the subject’s BAC, the breathalyzer’s design assumes that after blood enters the blood stream, a portion of alcohol dissolved in the blood passes through the alveolar cells in the lungs. Breath that is exhaled has passed through the alveolar cells and has absorbed some of the blood into which alcohol has been dissolved. The breathalyzer is based upon the assumption that the ratio of blood to breath is 2100:1. This ratio was developed based upon test results from many thousands of subjects.

Police departments like to present the results of a breathalyzer test as consistently accurate and reliable, but knowledgeable defense attorneys know better. A capable defense attorney will know many methods of challenging the evidence from a breathalyzer test, and the skilled application of these tactics can prevent a conviction for DUI that can plague a person for several years.