Several different types of DUI breath tests can be used to detect levels of alcohol present on a person's breath. Preliminary Breath Tests are most commonly used on the scene of a DUI incident. Preliminary Breath Tests (PBTs) are portable handheld devices. They typically have an LCD readout, and lack the ability print the result. The results of a PBT are generally not admissible in court, but are instead used to help the officer determine whether or not probable cause exists for your arrest. This means that the prosecutor and police can not use the results against a person accused of DUI in trial. Furthermore, a suspect usually has an absolute ability to refuse to submit to this test and it can not legally be held against them.
Even if you know you are under the legal limit, you shouldn't blow into a PBT. These machines are inaccurate (that's why they can't be used against you in court), and if you blow it could make the difference between you being arrested or not. On the other hand, the officer will probably arrest you if you refuse. Nevertheless, we think the better course of action is to politely refuse and let a blood test sort it out. PBTs are good party entertainment, and they are very useful in determining if there is alcohol in a persons system. However, they are not good at predicting a specific alcohol level, and they are not designed to detect impairment.
Evidentiary breath tests are breath tests taken on larger, non-portable machines that generally produce results that are admissible in court. A DUI suspect will blow a sample of his or her breath through a tube into a chamber where it is analyzed and measured by the machine. It is not uncommon for more than one sample to be taken. A suspect may be requested to blow into the machine 2-3 times. Multiple samples are taken and compared to ensure the most reliable and accurate result. A print out reading will be provided at the conclusion of the test.
Several types of these machines exist, but the ones most commonly used are the Intoxilizer and the DataMaster (or DataMaster CDM, pictured to the left). Check your local state laws to determine what machine is used in your jurisdiction.
The Different Types of Breath Analysis
There are two main DUI breath test technologies in common use today, fuel cell and infrared. Of the two, infrared is most commonly used and generally accepted in the scientific community for use in court.
In infrared breath testing in DUI cases, a light is passed through the sample and then measured. Various machines approach this measurement in different ways. Infrared light absorption is invisible to the human eye. Infrared radiation absorbs alcohol chemicals present in a person's breath at certain wavelengths. That absorption rate can be measured and transformed into a BAC reading.
Infrared Spectroscopy testing requires a person to blow into a tube connected to the breath machine. The breath sample flows through the tube to a testing chamber that contains infrared light. Alcohol chemicals present in chamber absorb infrared radiation in the light. The breath testing machine then measures the amount of infrared light that passes through the chamber or reflects off the breath. The machine then prints out a reading that converts the absorption into a BAC.
In fuel cell testing, a process called electrochemical oxidation converts alcohol to acetic acid. Two electrons for each alcohol molecule are generated that spark an electric current that supposedly can be used to measure the amount of alcohol present on a person's breath. This method is not as reliable for testing purposed and is thus not often used.
In addition to attacking the method of measurement in your DUI case, an experienced DUI defense attorney will also spend time challenging the procedures used to take and measure your sample. Regardless of which type of technology was used in your case, several human factors may also exist that would interfere with your result.
If you have been arrested for DUI and submitted to a chemical test of your breath for purposes of determining your BAC content, contact an attorney in your jurisdiction for more information on the admissibility of it's results and the impact it may have on your particular case.