The most common method used by law enforcement to obtain evidence of a person’s blood alcohol concentration is by use of breath testing. Breath testing is not direct evidence of the blood alcohol level. It is an estimate based on a complex equation called a partition ratio – Concentration of alcohol in an area of air above a liquid (the blood vessels in the lungs).

The first breath test usually given is at the side of the road after the field sobriety tests have been given. The name of the device used in most of Southern California is the Alco Sensor IV. The test is commonly referred to as a preliminary alcohol scan (PAS) test or a preliminary breath test (PBT). Of all the breath tests, the PAS test is the most unreliable because the Alco Sensor IV does not have various error checking safeguards to protect against the presence of mouth alcohol or other foreign substances in the mouth. Also, many officers fail to wait and observe the required 15 minutes before giving this test as required by the manufacturer.

The Alco Sensor IV operates by using a fuel cell. The fuel cell generates energy as alcohol molecules pass over it. Another problem is that the fuel cell can identify other things as alcohol such as bread residue and other molecules that have a similar structure to alcohol. Fuel cells also degrade over time and are subject to contamination.

It is possible to wash your mouth with mouth wash and then blow into an Alco Sensor IV and records a .40 BAC reading. Most law enforcement agencies do not rely on the results of the Alco Senor IV for prosecution. Only Ventura and Orange County appear to be trying to convict drivers with Alco Sensor IV readings.

The Los Angeles Police Department uses a breath machine called the Intox ec/ir. This machine also uses fuel cell technology. However, it also has an inferred device which is supposed to help ensure that ethyl alcohol is in fact being detected. The Intox ec/ir has a mouth alcohol detector to protect against a high reading caused by burping or other contamination.

The Los Angeles County Sherriff uses the Data Master CDM. This is the best machine in operation in Southern California. It does not use fuel cell technology. It is completely infrared, so there is usually no issue of interfering substances or other non alcohol substances showing up as alcohol. It also has an advanced mouth alcohol detector to prevent a burp or vomit from being read as a falsely high reading.

All breath tests have similar weaknesses. If the machine is not working properly, the test is not properly administered, or the person giving the test is not properly trained, the test result cannot be used. Also, some illnesses, a fever, and GERD can cause these machines to produce inaccurately high readings.

During the defense of a DUI case, I obtain the maintenance, calibration, accuracy checks, and tests results for a period before and after your arrest to determine if there is a reason to challenge the machine.

Another weakness of breath testing is that it gives a test result at the time of the breath test, not at the time of driving. What this means is that even if you have a breath test result of .08 or above, you may still be able to claim that you were under the legal limit at the time of driving. Remember, that in order to prove you guilty of DUI for having a BAC of .08 or above, the prosecution has to prove that you had that level at the time of driving.