What is Breath Testing - California DUI Law and Legal Information

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Anyone arrested for DUI in California is offered a choice of an evidential breath of or blood test. (Motorists are arrested for driving under the influence of drugs are offered a choice of a blood or urine test.) California criminal defense lawyers who concentrate on DUI defense have strategies to attack results of all of these tests.

Breath machines used in drunk driving investigations vary from jurisdiction to jurisdiction. The types of machines used include the E-PAS, a hand-held unit that is often administered roadside, or stationhouse breath-testing machines such as the Intoxilyzer 5000, the EC/IR, the Draeger, the BAC Datamaster, and many more.

Regardless of the type of machine used, all breath tests in Driving Under the Influence (DUI), or Driving While Intoxicated (DWI) cases are an indirect measurement of blood alcohol content (BAC). Breath testing devices engage in a conversion process that predicts BAC on the basis of certain scientific assumptions, which may or may not be applicable to the person being tested. These scientific assumptions include drivers' blood/breath partition ratio, their breath temperature, and many other factors.

Since all breath testing in a DUI investigation takes place after the time of driving - the relevant time in a driving under the influence (DUI) case - the number only becomes relevant through a process called retrograde extrapolation. Retrograde extrapolation is the process of looking backward to form an opinion about the allegedly drunken driver's alcohol level at the earlier time of driving.

This type of speculation has significant problems. Alcohol levels change over time, and the manner in which they change varies greatly from one individual to the next. Stomach contents, the amount of alcohol consumed, the length of the drinking period, body weight, gender, elimination or "burn-off" rate, and other personal metabolic factors all interfere with retrograde extrapolation in a DUI case.

Also, in order for retrograde extrapolation to be scientifically accurate, the expert must assume that the accused drunk driver is "post-absorptive" - an assumption not always true, since the absorptive phase can last for two hours or more.

There are two types of breath testing machines used in a California DUI investigation - infrared breath testing machines and fuel cell breath testing machines. When the infrared device is used, the driver blows into a collection tube. Light passes from one end of the tube to the other, and the machine measures the amount of light beam that is diminished as it passes from one side to the other in the light spectrum of the alcohol molecule.

When the fuel cell device is used, it measures the amount of oxidation that takes place on an electromagnetic chip, and then the amount of the electrical charge is converted to a number, which is supposed to represent the amount alcohol in the subject.

There are many challenges to breath testing in California DUI cases. Some defenses acknowledge the accuracy of the breath test result, but question whether the driver was under the influence of alcohol or exceeded the legal limit at the time of driving. Other defense strategies attack the validity of the breath test result itself.

The most fundamental challenge in a drunk driving case involving a breath test is the functioning of the machine itself. Whether the machine is an Intoxilyzer 3000, Intoxilyzer 5000, Intoxilyzer 8000, EC/IR I or EC/IR II, a Draeger or a BAC Datamaster, the machine must be working properly to give a reliable result. This means ensuring that the accuracy checks, calibration records, usage logs and maintenance history are all properly documented and don't reveal any problems.

An experienced Northern California DUI defense lawyer will work to have the breath test results excluded or attack the weight of the evidence. In either event, a criminal defense DUI lawyer will know how to use these issues to the driver's best advantage.