San Diego DUI Breath Test

 

Preliminary Alcohol Screening (PAS) Tests

 

A handheld device used by a police officer to give a breath test is called a Preliminary Alcohol Screening (PAS) device. PAS tests are given in the field, generally where an officer pulls over or arrests a suspect for DUI. PAS tests are far less accurate than tests give at a station or from a device installed in the trunk of the officer's police car.?

 

Stationary or Trunk Mounted Devices

 

Breath testing devices can be active or passive.  Active testing devices require a suspect to blow into the device in order to provide a breath sample. Passive devices just measure the air around a suspect's mouth, but do not require a suspect to breathe into the device.  As you might expect, active devices are far more accurate.  But there are several kinds of active devices.  They are infrared, electrochemical, and semi conductor devices.

 

The theory behind breath testing is that there the ratio between the concentration of alcohol in a person's breath and the concentration of alcohol in the person's blood is the same for all people at all times. This ratio is called the partition ratio.  The majority of breath testing devices assume that the concentration of alcohol in the person's blood is 2100 times the concentration of alcohol in the person's breath, so that the partition ratio is 2100:1. A breath testing device will measure the concentration of alcohol in a person's breath, multiply that concentration by 2100, and display the product of that multiplication as the person's blood alcohol concentration. This is a source of inaccuracy in the results calculated by breath test devices because the partition ratio is not the same for all people.  Unfortunately, the California state legislature passed a law saying that the ratio is 2100:1, totally ignoring individual variations in partition ratio. Therefore, it is rare that a judge will allow evidence that some people have a ratio other than 2100:1, or that a particular person's partition ratio has been tested and found to be something other than 2100:1. There are, however, plenty of other attacks that can be made against breath test devices.

 

A good San Diego DUI attorney will obtain the accuracy check records and calibration records for the device your arresting officer used to test your breath.  The accuracy check records and the calibration records are different things. When the police conduct accuracy checks, they start with a water-based solution of a known alcohol concentration.  They place the solution in a container, and heat the solution until it forms a vapor that contains water and alcohol. They expel the vapor into the chamber of the breath testing device, and look at the display of the breath testing device to see what the readout is. If the readout corresponds to the alcohol concentration of the solution used in the accuracy check, then the device has passed the accuracy check.

 

If a breath test device fails an accuracy check then it is calibrated.  To calibrate the device a San Diego Police officer or an employee of a San Diego crime lab will make adjustments to the device until the device displays results consistent with the alcohol concentration of the test solution.

It is uncommon for a police officer in San Diego to give a suspect a breath test on a device that has recently failed an accuracy check, and has not since been calibrated.  But it is less uncommon for an officer to give a suspect a breath test on a device that has not had an accuracy check for a while.  If a San Diego officer testifies at trial about the results of a breath test on a defendant, and the device used was not checked for accuracy recently enough before the test was given, then accuracy of the test results may be challenged.

 

Even members of law enforcement, who will argue that the variation between the partition ratio of different people is negligible, are more likely to agree that the partition ratio holds true only for air that comes from deep in the lungs.  The lungs have thousands of tiny little branches that take in air, and allow red blood cells to pick up oxygen and get rid of carbon dioxide.  These branches are called alveolar branches, and the sacks at the end of the branches are called alveolar sacks.

 

One argument can be made to attack the results of breath tests is to say that breath test devices measure mouth air, and not just alveolar air, the air from deep within the lungs.  The air from the mouth can have a much higher alcohol concentration than the air from the deep lung for a couple of reasons. One reason for this is that air that comes from the mouth can be contaminated by alcohol found in dental pockets or burps from the stomach.  Only certain breath testing devices are capable of distinguishing between deep lung air and mouth air.  A good San Diego DUI lawyer will challenge the accuracy of breath testing devices that are incapable of distinguishing mouth air from deep lung air.

 

Breath testing devices that can determine whether the air they are testing is the lung air or mouth air have what is called a slope detector.  A slope detector measures whether the alcohol concentration of the air the suspect is blowing into the testing device is increasing or decreasing over the course of the breath.  The air that goes into the device first comes from the mouth.  The alcohol concentration of the air in the machine will change as the mouth air at the beginning of the breath passes through the machine and is replaced by the deep lung air that comes in afterward. Because there is less mouth air than lung air, the phase during which mouth air is inside the testing chamber is shorter than the phase during which the deep lung air is in the chamber.  The device will then report the alcohol concentration of the air tested during the phase at which the alcohol concentration remained constant longest, the deep lung air phase.  Devices without slope detectors are incapable of making the distinction between mouth air and deep lung air because they only test the breath at one point in time. But a device with a slope detector will test the alcohol concentration of the air in the test chamber numerous times during the course of the breath, enabling it to distinguish between the mouth air phase and the deep lung air phase. If you were given a breath test using a device that does not have a slope detector, then the accuracy of that breath test may be challenged if it is offered into evidence at your trial.

 

Judges rarely completely exclude the results of preliminary alcohol screening device tests, despite their potential for inaccuracy.  Occasionally, judges persuaded by a good San Diego DUI lawyer will admit the results of these tests to show the presence of alcohol, but not to show the concentration of alcohol.  This can be very useful to a person who is asserting a rising blood-alcohol defense.

If a person is pulled over at one point in time, and is taken to the station where his blood or breath is tested an hour later, and the test at the station displays a result of more than .08%, then a person may argue that his blood alcohol level was lower at the time of driving than at the time of the station house test – the “rising” defense.  The prosecution will often argue that a person's blood-alcohol concentration was at a plateau at the time of driving or fell from the time of driving to the time the test was taken at the station.  One way they will try to show this is by saying that the concentration of alcohol in the breath, as displayed by the preliminary alcohol screening device, was higher than the concentration of alcohol measured by the blood or breath test at the station.

 

If you can get a judge to exclude evidence of the specific alcohol concentration displayed by the preliminary alcohol screening device, then you can undermine the prosecution's argument that the alcohol level had plateaued at or was falling at the time of driving, and have a better shot at persuading the jury that your BAC was rising.  Even if the judge allows into evidence the actual reading displayed by the preliminary alcohol screening device, it is still possible, through the use of your own expert witness, or cross examination of the prosecution's expert witness, to show that the results of the preliminary alcohol screening device tests are not accurate.  And one way to do this is to show that the preliminary alcohol screening device was not equipped with a slope detector.

Another factor that can cause variation in the blood-alcohol concentration of air is the temperature of the breath. At any given blood alcohol concentration, for every degree Celsius that air blown into the breath test device is warmer, the air can have a 7% higher alcohol concentration.  Breath testing devices assume that the temperature of the air that a person breathes out is 34° Celsius.  Just like the assumption of breath testing devices make about the partition ratio, this assumption does not always hold true.  It does not hold true from person to person, and it may not hold true at different times with the same person.  If a person has been exercising, like dancing at a club, or is sick, or just has an abnormally high body temperature, then the air the person breathes out may be greater than 34° Celsius.