Tuesday, December 1, 2009

Knowing a Little Science Never Hurts

What does
say about
DUI Breath Testing Instruments?

The more you know about what "they" know, the better able you will be to understand and protect your rights when the time comes.
The National Highway Transportation Safety Administration (NHTSA) annually publishes a list of breath testing instruments rigorously examined for accuracy and approved by NHTSA for their ability to accurately determine breath alcohol concentration, and thus blood alcohol concentration. The department of health or other appropriate agency in each state reviews the NHTSA list and test results, and issues a list of devices approved for use by law enforcement agencies in that particular state. The following is an excerpt from NHTSA.

Captured Samples
Exhaled air can be categorized into essentially three types of samples: tidal breath air, reserve breath air, and alveolar breath air. Tidal breath air is air exhaled in the course of normal breathing. It is the most shallow of the three types. Reserve breath air is exhaled when the body is exerted. It is produced through deeper breathing than tidal breath air, but great volumes of air are both inhaled and exhaled with little residence in the lung. Alveolar breath air is deep lung air. Since breath testing instruments are intended to measure indirectly the concentration of alcohol in the blood, it is essential for accuracy that the breath sample captured by the instrument for analysis be representative of the air in the alveoli of the lung, because it is in the alveoli that the 2100:1 equilibrium ratio between alcohol in the breath and alcohol in the blood occurs.

Infrared Instruments
Infrared breath measuring instruments operate on the principle that each chemical compound has unique infrared energy absorption characteristics. Ethyl alcohol absorbs energy in the 3.42 micron region of the infrared spectrum. The amount of alcohol contained in a sample can be calculated by observing energy loss when a known energy is applied to the sample. In the infrared devices, infrared energy is projected through a breath sample. A photo-detector identifies a decrease in wave amplitude caused by the absorption of energy by the alcohol. The amount of energy absorbed is equal to the breath alcohol concentration. The greater the alcohol concentration, the lower the wave amplitude. A computer on the instrument determines the breath alcohol content based upon the amount of energy loss, and then applies the 2100:1 conversion ratio to provide a digital readout of the suspect's blood alcohol content.

Because infrared instruments are based upon infrared absorption spectra, which are chemically unique, they cannot be influenced by compound such as acetone, which may have some chemical characteristics in common with ethyl alcohol. In fact, some infrared instruments also provide data on the concentrations of other compounds contained in the breath sample as well as that of alcohol.

Wet Chemical Instruments
When infrared instruments are not used, law enforcement generally uses wet chemical instruments, which operate on the basis of color changes produced through the chemical reaction of ethyl alcohol with chromate salts. These devices obtain a measured volume of alveolar breath and pass that sample through a known volume and concentration of a solution of chromate salt and acid. Chromate salt is yellow. As it reacts with the alcohol in the breath sample, it is chemically altered, resulting in a lighter color. The higher the alcohol concentration, the greater the color change.

A wet chemical instrument measures the difference between the light transmittance of a standard chromate\acid solution and the light transmittance of a sample solution. The difference in transmittance measured is directly proportional to the amount of alcohol in the breath sample.
Preliminary Breath Testing Instruments

PBT instruments are portable instruments for the purpose of BAC screening as part of the pre-arrest field testing. The suspect driver blows for several seconds through a plastic or glass tube, and the PBT provides an instantaneous determination of blood alcohol content.

In most jurisdictions, the legal basis for the use of these instruments is contained in the implied consent laws. While results of a PBT generally are not admissible as evidence of DWI, they do provide officers with additional objective information to establish probable cause for arrest and further chemical testing. They also help to detect persons who may be suffering from an illness or injury such as diabetes or head injury and are in need of chemical treatment, but would otherwise be mistaken for an intoxicated person. There are essentially three types of PBTs: electro-chemical, semi-conductor, and disposable chemical.

In electro-chemical PBTs, alcohol in the breath is absorbed into a fuel cell where it is oxidized, producing electrical current. The higher the alcohol content of the breath, the greater the current output of the fuel cell. By measuring the current produced, the instrument determines the breath alcohol content, and the BAC conversion is displayed with the aid of a computer chip. In semi-conductor PBTs, alcohol increases the electrical output of the semi-conductor. By measuring the voltage output, the breath alcohol content can be determined and the BAC conversion is displayed.

Disposable chemical PBTs are glass or plastic tubes containing a measured amount of the chemical, which is reactive with alcohol. As the suspect exhales through the tube, alcohol contained in the breath reacts with the chemical contained within. The greater the breath alcohol content, the greater the chemical reaction observed.

Non-Invasive or Passive Alcohol Sensors
Passive alcohol sensors (PAS) are instruments that detect the presence of alcohol in normally expelled breath. They require no cooperation from the driver. During the roadside interview of the driver and examination of documents, the officer places the PAS within six inches of the driver's mouth. It contains a small fan which samples the ambient air for examination. An electro-chemical mechanism analyzes the air for the presence of alcohol. Some instruments are concealed within a flashlight and can be used as a passive or active detector. NHTSA studies indicate these devices are effective during sobriety checkpoints when the decision whether or not to continue breath testing must be made quickly.

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