The Machines There are a wide variety of different makes and models of "chemical breath test" devices used in the
There are a wide variety of different makes and models of "chemical breath test" devices used in the
DWI Chemical Breath Test Devices
How The Machines Work
Both the AlcoTest 9510 and the DataMaster primarily use a technology known as "infrared absorption." This is based upon the scientific principle that different gases will absorb infrared (IR) light at different wavelengths. The machines operate by having a breath sample brought into a chamber where an IR light source passes through the breath sample and a mechanism then measures how much light is absorbed at certain wavelengths that ethanol (alcohol) is known to be absorbed at. The more alcohol vapor in the chamber the greater the amount of IR light absorption that occurs.
Based upon the measured amount of light absorbed at the prescribed wavelength machines then calculate the amount of alcohol vapor is present in the breath sample. They then convert that measurement into a percentage of liquid alcohol in the blood using a fixed formula. This conversation relies on an accepted scientific rule known as "Henry's Law," which postulates that the relationship of a chemical in a vapor found above a liquid will be proportional to the amount of liquid of that chemical in the total volume of the liquid. Thus, Henry's Law says that if the vapor above a liquid contains 10% of a certain chemical's vapor that chemical must make of 10% of the total volume of the liquid.
The DataMaster units rely totally on IR absorption to calculate BrAC. However, the AlcoTest units utilize a second system that operates in parallel to the IR system. The second system uses a different technology called "Fuel Cell" conductivity. This system uses the ability of a Fuel Cell to increase or decrease its ability to conduct electricity. The more alcohol that is present in the sample the greater the change in conductivity of the Fuel Cell. Again, like the IR system once it has calculated the amount of alcohol in the breath sample it relies on Henry's Law to convert that to a blood alcohol percentage.
Both the DataMaster and the AlcoTest attempt to verify that they are working properly by going also testing a known sample of alcohol vapor either prior to the driver's breath test and/or immediately after. That sample (known as a Simulator Solution or Simulator Sample) is generally tested by a laboratory before it is sent to a police station. The samples are usually set to contain .10% alcohol and the machines should read that sample within + .01% of the laboratory tested amount.
Problems With Chemical Breath Test Devices
While the police and prosecutors frequently refer to chemical breath test (
For example, all
Second, the manufacturers of the
Third, the machines rely on Henry's Law which only deals with liquids. However, blood is made up of both liquids and solids (white and red blood cells). The amount of the blood that is made up of solids varies greatly from one person to another. But, the machines assume that everyone is the same.
Fourth, the machines supposedly only measure alcohol vapor from the "deep lung" of the driver because alcohol in much greater concentrations may be found in the mouth (especially for people wearing dentures), in the throat or the bronchial passages. The machines supposedly can correct for alcohol that is not from the deep lung, but that can't really be proven.
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