Friday, June 27, 2008

More On Henry's Law

Earlier this week I posted here about the problems with the Intoxilyzer 5000. This machine is used nationwide by law enforcement to obtain breath samples from DWI suspects and to analyze the sample for ethanol concentration. More on Henry's Law.

The reference sample device on the Intoxilyzer is designed to deliver a sample of vapor containing a known or predicted amount of ethyl alcohol (ethanol). The reference sample device is supposedly used to verify the accuracy and calibration of the machine. A reference analysis is conducted as a part of each subject test to ensure the machine is properly calibrated.

The theory of operation of the reference sample device is based upon Henry's Law. In a closed system, like the diagram above, the amount of ethanol in the airspace above a liquid (lavender dots) is proportional to the amount of ethanol in the liquid (blue dots). Henry's law applies to closed systems at a given temperature and pressure. The Intoxilyzer 5000 does a good job at accurately predicting the amount of ethanol in the reference sample.

However, when the Intoxilyzer 5000 is used to predict the amount of ethanol in human breath the situation changes dramatically. If you assume the liquid in the closed system illustrated here is human blood, and the airspace illustrated is the air in the lungs, even a lay person can quickly recognize potential problems.

First, the human lung is not a closed system. Pressure in the lungs is constantly changing as we inhale and exhale. As the pressure changes so does the amount of ethanol in the airspace above the blood in our lungs. Furthermore, the temperature of the system is critical. If the solution temperature is low, the results will be low. If the solution temperature is high, the results will be high.

The problem with the Intoxilyzer 5000? It assumes a constant pressure and it assumes a predicted temperature within the system. If the pressure is changing then Henry's Law can only be used to approximate the concentration of ethanol in human breath. Moreover, the Intoxilyzer does not measure the temperature of the suspect's breath sample. Without knowing the precise temperature the Intoxilyzer can only make assumptions that might not bear out in a particular case. If the temperature of the person's breath is different than what the machine assumes it is, then the results obtained will be erroneous.

More about temperature problems in a later post.

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