Monday, September 22, 2008

Math, DWI style

Just how precise is that breath machine at the police station?  That's the question you should ask yourself before you agree to provide a breath sample should you be arrested for suspicion of DWI.

According to the Texas DWI statute, a person is considered intoxicated if his alcohol concentration is .08 or higher.  The state has dictated that if a person has .08 grams or more of alcohol per 210 liters of breath, that person is per se intoxicated.

The state's breath test machine does not measure 210 liters of breath.  The machine measures about 81 cubic centimeters of a person's breath in a sample chamber that's about 11 inches long and 1/2 inch in diameter (about one-third the size of a coke can).  210 liters, on the other hand, is about 55 gallons -- the size of a used oil barrel or a nice smoker.

A packet of sugar at a restaurant contains one gram.  To get an idea of what the law defines as an illegal alcohol concentration, pour 1/12 of that packet of sugar into a 55 gallon drum filled with water.

By the way, average total lung capacity is 5.8 liters.

Since we are in Texas and use ounces, one ounce is equal to a little more than 28 grams.

It would take approximately 2500 sample chambers from the state's breath test machine to equal 210 liters.

So, to determine how much alcohol is being measured in the sample chamber for a person who blows a .08, we take .08 grams and divide by 28.  That gives us .0028 ounces of alcohol.  Then we divide that by 2500 to determine how much alcohol is in the sample chamber.  The answer is .000001.  That's 1/one-millionth of an ounce of alcohol.

If you believe that the state's breath test machine is capable of making such a precise measurement, consider this -- the machine is serviced no more than once a month, the state's technical supervisor conducts a visual inspection of the sample chamber and the machine is kept (usually) in a beat-up wooden cabinet in an unsterile environment.

You have the right to refuse to submit to a breath test.  It's in your best interest to exercise that right.

If you've been charged with driving while intoxicated, contact my office and remember -- don't blow it!

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