Measuring the Invisible Breath Sample
Information courtesy of Lawrence Taylor - DUIblog
Many of my recent posts have been about the incaccuracy of breath machines used in DUI cases. But just how accurate do they have to be? Whats so special about measuring some alcohol?
Well, consider the amazingly tiny amount of alcohol these small machines are trying to measure, and the need for extreme precision becomes apparent -- a precision which cannot be found in these portable, relatively inexpensive police-operated devices....
Lets assume that a breathalyzer reading is .10%. This means that the suspects blood contained .10 grams of alcohol in 100 milliliters (cubic centimeters) of blood, or .001 grams per cubic centimeter of blood. A typical breath machine such as an Intoxilyzer 5000 captures 50 cubic centimeters of the suspects blood. Applying Henrys Law, this means that the equivalent of 1/40th of a cubic centimeter of blood is represented by this breath sample. Since a cubic centimeter contains 20 drops, we can say that 1/40th of a cubic centimeter contains half a drop.
Assuming a .10% reading, then, the machine is attempting to measure five one-hundredths of one percent of a drop of alcohol -- an amount invisible to the naked eye!
To express this graphically, imagine a 55-gallon drum filled with water. This represents about the same capacity as 210 liters of breath -- the volume used by law in determining breath-blood analysis. Then imagine taking an eyedropper with only one-tenth of a gram of alcohol in it, and adding this tiny amount into the drum. Now imagine trying to measure the amount of alcohol in the 55-gallon drum.
Now try it with a small, relatively cheap, portable machine maintained, calibrated and operated by police officers.....a machine that is not even warranted by its manufacturer to measure alcohol (see "Breathalyzers: Why Aren't They Warranted to Measure Alcohol?").
The government calls this "proof beyond a reasonable doubt".