Measuring the Invisible Breath Sample

Posted by Lawrence Taylor on August 21st, 2006

Many of my posts have been about the incaccuracy of breath machines used in DUI cases (for example, “How Breathalyzers Work — and Why They Don’t”.) But just how accurate do they have to be? What’s so special about measuring 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 simply cannot be found in these police-operated devices.

Let’s assume that a breathalyzer reading is .10%. This means that the suspect’s 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 suspect’s blood. Applying Henry’s 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 machine maintained, calibrated and operated by cops…..a machine that may not even be 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”.

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