How Does an Alcohol Test Work?
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Basic Premise
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While many different manufacturers make devices that measure blood alcohol content (BAC), these devices are typically classified under the name "breathalyzer." In its most basic form, the breathalyzer acts as an estimator for BAC, but are not always as conclusive as a blood sample.
The test is able to measure a person's BAC level because alcohol in a person's bloodstream also has to pass through a person's lungs. Therefore, the presence of alcohol can be tested by measuring breath. The rate of breath alcohol to blood alcohol is estimated at 2,100:1, meaning every 2,100 ml of air will equal 1 ml of blood. Therefore, if a person measure above a legal limit such as 0.08, this equals 0.08 grams of alcohol per 100 blood ml.
Breath Mixes with Vial 1
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A breathalyzer (particularly in the field of law enforcement) is a handheld device in which a person blows into a hose. The air then travels into the machine, where the breath sample feeds into a vial containing the chemicals sulfuric acid, potassium dichromate, silver nitrate and water. Once the breath hits the vial, the sulfuric acid separates the parts of the breath that contain alcohol. Then, these parts react with the potassium dichromate, which breaks up the molecule into several parts. One of those is (if it reacts with alcohol) a chromium ion that is green in color.
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Vial 2 Is Introduced
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This mixture then goes into an additional vial of the same ingredients. This creates an electric current that makes a meter reading on the handheld device. The person operating the device rotates a knob to bring the meter back to zero. The further the knob must be wound back, the greater the amount of alcohol present in the body. This method is known as infrared spectroscopy. While it is not considered a perfect process, each breathalyzer must pass a certain amount of tests before it can be used in the field.
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