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How does a breathalyzer work?

On Behalf of | Apr 20, 2021 | DUI/DWI Defense |

Any person operating a motor vehicle on the highways of Alabama is deemed to have given the state permission to ask the person to submit to a test of the amount of alcohol in his or her blood stream if a police officer has reasonable cause to believe that the person was driving while intoxicated. This test is administered by having the driver breathe into a device that measures the amount of alcohol in the person’s blood stream. This measurement of the blood alcohol content (BAC) is then used to determine whether the person was intoxicated. The device used to make this measurement goes by several different trade names, but they are commonly called “breathalyzers,” and they all work on similar scientific principles.

The basic principle

When a person consumes alcohol, trace amounts of the alcohol are absorbed by the lungs and then exhaled. When the alcohol is in the lungs, it crosses the lung’s air sacs (alveoli) because it is volatile. The concentration of alcohol in the alveolar air is directly related to the concentration of alcohol in the blood. Many scientific measurements have determined that the ratio of breath alcohol to blood alcohol is 2,100:1, meaning that 2,100 ml of alveolar air contains the same percentage amount of alcohol as 1 ml of blood. The exhalation of breath into the breathalyzer provides police officers with a means to measure a person’s BAC without requiring either a urine or blood test.

How does a breathalyzer work?

A breathalyzer contains a system to obtain and preserve a sample of the breath from the suspect, two glass vials containing chemicals whose reaction will vary depending upon the BAC, and a system of photocells that will measure the color change and provide the measurement in percentage terms on a gauge.

The breathalyzer uses sulfuric acid to remove the alcohol from the breath into a liquid solution. The solution contains a chemical that reacts with the alcohol in the sample. The reaction changes the color of the chemical from red to green. The degree of color change depends upon the concentration of alcohol in the blood. As the percentage of alcohol increases, the color change from red to green becomes more vivid. The breathalyzer compares the degree of change with liquid that has not undergone the same reaction and generates an electric current that causes the needle in the meter to move. The breathalyzer is calibrated so that the movement of the needle corresponds to the BAC.

Challenging a breathalyzer BAC measurement

As can be seen from this brief summary, a breathalyzer test can be very complex. Even an expensive breathalyzer can malfunction and provide an incorrect reading. Anyone who is facing a DUI charge based upon a breathalyzer reading may benefit from consulting an experienced DUI defense attorney. A knowledgeable attorney lawyer can review the test procedures and other evidence and spot holes in the prosecution’s case.