How Do Breathalyzers Work?

[jaci]

I found a neat article that explains how breathalyzers work. You know what a breathalyzer is if you have gotten a DUI, but you may not know what makes them tick. I hope this helps someone.

Alcohol that a person drinks shows up in the breath because it gets absorbed from the mouth, throat, stomach and intestines into the bloodstream. Alcohol is not digested upon absorption or chemically changed in the bloodstream. As the blood goes through the lungs, some of the alcohol moves across the membranes of the lung's air sacs (alveoli) into the air because alcohol will evaporate from a solution -- that is, it is volatile. The concentration of the alcohol in the alveolar air is related to the concentration of the alcohol in the blood. As the alcohol in the alveolar air is exhaled, it can be detected by the breath alcohol testing device. So instead of having to draw a driver's blood to test his alcohol level, an officer can test the breath on the spot and determine whether there is a reason to arrest the driver.

There are three major types of breath alcohol testing devices based upon different principles:

* A Breathalyzer uses a chemical reaction involving alcohol that produces a color change.
* An Intoxilyzer detects alcohol by infrared (IR) spectroscopy.
* The Alcosensor III or IV detects a chemical reaction of alcohol in a fuel cell.

Regardless of the type, each device has a mouthpiece or tube through which the suspect can blow air and a sample chamber where the air goes. The rest of the device will vary with the type.

How is it possible to determine the level of alcohol concentration in the blood by using a person's breath? Unlike milk or fruit juice, the body does not digest alcohol. Rather, it is absorbed through the membranes in a person's mouth, throat, stomach, and intestines. Once absorbed by the body, the alcohol passes immediately into the bloodstream, where it circulates until it is expelled through evaporation in the lungs. Evaporation occurs because alcohol is "volatile" in a solution, meaning that its molecules do not combine with the liquid that it mixes with.

Due to this volatility, as the blood passes through the lungs, some of the alcohol passes over the alveoli (the lungs' air sacs), allowing it to be released by the person's breath. The expulsion of the evaporated alcohol through the breath permits the BAC to be accurately measured, since the percent alcohol being expelled contains the same level of alcohol that is contained in the blood. The amount of alcohol in 2,100 ml of expelled breath is exactly equivalent to the amount of alcohol in 1 ml of blood. With these equivalent measures in mind, it is possible to attain an accurate measure of the driver's intoxication, based on the figures set for legal limits. But how is the concentration of evaporated alcohol in the breath measured?

A breathalyzer consists of a collection device, a "straw" attached to a cylinder. The cylinder contains two vials, which contain a solution of sulfuric acid, potassium dichromate, silver nitrate and water. The individual being tested blows into the straw for approximately two to four seconds. The object is to test the air from deep in the lungs, which will produce the most accurate reading. The expelled air travels into the vials, where the silver nitrate acts as a catalyst to initiate and speed up the analysis. The first thing that happens is that the sulfuric acid removes the alcohol from the air. The sulfuric acid potentially produces the acidic condition necessary for the remainder of the process to take place.

The breathalyzer operates on a similar principle as diabetic testing strips or pool testing kits: a certain level of alcohol produces a chemical reaction that causes a color change on a piece of paper or in a solution. The color changes produced by these testing kits are usually interpreted by comparison to a color chart. This comparison is subjective, based on the observation of the individual performing the test. In the case of the breathalyzer, however, the expelled breath travels through a chemical solution, creating the color change that is interpreted objectively using a device that police officers are trained to use.

The alcohol found in alcoholic beverages is ethyl alcohol (ethanol) . The molecular structure of ethanol looks like this:

H
H 3 C - C - O - H
H

where C is carbon, H is hydrogen, O is oxygen and the hyphen is the chemical bond between the atoms. The bonds of the three hydrogen atoms to the left carbon atom are not shown for clarity.

The OH (O-H) group on the molecule is what makes it an alcohol. So, there are four types of bonds in this molecule:

* carbon-carbon (C - C)
* carbon-hydrogen (C - H)
* carbon-oxygen (C - O)
* oxygen-hydrogen (O - H)

The chemical bonds between the atoms are shared pairs of electrons. Chemical bonds are much like springs. They can bend and stretch. These properties will be important in detecting ethanol in a sample by infrared (IR) spectroscopy .

With all breathalyzers you must wait about 20 minutes after your last alcoholic drink before testing. This is because any alcohol that is left in your mouth after your last drink is at a very high concentration when compared to the alcohol that is in your exhaled breath. Alcohol in your mouth will result in erroneous blood alcohol readings, so you must wait for the mouth alcohol to dissipate before testing.

Professional quality breathalyzers require that you blow for a minimum amount of time at a minimum flow rate to assure that you expel enough air out of your lungs to get to the "deep-lung" air, where the most accurate readings can be obtained.

DUI lawyers will often challenge the results of the breathalyzer test in your DWI defense case.