Police departments around the United States use a machine called the Intoxilyzer 5000 as a breath testing instrument. This macine has been the standard DWI breath test machine for years, and is used by cities across Tarrant County, including Fort Worth and Arlington.

Breath tests are still a huge part of DWI Evidence. If the police are not satisfied with how you did on the field sobriety tests, they will ask you for a breath test. If you say no, they will get a warrant for you blood. For more on blood draws and blood testing, click here and here. for In a DWI trial, the prosecutor must call the person who administered the test as a witness to admit the results. So how does it work?

Henry’s Law

William Henry was an English chemist 200 years ago, but don’t worry about that. Henry’s law is the science that the Intoxilyzer 5000 is based on. Henry’s law states that when alcohol dissolves into the blood, and comes into contact with a closed airspace, a stable ratio establishes between the alcohol in the air and the alcohol in the liquid.

Imagine Filling an Empty Coke Bottle with Whiskey and Coke

Imagine a plastic bottle of coke that you have emptied out and filled halfway with coke and whiskey. Then you twist the cap securely back on it. If you do that, there will be alcohol in the liquid and alcohol in the airspace. Alcohol is volatile, which means that it wants to escape into the air. But it cannot. The coke bottle is a closed system. If the temperature and pressure are also constant, you can measure the airspace to gauge the amount alcohol in the liquid.

This is how the Intoxilyzer 5000 works. Unfortunately, the lungs are not like the coke bottle. Your lungs are constantly changing pressure, and temperature. Your lungs are an open system, not a closed one like the coke bottle. Still, the Intoxilyzer treats your lungs like the coke bottle.

They Test My Breath to Measure My Blood—Wait, What?

Yes. The Intoxilyzer 5000 has determined that for every 210 liters of breath is the equivalent of 100 milliliters of blood. This creates a partition ratio of 2100:1. Unfortunately, this is an assumption. The machine doesn’t know anything about you! Because of the changing nature of the lungs, no stable partition rate exists for any person, and no stable partition rate can be predicted for any person either.

People differ from one another in every way imaginable, and partition ratio is no different. People partition alcohol from their blood at different rates.

Some researchers have argued that the partition rate should be lower, such as 1880:1 or even as low as 1720:1. The majority of the time, the ration assumed by the Intoxilyzer 5000 will be incorrect for any given person.

When You Had Your Last Drink Can Make a Big Difference

The difference between passing and failing can depend on when you had your last drink. When you have a beer or a glass of wine, it takes some time for your body to absorb the alcohol into your bloodstream through your small intestine. When your body is done absorbing the alcohol, it will begin eliminating it through your liver. It can take an hour or 90 minutes to fully absorb alcohol. How quickly you absorb alcohol depends on what you’ve had to eat. Your body absorbs alcohol much faster if you haven’t eaten in a while. You may have felt this if you’ve ever had any alcohol on an empty stomach.

The Intoxilyzer Assumes that Your Body is Done Absorbing Alcohol, but that Isn’t Always the Case

The Intoxilyzer 5000 assumes that you are done absorbing alcohol when you take it, but this is not necessarily true. You may be, but police and the Intoxilyzer have no way of knowing that. If you tell them when you had your last drink and what you’ve had to eat, they have a way of guessing using your body weight, gender—but that’s all they can do. If you are still absorbing alcohol when you take the Intoxilyzer, your partition rate is going to be lower. This means could give a false positive or a falsely high reading! When the difference between testing .07 and .08 can mean getting a DWI conviction, this means everything.