Every breath test result depends on a number you never agreed to. To turn the alcohol in your breath into a blood-alcohol reading, the Intoxilyzer 8000 multiplies by an assumed blood-to-breath ratio of 2,100 to 1. That ratio is treated as a constant. It is not.
As an ACS-CHAL Forensic Lawyer-Scientist who sits on the National College for DUI Defense (NCDD) National Task Force on alcohol and drug testing devices, I take this assumption apart, because for many people it is the single reason a reading lands over the limit.
What the Partition Ratio Is
When you drink, alcohol moves from your blood into the air in your lungs at a rate that depends on your physiology. The partition ratio is the conversion factor between the two. A breath machine cannot measure your blood. It measures your breath and multiplies by an assumed ratio to estimate what your blood would show. Florida’s machine uses 2,100 to 1 for every person who blows into it.
The machine multiplies your breath alcohol by a single number, 2,100 to 1, to estimate your blood alcohol. Real people fall across a wide range. When your true ratio sits below 2,100, the machine reports more alcohol than you have.
Do not miss this
A breath reading at or above 0.08 also starts the 10-day license clock.
A breath result over the limit triggers an administrative suspension on top of the criminal case. You have 10 days from the arrest to demand a formal review hearing with the DHSMV in Clearwater, which protects your license and can secure a 42-day permit. We file that request the same day you hire us. Call or text (727) 761-4318.
Real People Are Not Average
The 2,100 to 1 figure is a population average chosen decades ago. Real ratios vary widely. Dr. Kurt Dubowski, one of the most cited researchers in the field, found that the true average is closer to 2,375 to 1 and that individual ratios run from roughly 1,100 to 1 up to 3,200 to 1. His research suggests that for as many as 14 percent of people, a breath reading can overstate the real blood-alcohol level by as much as 55 percent. Breath temperature, breathing pattern, red blood cell volume, sex, and the timing of your last drink all move the ratio. When your true ratio sits below 2,100 to 1, the machine reports a blood-alcohol level higher than the one you have.
It Even Changes Within You
Your own ratio is not fixed. It shifts as you move from absorbing alcohol to eliminating it, and it changes with your breath temperature. A reading taken while you are still absorbing, with a slightly elevated breath temperature, can sit well above your true blood-alcohol level. This is one reason the timing of the test matters so much, which is the heart of rising BAC and breath temperature.
Why This Matters in Your Case
A jury can understand this. The State’s number looks exact, and the science underneath it is an average applied to a person who may be nothing like average. Showing the jury that the 2,100 to 1 assumption may not fit you, with the records and the testimony to back it, turns a hard number into a real question.
Where 2,100 to 1 Comes From
To turn a breath measurement into a blood-alcohol number, the machine assumes a fixed relationship between the alcohol in your breath and the alcohol in your blood: 2,100 parts in the blood for every 1 part in the breath. That figure is a population average chosen decades ago to be conservative. It is baked into the instrument and applied to everyone, from a small person who drank on an empty stomach to a large person who ate a full meal. The machine does not learn your ratio. It assumes it.
Studies put real ratios across a broad band. When your true ratio is below 2,100 to 1, the machine converts your breath into a blood number that is higher than your actual level.
Your Ratio Is Not the Average
Real blood-to-breath ratios vary widely from person to person and even within the same person across a night. Published values range well below and above 2,100 to 1. The direction of the error matters: when your true ratio is lower than the assumption, the machine overstates your blood-alcohol level, because it credits your breath with more blood alcohol than it represents. A great many ordinary people carry a ratio under 2,100 to 1, and for them the assumption does not run neutral, it runs against them.
What Pushes a Reading Up
Body temperature, breathing pattern, the phase of absorption, and individual physiology all shift the real ratio away from the assumed value. That is why the partition ratio rarely stands alone. It travels with breath temperature, breathing pattern, and the margin of error, each of which can move a borderline reading. We present the assumption for what it is, a one-size-fits-all number that the science says does not fit everyone.
The partition ratio is the assumption the whole breath number rests on, and it is one I know how to take apart. The machine did not measure the ratio in your body, it borrowed an average and applied it to you, and the research says that average is wrong for a real share of people by a wide margin. I put Dubowski’s actual numbers in front of the jury, the true average, the range, and the share of people the machine overstates, and I make the State own the fact that it convicted a person on a value it never measured.
I started out as an Assistant Public Defender in Florida’s Thirteenth Judicial Circuit, in Tampa, and today I am one of six ACS-CHAL Forensic Lawyer-Scientists in Florida, with forensic training in how these instruments work. A breath result is an estimate produced by a machine, and I read its calibration history, its agreement checks, and the assumptions built into the number, so I can show a jury where it does not hold up. Learn more about my background.
Questions About the Partition Ratio
What is the partition ratio in a breath test?
It is the conversion factor a breath machine uses to turn the alcohol in your breath into an estimated blood-alcohol level. Florida’s Intoxilyzer 8000 assumes a ratio of 2,100 to 1 for everyone who blows into it.
Is the 2,100 to 1 ratio accurate for me?
It may not be. It is a population average. Real ratios vary from person to person and within the same person. When your true ratio is below 2,100 to 1, the machine reports more alcohol than you have.
What changes a person’s partition ratio?
Breath temperature, breathing pattern, red blood cell volume, sex, and whether you are still absorbing alcohol or eliminating it can all move your ratio away from the assumed value.
Can the partition ratio be used to challenge my breath test?
Yes. When the facts suggest your true ratio differs from the machine’s assumption, that difference can be presented to the jury, sometimes with expert testimony, as a reason to doubt the reading.
Related: the main breath test defense page, how we challenge a breath test, and how the Intoxilyzer 8000 works.
This page is general information, not legal advice, and it does not create an attorney-client relationship. Breath testing in Florida is governed by Fla. Stat. 316.1932 and 316.1934 and the Florida Administrative Code chapter 11D-8. Procedures and rules change, and every case turns on its own facts. Past results do not guarantee a similar outcome.
