Five Possible Breathalyzer Flaws

| Mar 7, 2020 |

If a defendant provided a breath sample, the DWI conviction rate is significantly higher. But that’s primarily because of the way the law is written, and not because Breathalyzers are bulletproof. In Texas, if the defendant’s BAC level was above the legal limit, the defendant could be guilty as a matter of law.

So, it’s especially important for a Fort Worth criminal defense lawyer to challenge Breathalyzer results. These challenges are particularly effective in borderline BAC cases, such as a .08 or .09. If an attorney partners with a degreed chemist who explains Breathalyzer flaws to the jury, that’s even better.

Ethyl Group Misreading

Breathalyzers count the number of ethanol particles in a breath sample and use that number to estimate the subject’s BAC level. That process seems rather straightforward, but upon closer examination, there are problems.

Many Breathalyzers count any particle in the methyl family as ethanol. Roughly 70 percent of all breath particles have this molecular configuration.

Another way to look at this issue is that the Breathalyzer’s accuracy works against it. The more particles it reviews, the more particles it counts as ethanol, even if they are something else. So, Breathalyzer techs who crow about the gadget’s ability to examine millions of particles might be eroding the prosecutor’s case.

Acetone Levels

This flaw is an offshoot of the methyl miscalculation issue. Diabetics, smokers, and some other individuals have unnaturally high acetone levels in their bodies. Acetone particles are molecularly similar to ethanol.

This flaw is a good one to present to a jury. Jurors clearly see the problem, even if they do not know a lot about science.


A hematocrit is the volume of red blood cells in human blood. The higher the blood cell count, the higher the BAC level. Most Breathalyzers assume a person’s hematocrit count is 47 percent. But the average hematocrit count is about 45 percent for men and 40 percent for women. Lower counts are common.

Frequently, attorneys have defendants undergo blood tests to determine their actual red blood cell levels. If the actual level varies from the presumed level, many jurors will see the results as inaccurate.

In other cases, it’s best to expose the jury to the theory. Many police Breathalyzer techs do not know what a “hematocrit” level is, or they at least do not know how to explain it to jurors. Such inability makes the tech appear incompetent.


Scientific instruments like Breathalyzers are very sensitive to air temperature. In North Texas, air temperature can vary significantly from one day to the next or even one hour to the next, especially during certain times of year. Yet most Breathalyzers are only calibrated once a month.

The subject’s body temperature is important as well. Generally, every one degree above normal triggers about an 8 percent increase in the BAC estimate.

Mouth Alcohol

Texas law requires officers to monitor suspects for at least fifteen minutes before they provide breath samples. Courts have watered down this requirement so much that the monitoring period is basically a waiting period.

In this case, the broad interpretation hurts prosecutors. There’s usually no way to know if the defendant burped or vomited in the fifteen minutes prior to the test. In these situations, alcohol particles from the stomach flood the mouth, causing an artificially high BAC estimate.

Breath tests are nowhere near 100 percent accurate. For a free consultation with an experienced criminal defense attorney in Fort Worth, contact Herreth Law. We routinely handle matters in Tarrant County and nearby jurisdictions.