Intoxilyzer Math – Reducing a Breath Test Score in Four Easy Steps

Youve got a client charged with driving while intoxicated. There are no bad driving facts to speak of – just changing lanes without signaling. You have a decent video. But your client decided to blow into the states breath test machine and the test slip indicates an alcohol concentration of .14. Youre smack dab in the middle of no mans land. The test consequence is too high to argue accuracy but too low to ask the jury whether they believe what they see or what a machine says. What do you do?

My suggestion is to use some Intoxilyzer math.

When a motorist arrested for drunk driving is asked to submit a breath sample on the Intoxilyzer 5000 (the machine of choice in Texas), the machine operator will have him blow into the machine twice. In order for the test consequence to be valid, the two scores must be within .02 of each other. In other words, if the first blow registers a .10, the second blow must be somewhere between .08 and .12 for the test to be a valid test.

At four different times during the test the Intoxilyzer will suck in outside air in order to purge the sample chamber of any residual alcohol. During these air blanks, the machine will read .000. The machine is programmed to characterize .000 during an air blank unless the remaining alcohol concentration in the sample chamber is .01 or higher at which time the machines characterize will indicate an ambient air failure.

The rule behind the breath test machine is Henrys Law which states that, under pressure, the concentration of alcohol in a known amount of liquid, will be the same in the headspace gas as it is in the liquid itself. consequently, the machines algorithm is the concentration of alcohol in ones breath is the same as it is in ones blood since air and blood mingle in the lungs.

Attached to the back of the Intoxilyzer is a device known as the simulator. Inside the simulator is a combination of distilled water and alcohol. The headspace gas in the simulator is sucked by the Intoxilyzer during a breath test to check the machines calibration. This calibration check is performed between blows.

The alcohol combination in the simulator is mixed at either a .08 or a .10 concentration and is heated to 34.2 degrees centigrade (about 94 degrees Fahrenheit). In order for the breath test to be a valid test, the machine must report the alcohol concentration of the simulator solution within .01 percent of the actual concentration.

One problem with this procedure is the simple fact that our chief body temperature is about 37 degrees centigrade (98.6 degrees Fahrenheit). consequently, the alcohol sample the machine is using to check its own calibration is cooler than the actual breath specimen. If we were to raise the temperature of the simulator, molecular activity in the liquid solution would increase and more alcohol particles would be expelled into the headspace gas. This would raise the alcohol concentration of the headspace gas.

In addition, the breath tube and sample chamber in the Intoxilyzer are heated to prevent condensation inside the machine. consequently when the accused blows into the machine his breath sample is heated before the machine measures the alcohol concentration in that sample.

Now its time to do some Intoxilyzer math.

If we take our .14 breath test from the example above, we know test is considered valid as long as a second blow falls somewhere between .12 and .16. consequently we can subtract .02 from our test score and nevertheless have a valid test, leaving us with a .12.

Since the machine will characterize .000 during an air blank as long as the actual alcohol concentration is under .01, we need to subtract another .01 from our test score, reducing it to .11

Next we can deduct another .01 from our score as a consequence of the allowable margin of error for calibrating the machine using the simulator solution. This reduces our breath test score to a .10.

Under oath the states expert observe will testify that for every degree you raise the simulator solution, you increase the alcohol concentration by 6.5%. Since chief body temperature (the temperature at which the blood-oxygen move in the lungs takes place), is 3 degrees centigrade higher than the temperature of the simulator solution used to calibrate the machine, we can reduce our score by another .027 percent (19.5% X .14).

consequently, a breath test consequence of .14 has been reduced to .07 (the machine lops off the third digit), below the states legal limit of .08 percent. This method is enough to raise reasonable doubt in the mind of the right juror.

If youve been wrongly arrested for DWI, you need an experienced DWI attorney to defend your rights and restore your reputation. Contact Houston DWI Attorney Paul B. Kennedy for more information and a free consultation.