Intoxilyzer Math - Reducing a Breath Test

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


You've 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

state's breath test machine and the test slip indicates an alcohol concentration of .14. You're smack dab in the middle of "no man's land." The test result 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 result 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 display .000 during an air blank unless the remaining alcohol concentration in the sample chamber is .01 or higher at which time the machine's display will indicate an "ambient air failure."

The principle behind the breath test machine is Henry's 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. Thus, the machine's algorithm is the concentration of alcohol in one's breath is the same as it is in one's 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 mixture of distilled water and alcohol. The headspace gas in the simulator is sucked through the Intoxilyzer

during a breath test to check the machine's calibration. This calibration check is performed between blows.

The alcohol mixture 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 core body temperature is about 37 degrees centigrade (98.6 degrees Fahrenheit). Therefore, 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. Thus when the accused blows into the machine his breath sample is heated before the machine

measures the alcohol concentration in that sample.

Now it's 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. Therefore we can subtract .02 from our test score and still have

a valid test, leaving us with a .12.

Since the machine will display .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 result 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 state's expert witness will testify that for every degree you raise the simulator solution, you increase the alcohol concentration by 6.5%. Since core body temperature (the temperature at which the

blood-oxygen transfer 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


Thus, a breath test result of .14 has been reduced to .07 (the machine lops off the third digit), below the state's legal limit of .08 percent. This method is enough to raise reasonable doubt in the mind of the

right juror.

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