Three ways Science Impacts the Breath Test


A DWI/DUI defense attorney needs to understand science. We know different volumes of expelled lung capacity, affected by different body temps, at differing pressures of force of blow, will directly affect the number and density of ethanol molecules in the sample tested by the machine.

Avagadro failed the breath test

Avogadro’s law affects the veracity of every breath test conducted. Avogadro’s law states that, “equal volumes of all gases, at the same temperature and pressure, have the same number of molecules”. For a given mass of an ideal gas, the volume and amount (moles) of the gas are directly proportional if the temperature and pressure are constant.

Avagadro’s Law informs us that aside from volume, the primary two variables impacting every breath sample are temperature and pressure. Those three variables are different for every person taking a breath test, which means that every sample will be scored with different criteria. Let’s look at the variables.

Volume of air in the sample.

“Keep blowing. Blow. Blow”

The officer sounds like a cheering squad. And why wouldn’t he? He is cheering for you to go to prison. It is all about the volume of air. Most breath testing machines have a pressure transducer to measure volume — but did you know that the pressure transducer is not an item checked during calibration or regular maintenance service?

Very few law enforcement officers who have been trained on the machine even know what the pressure transducer is and fewer still know where it is supposed to be on the machine. A recent review of the pressure transducer of 231 of Florida’s Intoxilyzer 8000s has revealed that 40 percent of them record the wrong breath volume. Breath volume is a critical factor in determining blood alcohol content based on breath.

Why is volume so important?

The breath test machine use either a fuel cell or an infrared cell, which passed infrared energy through the sample and measures the unabsorbed energy. Ethanol absorbed infrared energy. Then the software calculates the difference, applies a formula known as Henry’s Law and then issues a number.

Henry’s Law states that in a closed system, at any given temperature, the concentration of a volatile substance in the air above a fluid is proportional to the concentration of the volatile substance in the fluid. (For the science minded: Wt. of Alcohol per Volume of Air = K/ Wt. of Alcohol per volume of Water)

This is also described as the partition ratio. The average temperature of breath as it leaves the mouth is 34 degrees Celsius.At that temperature, research demonstrates that 2,100 milliliters of deep lung air contain about the same quantity of alcohol as one milliliter of arterial blood. Accordingly, breath alcohol instruments calculate the amount of ethanol per 210 liters of air. And they are testing for .08 grams of ethanol in 210 liters of air. Ironically, research has shown this partition ratio to be inaccurate in calculating blood alcohol levels.

Research by Dr. Michael Hlastala of the University of Washington School of Medicine found that the average directly measured partition coefficient for alcohol in blood at 98.6 degrees Fahrenheit is 1756:1, not 2100:1. Also he discovered that blowing a volume beyond the minimum required for a breath testing device results in an increased breath alcohol reading and a lower blood-to-breath partition ratio.

More volume = lower partition ratio = higher breath alcohol result.

You got me under pressure

Pressure affects the breath test in several ways. First as mentioned above, pressure is one of the constants required under Avagadro’s Law. Second, changing pressure results in temperature changes, the effect of which we’ll discuss in the next section. Finally, a higher pressure blow — especially toward the end of your lung capacity — significantly increases the chances of expelling droplets of spit or condensed vapor as a result of the rapid cooling from pressure changes between the lungs and machine. Colder air cannot support the same humidity levels.

First, the volume of a gas (210 liters) will have have the same number of molecules at the same temperature and pressure. When you change temperature or pressure you also change the volume or the number of molecules within that volume. The amount of ethanol being searched for in minuscule; so even a small increase in the number of molecules in the sample can have a big impact on the results.

Baby, its hot in hereRemember as we move into the temperature section, that nothing in the system acts independently. A change in temperature must either affect volume or pressure. An increase of only one degree Celsius (1.8 degrees F) can increase test results by 7 percent.

Dr. Hiastala first stated the possibility of a 7% increase in breath-alcohol test results in 1985. Based on Dr. Hlastala’s findings, the partition ratio for alcohol in the blood is affected by the actual body temperature compared to the normal body temperature. A healthy individual may fluctuate 1 degree Celsius (1.8 degrees Fahrenheit) from the average of 37 degrees Celsius (98.6 degrees Fahrenheit).(Physiological Errors Associated with Alcohol Breath Testing, 9(6) The Champion 18.)

Those of us from humid places understand that with heat comes humidity. The warmer the air, the great amount of moister that it can absorb. Think about the dry winter air up North. Temperature and humidity as a result of that temperature can significantly affect the 0.000038095 grams of ethanol per milliliter they need to prove you are over the limit.

Bonus Section: The difference between Henry’s Law and Dr. Hiastala’s research is:

Henry’s Law is based on a partition ration of 2100:1. This is the ratio used by formula in the breath test machines. As we learned earlier, Dr. Hiastala, with the University of Washington Medical School, placed that ratio more accurately at 1756:1

At 2100:1 The legal limit of .08 equals 0.000038095 grams of ethanol per milliliter of air

At 1756:1 The legal limit of .08 equals 0.000045558 grams of ethanol per milliliter of air

The difference between the two equates to the machine testing 17 percent higher than your breath result actually is. Add in a slight increase in body temperature (1.8 degrees) and your breath test result is 24 percent higher than it actually is. There is no way to account for the difference due to increase lung capacity or a higher volume being delivered other than to say, we can comfortably say the possibly increase is at least one percent.

So your breath test may be incorrectly reporting your breath alcohol by 25 percent higher than it should be. Something you know because of science.