Henry’s Law and Breathalyzers?

Filed under: DUI by Contributor @ February 20, 2016


You have been pulled over for suspicion of driving under the influence of alcohol. Rather than taking you to the local hospital for a blood alcohol test, the officer decides to administer a breath test using one of the devices approved for testing in the Commonwealth of Pennsylvania. After the second test, the officer informs you that you are over the legal limit for driving and informs you that your blood alcohol content is .085, just over the limit of .080. While you did have a few drinks, you do not believe that you had enough to push you over the limit. So why have your results shown that you are over the limit? One reason may have to do with Henry’s Law.

Henry’s Law is a relationship between the concentration of dissolved gasses in a liquid to the concentration of the air above the liquid, or the saturation ratio of alcohol in the air in which the gas is contained. This is easy to understand when you think of how a person smells a scent such as tomato sauce when someone is making dinner. The pressure above the sauce is constant, the heat is constant, and the sauce is in direct contact with the air in which the scent is traveling. As the gas escapes the sauce, the saturation ratio in the air increases and the smell of dinner becomes perceivable.

Henry 2The proportionality constant between the liquid and air concentration in Henry’s Law is the partition ratio. The partition ratio varies for different liquids and is strongly temperature dependent. A breath testing instrument assumes a partition ratio of 2100, and that value is used for calibration of breath testing instruments and is considered the equilibrium ratio of alcohol between a liquid and air.

Variations in temperature have a profound effect on partition ratios. An average human being has a body temperature of 98.6 degrees Fahrenheit, but that can vary by as much as 1.8 degrees. This temperature variation may not seem significant to us most of the time; however, a 1.8 degree temperature increase can increase the partition ratio and increase breath alcohol concentration by as much as 6 1/2 per cent. This means that a breath concentration level of .080 can rise to a level of .085 with a body temperature increase of 1.8 degrees. Interestingly, it is normal for a person’s body temperature to vary by 1.8 degree throughout the day, and women normally experience a 1.8 degree variation during their menstrual cycle. Other factors can increase a person’s temperature including disease, physical trauma, or emotional trauma.

Blood hematocrit also significantly influences the proportion ratios of breath alcohol concentrations. Hematocrit is the amount of blood that is cellular and normally varies. When alcohol dissolves in the blood, it tends to go into the plasma, which is more watery, more than in the blood cells. Individuals with a higher hematocrit have a lower partition ratio and a higher breath alcohol concentration.

Currently, breath alcohol testing does not require measurement of either blood hematocrit or body temperature. For these reasons and more, testing an individual’s breath is far less accurate than testing a person’s blood. If you have been charged with driving under the influence of alcohol after the results of a breath test, schedule a consultation with one of our experienced attorneys. These experienced attorneys know best what challenges to raise to the tests that were administered.


Leave a Reply

Required fields are marked *

Or contact me privately:

(215) 997-1000