When someone is pulled over for DUI, it is far easier and cheaper to have him blow into a tube than it would be to draw his blood and have that blood analyzed in a laboratory. For that reason, breath testing is very attractive to law enforcement. Earlier this month I wrote about how alcohol breath testing machines generally work. Today I would like to focus on one specific part of that operation, partition ratios.
Alcohol concentration in breath is not equal to alcohol concentration in blood. Rather there is a ratio between the alcohol level in someone's breath and the alcohol in someone's blood. This ratio is called the partition ratio. Based on Henry's law, breath test manufactures assume that all people have a partition ratio of 2100 to 1.
There are, however, a number of factors that can affect an individual's partition ratio. Some of these are body temperature, atmospheric pressure, cell count in blood, physical activity, and hyperventilation. Surprisingly, when police arrest someone for DUI they do not take the person's temperature, or record the local atmospheric pressure. Both of these measurements would be simple to make with a thermometer and barometer. Other factors which might effect a person's partition ratio are not as easily measured.
As I have discussed earlier, it is well documented that breath testing is inaccurate up to 20% in either direction. This inaccuracy is largely because of partition ratios. If a person does not have a partition ratio of 2100:1, the results of the test will be inaccurate. The farther away from that assumed ratio a person is, the more inaccurate the test will be. Of course it is impossible to determine what someone's partition ration was when they were tested. This is because partition ratios not only differ from person to person. They also differ over time for any given person.
