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DUI Issues &Technology bruce on 18 May 2011 09:02 am

3 Physical Reasons Breath Machines Can Overstate Blood Alcohol Level

This post is the first of a three part series on physical reasons breath machines can overstate a person’s true blood alcohol level.

Reason One: Breath Temperature

Most people will realize that measuring a person’s breath to determine their true blood alcohol level is an indirect measurement, whereas measuring the blood is direct.  What is ignored by the government witnesses are the variables among individuals which can change the predetermined conversion factors from breath to the blood.  In other words, the breath machine assumes that all of us have the same physical features, such as body temperature, weight, breathing patterns, etc.  We know this is not true.  This note will explain why and how this matters.

In order to measure a gas which has evaporated from a liquid, we use Henry’s law.  Simply put this scientific rule says that if we know the amount of the alcohol in the blood, and we know the temperature the blood is heated to, then we can use a set formula to determine how much of the alcohol in the blood would evaporate in to the air.  Working backwards, if we measure the alcohol in the breath, we can estimate the alcohol in the blood, BUT WE MUST ASSUME A SET TEMPERATURE.  In state run labs the assumption is that the breath is at 34 degrees centigrade.

Henry’s law dictates that at 34 degrees centigrade, the breath will contain 82.7% of the alcohol in the blood.  How is  this important for most people?

The research upon which the 34 degree premise was predicated was conducted in 1950 by Dr. Harger.  Unfortunately he only used SEVEN SUBJECTS.  More recent research, conducted by several different scientists using HUNDREDS of subjects has determined that TRUE breath temperature is 35 degrees Celsius.  While one degree may not seem like much, using Henry’s law, for every ONE DEGREE increase in breath temperature, the results will OVERSTATE the true blood alcohol by about 6%.

Standing alone this may not seem like much, but when added to the other physical factors I will discuss in upcoming entries, the breath can be as much as 35%.


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