Breath testing machines are used nationwide by law enforcement to obtain breath samples from driving while intoxicated (DWI) suspects and analyze the breath sample for alcohol concentration. Henry's Law determines how much volatile chemical (eg, ethanol) present in a liquid will be present in the airspace above the liquid.
The reference sample chamber on police breath machines is designed to create a sample of vapor containing a known amount of ethyl alcohol (ethanol). The reference sample is supposedly useful to verify the accuracy and calibration of the breath testing machine. A reference analysis is always conducted as part of each test to ensure the machine is properly calibrated.
The theory of operation of the reference sample device is based upon Henry's Law. In a closed system the amount of ethanol in the airspace above a liquid is proportional to the amount of ethanol in the liquid. Henry's law applies to closed systems at a given temperature and a given pressure. Police breath testing machines do a good job accurately predicting the amount of ethanol in the reference sample.
However, when the machines are used to predict the amount of ethanol in a person's breath the situation changes substantially. If one assumes the liquid in a closed system is human blood and the airspace is the air in the lungs … even a lay person can quickly recognize critical problems.
Most importantly, the human lung is not a closed system. Pressure in the lungs constantly changes as people inhale and exhale. As the pressure changes in the lungs, so the amount of ethanol in the airspace above the blood in the lungs also changes. Further, the temperature of the system is critical. If the solution temperature is low, the results will be lower. If the solution temperature is high, the results will be higher.
So what's the complication with the police breath testing machines? They assume a constant pressure. They also assume a specific temperature within the system. If the pressure is changing then Henry's Law can only be used to approximate the concentration of ethanol in human breath. Moreover, the machines doe not measure the temperature of the suspect's breath sample. Without knowing the precise temperature the machines can only make assumptions. These assumptions might not bear out in a particular case. If the temperature of the person's breath is different than what the machine assumes it is, then the results obtained will be erroneous.
If you are accused of DWI and provided a breath sample to the police, please contact a good DWI attorney to assist in your defense. In particular, an attorney who understands the science behind these machines and one who can effectively challenge the testimony of the prosecutor's experts.