Alcohol Facts by National Draeger
The principles that the simulator incorporates in its design have been accepted concepts for many years. The basic law involved with the theory of this system was discovered in 1803 by William Henry, a British Chemist. Henry's law is stated as follows:
"When the water solution of a somewhat volatile chemical compound is brought to
equilibrium with air, there is a fixed ratio between the concentration of the compound in the air and its concentration in the water, and this ratio is constant for a given temperature and atmospheric pressure."
This same physical law applies to the human body between the concentration of alcohol in the blood and its counterpart in the breath. The simulator was designed to simulate this effect and give the breath test field a tool for instrument calibration.
The temperature of the simulator is elevated to 34 C ± .2 C which is directly related to the temperature of a breath sample as it leaves the mouth of the subject. So by heating the solution up to this specific temperature and maintaining it there, and by knowing the concentration of alcohol which is in the solution, we can pre-calculate the amount of alcohol which will be in the head space above the solution. By introducing a direct alcohol-free breath sample into the simulator and forcing the head space air into the Breathalyzer, we can predetermine what the reading of the instrument should be.
The first device which was widely used for this purpose was known as the Equilibrator, invented by Dr. Polla N. Harger in the 1930s. The same concept and principles hold for this device as we are using currently with the simulator except for one change. The Equilibrator did not maintain a constant temperature and was operated at room temperature. Room temperature varies greatly from minute to minute. The temperature had top be closely monitored so a calculation could be made to alter the results to the equivalent of 34 C. The Equilibrator was attached to a Breathalyzer and room air was pumped through the solution. The temperature would be monitored at that time. The Breathalyzer would go through its procedures and indicated a result. The result would then go through a mathematical equation to calculate back to the equivalent of 34 C. The Simulator eliminates a lot of work on the part of the technician or operator, makes the process much more accurate and expedites the entire process.
The solution which is placed in the simulator can be of a varying concentration. At no time should the concentration be greater than 0.40% because the system is designed to measure only up to the maximum scale limit of 0.40%. The accuracy of any analysis completed over 0.40% would greatly increase re-establishing a balance.
If we take a look at the concentration of alcohol in the aqueous phase and the concentration of alcohol in the vapor phase, we will find that there is a difference. This difference is dependent upon the ratio which is controlled by a specific temperature and atmospheric pressure. For example, if we prepared a solution which would deliver a 100 milligrams percent (0.10%) vapor sample, the concentration of the aqueous phase must be 120 milligrams percent )0.12%). This relationship holds true throughout the entire spectrum of concentrations. If you are ever questioned about this situation, you must explain that you are measuring the vapor concentration of ethyl alcohol, and not directly measuring the concentration of solution.
