The breathalyzer, actually called an Intoxilyzer in Texas, works by having a person blow some of their air from their lungs into a machine, which shines a light through a sample chamber, it’s a little piece of pipe, to see how much light is absorbed by what they assume are alcohol molecules. Then, the reduction in that amount of light that gets through the other end of the tube tells them how much alcohol is in that sample.
But, it’s important to understand that that sample chamber is very small; it takes a very minuscule amount of alcohol molecules to equal 0.08 or greater in that sample because it only takes 0.00003 grams, that’s three one-hundred thousandths of a gram of alcohol, in that sample chamber to equal 0.08 for the test that they’re administering, because it’s based on a larger sample of air.
So, it works by assuming that there’s a correlation between how much is in that sample chamber and how much would be in 210 liters. But, the test is widely inaccurate and I could give you a 30-minute lesson all of the ways that it’s inaccurate, but your eyes might glaze over and your head might start spinning so we’ll pass on that right now.