How Breath Testing Works
Measuring Alcohol
The scientific principle behind the measurement of alcohol by a breath testing device used for court evidence is straightforward. When a breath sample is blown through the machine, it passes through a sample chamber. That chamber has a source of light at one end and a light-measurement device on the other end. The difference in the amount of certain infrared light from one end of the chamber to the other represents information about a concentration of alcohol.
This difference in light from one end of the chamber to the other is caused by the absorption of light by a carbon-oxygen chemical bond in ethanol molecules as they pass through the chamber. Etanol is alcohol for drinking.
This technique is known as “infrared spectrometry.” Light exists in a broad spectrum, and the machine measures a narrow range of that spectrum relating to ethanol, the alcohol used in beverages. A carbon-oxygen chemical bond on the ethanol molecule absorbs a bit of light as it passes through the sample chamber. The detector on the opposite end of the sample chamber from the light measures the amount of light reaching it. The information gathered is used to calculate a ratio into the grams-to-210 liters (g/210L) format]]
If you want a sense of the history of breath testing, check out the National Safety Council report.
.08 of What, Exactly? Legal limit of .08 in breath testing
Everyone has heard of “.08.” That’s the so-called “legal limit” of alcohol. That number is part of a formula. Please don’t think you need to understand math, but the expression is .08g/210L. Another way to express it is as follows: intoxication is established when at least .08 grams of alcohol is found in 210 liters of a person’s breath.
Don’t get overwhelmed by that. It’s not a hard concept, if you break it into its parts.
First Number: .08 = grams (g) of weight of alcohol
The first part may be puzzling because few people are in the habit of thinking of a liquid like alcohol in a weight like grams, rather than a familiar volume like 12 fluid ounces of beer. You’re also probably having a hard time imagining the weight of .08 of a single gram, but there’s no need to worry.
Let’s start with what we all have seen: a small packet of Sweet ‘N Low or Splenda. If you pour the contents of the packet onto a dark table, you see one (1) gram of white powder. If you divide that pile by half, then divide that pile by half again, then divide that pile by half yet again, THEN remove about 1/3 of what remains, you have about .08 grams. It’s a tiny amount of powder and weighs next nothing. Bottom line: .08 gram is a speck of alcohol.
Second number: 210 = liters (L) of breath
Imagine you are looking for that tiny trace of evaporated alcohol in a 55-gallon drum, which is about 210 liters (L) of volume. It may be pretty hard to imagine because you wouldn’t see the alcohol any better than you could see the air.
Also, no pair of human lungs can blow 210 liters in one breath. Our lung capacity is roughly 6 liters. Since people cannot provide a 210L breath sample, breath test machines are designed to process a small breath sample.
They Add a Dash of Math
They collect data from the sample flowing through the machine, then multiply the amount of breath and alcohol by a formula which can convert the smaller volume of breath to 210L. The exact formula is not disclosed and is considered a trade secret so valuable manufacturers won’t disclose it.
In the previous breath test device Texas used, the Intoxilyzer 5000, the sample chamber was 81 cubic centimeters (also known as milliliters) in volume. That means the largest sample it could possibly see at a time is 81 cc. That volume must be multiplied by 2592 to reach 210 liters. (I do not have access to a reliable source of the 9000’s sample size, or I’d provide the associated multiple.)
Here’s a takeaway everyone should know about breath testing: the slightest imperfection at any stage of measuring alcohol in breath is multiplied thousands of times. As you can imagine, such a result can be false and devastating. This is a systemic problem and realistically cannot be avoided, due to a biological limitation of humans. The state is perfectly willing to take that risk with your freedom, reputation, and life.
What is a breath to blood ratio, and why do we need it?
An ideal objective of a breath testing program is to accurately identify alcohol levels in a person’s breath which correspond to the person’s blood alcohol levels. Alcohol in the blood is the source of alcohol in the breath and the reason for symptoms of intoxication. Thus, trying to ground the process in blood makes good sense.
Establishing a numerical relationship between a breath sample and blood concentration presents more of a challenge than you may realize. For starters, the human population has a broad range of breath to blood ratios, having been detected as low as 685:1 and as high as 3100:1. We know this from almost 100 years of extensive research about the effect of alcohol on humans. Breath to blood ratio also can vary even for the same person at different times.
Quantifying an individual’s true breath to blood ratio would require taking simultaneous samples of breath and blood, then comparing their concentrations. That’s complicated, time-consuming, and expensive. Besides, if you are taking a blood sample, why would you need a breath sample?
You might think that seemed like an unresolvable problem with breath testing. Nope. It’s not a problem at all because laws don’t have to follow the science.
How They “Fixed” The Problem
The dilemma was resolved by simply setting a default “one-size-fits-all” breath to blood ratio for everyone.
All breath test devices approved in the United States use a standard breath to blood ratio of 2100:1. If the ratio is true, an equal amount of alcohol should be in 2100 units of breath and 1 unit of blood.
By programming their machines with that ratio, breath test manufacturers guarantee a sizable portion of the people who blow into the machine will have inaccurate and unreliable results. Law enforcement officials are fully aware of this problem, but breath testing is particularly convenient and easy for the government. It helps them win cases with minimal effort and expense.
To me, the breath to blood assumption in breath testing alone is enough to consider breath testing unreliable and unsuitable for purposes of evidence in a criminal case. Decision-makers for breath testing programs obviously consider it acceptable to create programs they fully know falsely elevate reported alcohol levels for some arrestees.
So, if your innate biology is such that alcohol is overrepresented in your breath, a .08 or more breath test may convict you when your blood alcohol level was truly below a .08.
That’s not to say all people are prejudiced by the assumption. Some will register higher than their true alcohol level, and some will register lower. Which one you are is based on luck of the draw.
Luck of the draw is a terrible way to gather evidence to use against someone in court. It’s even worse when dressed up as if it’s the scientific “truth.” Instead, what we have is built-in injustice to fight.