Can you be too drunk to register on a breathalyzer?

Police in Iowa were investigating an accident and suspected the driver was intoxicated. They had him blow into a portable breathalyzer, and the first result was .467. When he blew the second time the machine simply registered "Hi". According to the police, the man was too drunk to register on the breath tester. This was a story that got a lot of attention, with most writers assuming the man "blew up" the machine because he was so drunk.

Apparently, no one has suggested that the the problem was with the machine. It did produce a reading the first time, so why couldn't it produce a reading again. If it's a valid scientic instrument, the amount of alchohol shouldn't effect the reading. You would think someone might suspect a problem simply from the first reading; it was almost 6 times the legal limit of .8. According to the company that manufactures the breahalyzer, .45 is enough to cause death in most people. Yet this individual was walking and moving around.

Most people have no idea how a portable breathalyzer works. It's different from an intoxilzyer, which is the unit that is usually at the jail. Intoxilzyers are not portable, so they can't be  used in the field. That's why officers will use portable breathalyzers. Here's an explanation of a breathalyzer:

The Breathalyzer device contains:

  • A system to sample the breath of the suspect
  • Two glass vials containing the chemical reaction mixture
  • A system of photocells connected to a meter to measure the color change associated with the chemical reaction

To measure alcohol, a suspect breathes into the device. The breath sample is bubbled in one vial through a mixture of sulfuric acid, potassium dichromate, silver nitrate and water. The principle of the measurement is based on the following chemical reaction:

In this reaction:

  1. The sulfuric acid removes the alcohol from the air into a liquid solution.
  2. The alcohol reacts with potassium dichromate to produce: chromium sulfate potassium sulfate acetic acid water

The silver nitrate is a catalyst, a substance that makes a reaction go faster without participating in it. The sulfuric acid, in addition to removing the alcohol from the air, also might provide the acidic condition needed for this reaction.

During this reaction, the reddish-orange dichromate ion changes color to the green chromium ion when it reacts with the alcohol; the degree of the color change is directly related to the level of alcohol in the expelled air. To determine the amount of alcohol in that air, the reacted mixture is compared to a vial of unreacted mixture in the photocell system, which produces an electric current that causes the needle in the meter to move from its resting place. The operator then rotates a knob to bring the needle back to the resting place and reads the level of alcohol from the knob -- the more the operator must turn the knob to return it to rest, the greater the level of alcohoYou.

You also have to  understand how you measure alcohol in your breath. The legal test is based on blood alcohol level, which is the amount of alcohol in the blood. Breathalyzer's don't test blood, so you have to perform a conversion. That conversion is based on an assumption, which is that a millleter of alcohol in the blood is equivalent to 21,000 milliters in air from your lungs. That's an average, which means that applies to some people, and not to others; so if you're in not in the average the reading isn't going to be accurate. There's another concept addressing why alcohol is present in the air from your lungs, but we will skip that for now.

Everyone has covered this story as driver who was so intoxicated the breahalyzer couldn't measure him. It's too bad no one has looked at the angle of whether the failure to get a result wasn't because he was too intoxicated, but because the machine was flawed. Maybe the calculations used by the PBT are simply flawed.

Portable Breath Testings are nothing more than a tool used by the police department. They provide nothing more than an estimate, which is why their results are not admissible in court. Maybe this story should be a reminder of that.

Be the first to comment!
Post a Comment