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More Than You Ever Wanted to Know About How South Carolina’s DUI Breath Test Machine Works

So you get arrested for DUI and it all leads up to the officer handing you a tube connected to a laptop. Then he says, “Blow into it.”

Blow into what? There begins the biggest problem with officers giving you these breath tests: they don’t tell you a single thing about the machine, including how it works. Assuming it does work, that is. And that can be a mighty big assumption.

Here’s more than you’ll ever learn from the officer about South Carolina’s breath test machine.

The First Thing You Need to Know

Whatever your result if you take the test, the machine doesn’t test for the ultimate issue in a DUI case: whether you drove impaired. In my experience, most cases come down to how you looked and acted doing normal things, which is why video is vital to your DUI defense.

If you get charged with driving with an unlawful alcohol concentration, the breath test result is very important, but it can still be defeated.

That’s why we say there’s really no “legal limit” for alcohol levels in South Carolina.

That said, here’s the machine and how it works.

No, South Carolina, There Really Isn't a Breathalyzer 

If you want to amaze your friends at a party (or not), you can tell them the breathalyzer no longer exists in South Carolina. In fact, it hasn’t for decades. The official and only breath test device approved for use in South Carolina DUI cases is the DataMaster DMT. It’s the more updated version of the machine our state started using in 1997. The state began using the DataMaster DMT in 2009.

How the DataMaster DMT Works

In a nutshell, the DataMaster uses “infrared spectroscopy” to arrive at a reading. According to the State Law Enforcement Division (SLED), the agency charged with overseeing alcohol breath tests, alcohol has a unique pattern of absorbing infrared light. The machine shoots infrared light through your breath sample to measure the alcohol in it. Whether that's accurate- or even matters since it doesn't prove impairment- is another matter.

Let me demystify that scientific-sounding stuff for you: this thing works like a grocery scanner. You know, the little red light you see in the self- checkout line that the reads the barcode so you don’t have to wait behind the lady with four children and two grocery carts? Yep, that’s what the state wants to hinge its case on.

Here are the basic steps the machine takes, according to SLED policies and the manufacturer’s owner’s manual:

  1. Purging. The machine clears itself by sucking air through the breath tube and pumping it throughout the machine.
  2. Ambient zeroing. The machine tests that air to make sure there’s no alcohol in it. The machine also sets itself to measure what 0 alcohol is, based on the room air inside it at the time.
  3. Blank test. This automatic test is intended to protect against contaminants within the machine or surrounding air.
  4. Internal standard check. The machine inserts a quartz plate into the infrared scanner path to check for accuracy. The quartz has a known infrared absorption the machine can measure. It compares that reading with the reading done when the machine was last calibrated.
  5. Simulator solution test. Attached to the machine is a mixture of pure water and ethanol. It’s supposedly mixed at a level of .076 - .084 to achieve close to a .08 reading for a pretest verification.  Vapor from that solution is pushed through a tube connected to the DataMaster for testing. If the reading for that is between an acceptable limit of .076 - .084, the test can proceed. Notice the state gets a margin of error with its test that you don’t. Brings a whole new meaning to “close enough for government work,” huh? And by the way, since when is breath ever like pure water?
  6. Ready phase. Anyway, now the machine is ready for a breath sample—if you decide to give one. You have two minutes to do it.
  7. Sample processing. Here’s what happens when your breath goes in it. Your breath goes down the tube and inside the machine, where it enters the sample chamber. The sample chamber is designed for the breath to bounce off several mirrors as it goes through the infrared scanner, allowing the breath to be tested multiple times. By the way, the goal for your sample is to get “deep lung air”—so you can give the most alcohol-saturated sample possible. That's right, the machine is designed to cherry-pick the highest possible reading.
  8. Alcohol detection. The machine “sees” alcohol based on its perceived decrease in the amount of light striking the detector after it passes through breath.
  9. Posttest measures. The machine runs another blank test and internal standard.
  10. Ticket. After all that, it spits out a ticket summarizing the test and its result.

What This Means for You

Even if you get a high reading, the test can create more questions than answers, which can give you a solid defense at trial. It’s especially important to contest the charge if your reading is lower because your defenses are often that much stronger.

If you’ve gotten a DUI, the clock is ticking—you need to take quick action to protect your rights because you’re likely coming up on an important deadline that could cause you to lose your rights forever. Call us right now where you are to set up a question and answer session so you can get to work defending yourself against this charge that can be so damaging to your freedom and reputation. I work hard to make this meeting painless for you. If you’re anxious about it, here’s a video on what it’s like.

Rob Usry
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Rob is a South Carolina personal injury and criminal defense lawyer.

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