How are Preliminary Breath Tests Calibrated for Accuracy?

stopped for DUI

If you’re on trial for DUI in Virginia, you should know how the preliminary breath test (PBT) must be checked for accuracy and calibrated. If that device can’t be proven to be in proper working order, it shouldn’t be allowed into evidence at trial.

What is the preliminary breath test?

It’s the box that the officer stuck in your face on the side of the road. They tell you to take a deep breath and blow into it.

Officers sometimes tell you that the PBT is just for their use on the side of the road, which is a complete lie. The PBT is a vital tool to prove whether or not the officer can arrest you for DUI.

Do we know it’s accurate?

Just like any scientific test, we can’t blindly trust a box to produce the right measurement. We need to know for sure that the PBT is accurately testing the level of alcohol in your blood by measuring a breath sample.

If the officer can’t prove that it’s accurate, the PBT should NOT be admitted against you at trial. That would be just like an officer trying to testify about a radar reading when he can’t prove that the radar was working properly – not allowed.

How does an accuracy test work?

PBTs are checked for accuracy with a “dry gas standard.” This is a canister of gas that has a known concentration of alcohol in it (typically 0.10%).

Each PBT manufacturer specifies their accuracy test procedures. But essentially the officer must shoot the dry gas standard through the PBT and check that the PBT reading matches the dry gas alcohol value.

If the PBT is off, it must then be calibrated to match the expected value.

Once the PBT is reading the correct dry gas alcohol value, the officer can testify in court that the PBT is working properly.

It’s important to remember that the PBT must have regular, periodic accuracy checks, such as every 30 days. They notoriously fall out of calibration over time, so it must be maintained correctly to be accurate for use.

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Andrew Flusche

My name is Andrew Flusche. I am a traffic and misdemeanor defense lawyer in Virginia. I limit my practice to traffic tickets and misdemeanor defense, so I know the ins and outs of these offenses. I literally wrote the book on reckless driving in Virginia which you can get on Amazon here or download for free here. I opened my practice in 2008 after earning my Juris Doctor degree from the University of Virginia School of Law. Since then I have earned over 600 5-star reviews from happy clients on Google, Yelp, Facebook, and Avvo. If you’ve been charged with a misdemeanor offense in Virginia, please don’t hesitate to contact me. Your initial consultation is always free, and you'll talk directly with me about the details of your case.

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