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How do we know if breath testing machines are accurate?

When someone is arrested for DUI, one thing a defense attorney might look into is whether the breath test was accurate. These tests can be performed incorrectly, and the machines may be uncalibrated or calibrated improperly. Just as important, the machines are manufactured and marketed for profit, with source codes that are proprietary. That means that there could be hidden issues that lead to inaccurate test results.

Recently, the website ZDNet published an investigation into the Alcotest 9510, a commonly used breath testing machine. (The Alcotest 9510 is not the only breath testing machine to be challenged for potential inaccuracy.) The investigation found good reason to suspect the Alcotest 9510, as configured and used by the Washington State Patrol, may be capable of inflating breath-alcohol test results under certain circumstances. The Washington State Patrol's configuration and options are probably relatively common among police agencies.

The machine works by processing the inputs of two main sensors, an infrared beam and a fuel cell. The infrared beam measures how much light can go through the breath being tested. The fuel cell sensor measures the electrical current of the breath.

When two software engineering consultants were brought in to examine the machine's source code, they found that the machine can, under some conditions, return results inflated enough to push a person's test result over the legal limit.

Without getting into too much detail, there were a variety of problems with the algorithms used to correct for common issues during testing. For example, the manufacturer acknowledges that the machine may take inaccurate readings if it is used in an environment that is too hot or too cold, or when the breath being tested is too hot. One issue the consultants found was that a formula in the code meant to adjust for temperature variance appeared to be faulty.

The impact of that faulty formula could be substantial. A single degree centigrade over the normal breath temperature can inflate the test results by as much as six percent. That could easily push a person's results over the limit.

DUI defendants have the right to fully challenge the evidence being used against them. That means that defense attorneys can get hold of the source code for breath testing machines and hire experts to examine it. As in the Washington State case, this is usually done with a protective order maintaining the manufacturer's proprietary code.

It takes time and effort, but challenging breath testing machines' accuracy is becoming a major part of DUI defense across the nation. If you have been charged with DUI, you should discuss this possibility with your defense attorney.

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