How Accurate Are Breathalyzer Tests in Texas?

The Intoxilyzer 9000 — Austin’s Breath Testing Instrument

The Austin Police Department uses the Intoxilyzer 9000 for breath testing in DWI cases. The instrument does not directly measure blood alcohol concentration. It measures alcohol vapor in deep lung air and converts that measurement to an estimated BAC using a fixed 2,100:1 partition ratio — the assumption that 2,100 mL of alveolar breath contains the same alcohol as 1 mL of blood. That assumption is a population average. Research shows individual partition ratios range from approximately 1,700:1 to 2,900:1. For individuals whose actual ratio is higher than 2,100:1, the machine will report a BAC higher than they actually have.

Sources of Intoxilyzer 9000 Error

Partition ratio variation. As described above, the fixed 2,100:1 assumption can systematically overestimate BAC for individuals with higher-than-average partition ratios. This is not a malfunction — it is a built-in limitation of breath testing technology.

Mouth alcohol contamination. Alcohol from belching, regurgitation, recent mouthwash use, or dental work can artificially inflate the reading. The required 15-minute observation period before the test is specifically designed to prevent mouth alcohol from affecting the result. Any gap in that observation period creates a challenge to the result's reliability.

Radio frequency interference. The Intoxilyzer 9000 contains an RFI detector, but interference from police radios, cell phones, and electronic equipment in the testing environment can still affect accuracy.

Medical conditions. Diabetes, acid reflux, GERD, and low-carbohydrate diets can produce compounds that the instrument may read as ethanol. Isopropanol and acetone from these conditions can produce false readings.

Temperature effects. Elevated body temperature (fever) increases the breath-to-blood ratio, causing the instrument to overestimate BAC.

The Maintenance and Calibration Record

Every Intoxilyzer 9000 has a specific maintenance history — calibration logs, solution changes, accuracy checks, and error logs — obtainable through Texas open records requests. We request the complete maintenance file for the specific instrument used in your test. Common findings that support BAC challenge include:

• Missed calibration checks
• Solutions outside acceptable range
• Logged instrument errors near the date of your test
• Lapsed operator certification for the testing officer

Blood Tests Are Not Automatically More Reliable

Blood draws analyzed at the Travis County Medical Examiner’s lab are also subject to challenge. Improper preservative ratios, inadequate refrigeration, and chain of custody documentation gaps can all affect the reliability of the result. In-vitro fermentation — alcohol produced by bacteria in an improperly preserved sample — can produce a result higher than the actual BAC at the time of the draw.

What This Means for Your Austin DWI Case

A BAC result above 0.08 is not a verdict. It is a number produced by a machine operating under assumptions that do not apply equally to every person tested. A prepared Austin DWI attorney pursues the maintenance records, operator certification, and partition ratio analysis on every breath test case. The question is not whether the machine produced a number — it is whether that number accurately reflects your BAC at the time you were driving.