Weaknesses in the accuracy of the Intoxilyzer 5000

In our last blog entry, we discussed basic temperature issues with the Intoxilyzer 5000. The machine possesses a so-called electronic “eye” that searches for alcohol molecules in the breath sample. The data is sent to a microprocessor which then extrapolates the amount of alcohol content based on two hundred ten (210) liters of air. Remember that the basic formula is an alcohol concentration of 0.080 in 100 milliliters of blood which is supposed to correlate to 210 liters of breath – i.e. a BrAC formula of 0.080 g / 210L . Of course, the machine is nowhere near 210 liters and in fact the sampling chamber inside of the machine only fits 81 milliliters of air – less than the size of soda can.

The microprocessor contains a computer program, but the technical supervisor knows nothing about the actual program. CMI, the manufacturer of the Intoxilyzer 5000, hold the program as a trade secret. When CMI delivers the Intoxylizer to the technical supervisor, the computer program is already on the machine and technical supervisor cannot access the program. Nor is the program ever updated such as we typically expect for GPS or other programs that get internet updates. Once the technical supervisor takes possession of the machine, there is an assumption that the mathematical program used to calculate the breath alcohol concentration is accurate. A San Antonio DWI lawyer would point out to a jury that he cannot employ an independent computer expert to verify the accuracy of the program – that is a distinct advantage for the State and one that should make jurors wonder. If the manufacturer is the only entity that knows anything about the computer code and there can be inherent problems with computers, jurors will wonder.


The next key issue is a scientific principle known as retrograde extrapolation. According to Texas DUI law , the DA has to prove the driver was intoxicated at the time of driving. Yet the test on the breath machine reflects a breath alcohol concentration at the time of the breath test, not at the time of driving. Adding up all the time for the actual stop, conducting the field sobriety tests , the arrest, hauling away the driver’s vehicle, transport time to the precinct, reading of Miranda warnings, and the fifteen (15) minute observation period, more often than not at least an hour has passed from the time of driving until administration of the breath test. The passage of time then begs the question as to what was the breath alcohol concentration at the time of driving.

The DA often uses the technical advisor to give testimony as to the breath alcohol concentration at the time of the stop, using principles of retrograde extrapolation. There are multiple factors that go into retrograde extrapolation including the amount of alcohol consumed, type of alcohol consumed, rate of consumption, when it was consumed, how much food if any was ingested, the type of food ingested, when it was ingested, the last time the driver slept, and the length of sleep. Other factors include height, weight, and other physical conditions that could impact retrograde extrapolation.

There are several Texas DUI cases which discuss retrograde extrapolation. In Mata v. State, the Court of Criminal Appeals ruled that unless specific factors are analyzed properly, any calculations as to the breath alcohol concentration at the time of driving are not reliable. In Stewart v. State, the 4th Circuit Court of Appeals in San Antonio ruled that a breath test taken more than eighty (80) minutes after a stop would make retrograde extrapolation calculation inadmissible. The Court of Appeals reasoned that it would be impossible after 80 minutes to determine if the defendant was absorbing alcohol (meaning the breath alcohol concentration would be on the rise) or eliminating alcohol (meaning the breath alcohol concentration would be falling).

A San Antonio DWI lawyer will ask the court prior to trial to prevent the DA from using or suggesting retrograde extrapolation. Generally, the police ask the driver a series of questions designed to get information to do a retrograde extrapolation calculation. If the driver answers questions regarding retrograde extrapolation, then a San Antonio criminal defense attorney will need to employ an expert to challenge the calculation.

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