When you blow into the chamber, it may give the officer a firm-looking data point, but that Breathalyzer reading isn't set in stone. Breathalyzers fail, just like any device.
As breath testing machines have gained prominence in law enforcement, so has the idea that a positive test means proof of guilt. It's simply a piece of evidence and, depending how it was gathered, it may or may not be admissible.
How a Breathalyzer works
A breath tester is a machine that measures alcohol on the breath while giving a reading of blood alcohol content (BAC). It uses a conversion formula to determine how much you've had to drink. The science is well established but it's how the test is administered that makes the difference between a correct reading and a false positive. The California BAC limit for driving is .08.
As a machine, Breathalyzers are subject to electrical errors, improper technique or contamination. Regular maintenance requires regular calibration by lab technicians. Furthermore, readings can be inaccurate due to "mouth alcohol." Alcohol leaves the mouth 15 minutes after drinking, but mouthwash and certain foods can interfere with readings. A burp or vomiting will also compromise accuracy because it affects this measurement.
Human bodies come in all shapes, sizes and health conditions. A diabetic will have different acetone levels and may be victim of a false positive. There is a growing diabetes epidemic in the country and many with the disease are undiagnosed, meaning many will be mischarged as a result. Low carb diets like the Atkins Diet can have similar effects on acetone levels.
Breath patterns, use of an inhaler, smoking, and the consumption of sugar alcohols (often found in gum) can disrupt a reading and cause additional inaccuracies. While a Breathalyzer is a useful tool, it doesn't account for unique individuals.
In converting breath alcohol to a BAC reading, the Breathalyzer presumes that the conversion ratio from breath to blood is the same for all humans, regardless of body type and health condition. California's state courts will not hear arguments on the science of the tests but, as machines, they are subject to their handler's use.
Numerous police departments have been found using faulty equipment that wasn't properly stored or maintained, leading to case dismissals and retrials. A key issue is the application of the breath test itself.
As noted, an officer needs to observe the driver for 15 minutes prior to a test. Something as slight as a burp can disrupt the results. If you were cited for a DUI but the officer was not closely watching you during that time, the entire test may be bunk.
Solving the problem doesn't mean improper conviction
Drunk driving is a true problem in society and a threat to innocent drivers and passengers. However, the Breathalyzer alone is not proof of guilt. Officers use Breathalyzers in combination with field sobriety tests to gather evidence of impairment. As with any crime, the proof must be indisputable and properly gathered: guilty beyond reasonable doubt.
When administered free of contaminants and properly maintained, a Breathalyzer test can read the intoxication level of an average driver correctly. However, few things in life are this interchangeable and absolute. To prove guilt, the court needs to confirm the variables.