The crime of DUI in every state is committed by operating or being in control of a vehicle with a BAC of .08 or more. There are three ways to measure alcohol in the blood: A test of breath, blood or urine. A blood test is obviously the most accurate because the results do not have to be converted from either breath or urine content to blood content. A Breath test is the one favored by law enforcement and is also the least accurate. This is because the level of alcohol in a breath sample converts into blood alcohol content in different ratios for different individuals. This is called the partition ratio. The only practical and expedient way to quickly convert breath alcohol content to blood alcohol content is to take an average of lots of people and use that average for everyone. The average used by most if not all breathalyzers is 2100 to one, i.e., 2100 units of alcohol in the blood for every unit found in the breath. The machine automatically [but not always accurately] counts the number of units of alcohol in the breath sample [using a technology called "infrared spectroscopy”] and multiplies that number by 2100 to determine the number of units in the blood, then converts that to a percentage, i.e., BAC.
What if you are not average? You probably aren’t because almost nobody is. Likewise, the temperature on any given day is almost never the same as the temperature that is said to be “normal temperature for this date” by the television meteorologist. What they call normal is an average.
The alcohol partition ratio shows wide variances among individuals. Actual ratios vary from as low as 900:1 to as high as 3500:1; if a particular individual’s ratio is different, the BAC result will be different. For someone in the low range, say 1500, 10 units in the breathe would equate to 15,000 units in their blood. But the machine will multiply the breath by 2100 coming up with 21,000 units in the blood, a 40% increase. If that results in BAC percentage of .09, which is over the legal limit, that person’s actual BAC would be .064, within the legal limit.
So why can’t you get a certification of the your individual partition ratio and use that to challenge a charge of being over the limit? Your individual partition ratio may not be constant for one thing. But also because every DUI charge involves two separate criminal charges. One is for DUI, the other is called DUI per se. You can challenge the DUI charge because the prosecution must prove impairment to operate or be in control of a motor vehicle. The BAC reading of the Breathalyzer, along with the police officer’s testimony of your demeanor, odor of alcohol, slurred speech, etc., is the evidence used to convict you. Some states will allow you to challenge the accuracy of the machine on the DUI charge. If you do and are successful, that part of the evidence may be thrown out and the case will proceed on the officer’s testimony. You might…win.
On the other charge of DUI per se you will still lose because the charge consists not of whether you were impaired but on whether you blew a BAC of .08 or more. The accuracy of the machine, in many states, is not open to question. In that case, you will be guilty of DUI per se not because of an illegal BAC over the limit, but because the machine said you were over the limit. This is called conviction by machine, much like a red-light camera. It is also called the DUI exception to the United States Constitution. You see, maximizing DUI convictions constitutes a compelling state interest that justifies ignoring constitutional rights. To fully understand this one must, as always, follow the money.
The reason the DUI convictions in San Francisco may be overturned is because it was not the large breathalyzer at the police station that is under suspicion. It is the portable hand-held breathalyzers* the police officer used in the field to take a preliminary breath test. This test is for the purpose of giving the officer probable cause to arrest you for DUI, it is not used as evidence in court. The last remaining viable defense to a charge of DUI is that the police officer lacked probable cause to arrest you. In that case, everything that comes after the arrest is out the window. You get your conviction on both charges overturned. The next step in DUI jurisprudence will probably be to eliminate even this basic right from the Constitution, because DUI convictions are just too important. Then a charge of DUI will be equal to a charge of racism by a white person, no possible defense.
*portable hand-held breathalyzers use a different technology called “fuel cell analysis” which is even less accurate than “infrared spectroscopy” used by the larger machines at the police station. That’s why their results are not, as of now at least, admissible as evidence in court.
Other DUI facts: You don’t need to be driving to get a DUI. Under most statutes the crime is completed by either operating or being in control of a vehicle. Being in control can consist of being in vicinity of the vehicle and having possession of the keys even if the vehicle if parked with the engine not running. If your designated driver is going to drive you home, give him or her the keys before you leave the restaurant. The minute you step into the parking lot you can be detained if a police officer has probable cause to believe you might be impaired. Most cops will wait for you to start the car and perhaps even drive a ways, but they don’t necessary have to do that to make a charge of DUI against you. More likely is the case where you have no designated driver and decide to take a nap in the back seat for a couple of hours to let your body metabolize alcohol. I know of cases where a police office woke someone up, the odor of alcohol was present, they were asked to perform roadside sobriety tests which they failed [everybody fails], and arrested for DUI even though the engine was cold. The arrest and conviction were upheld.
Vehicle has a broad definition. You can be convicted of DUI for riding a bicycle with a BAC of .08 or higher. Also for rowing a motorless boat with a BAC, according to a breathalyzer, of .08 or higher. At least if you are on a public waterway, river or lake where other members of the general public may put in their boats. A good example would be the rowboats typically rented to paddle the lake at a public park or amusement park.