Machines have been created, such as the intoxilazer 5000 and intoximeters 7110, in order to determine the person's blood alcohol thru the examination of alveolar air from the person's lungs. Now as you may have guessed this requires some very complex calculations. It is for these reasons that one should never plead guilty to a DUI charge without first conducting a thorough examination of all maintenance records and quality control information.
Breath tests are the most inaccurate of blood alcohol measurement. A PAS breath machine can be off by many points and can even register alcohol despite the fact that the tested subject has not had a single drink. The breath machine (typically an Intoxilyer 5000) at the police station is off by .02 in either direction. This means that a reading of .08 is just as likely to be a .06 at its most accurate. However, there are also many things that can make a breathalyzer far more inaccurate. Certain food, gum, mouth wash, acid, diabites, reflux disease and others can create an abnormally high and inaccurate reading on a breath machine that can be used as a defense in a DUI case.
ALCO SENSOR 7110 there are benefits and detriments to requesting a breath test over a blood test. First, if you have drugs in your system that can cause impairment (pain medications, marijuana, valium, anti-depressants, etc.), a breath test won't be able to detect them. This is important because you can be charged with a DUI for driving under the influence of drugs, whether legally prescribed or not, if the officer believes that it has impaired your driving. Second, there are more ways of defending a breath test than there are of defending a blood test because of all the molecules that exists that can be measure as alcohol by the machine.
INTOX 5000One detriment to requesting a breath test is that it is almost always readily available, usually within 30 minutes of being arrested. A blood test can take substantially longer because a blood technician must be called in to obtain the sample. A delay in testing can help provide a defense to DMV (a test obtained beyond 3 hours from the point of driving is invalid for DMV purposes). A delay can also get rid of the 3 hour presumption in prosecution of the case in court.