California, United States of America
The following excerpt is from People v. Cazares, H044099 (Cal. App. 2018):
Under the Vehicle Code, drivers lawfully arrested for driving under the influence must submit to chemical testing to determine their blood-alcohol level. ( 23612, subd. (a)(1)(A).) "Whereas a blood test directly measures the subject's blood-alcohol level, a breath sample must be converted to derive a blood-alcohol percentage. The conversion factor, known as a 'partition ratio,' reflects the relationship between alcohol measured in a person's breath and alcohol in the blood. Breath-testing machines in California incorporate a partition ratio of 2,100 to 1, which means the amount of alcohol in 2,100 milliliters of breath is considered equivalent to the amount of alcohol in 1 millimeter of blood. It is undisputed, however, that partition ratios can vary widely, both in the general population and within an individual." (People v. McNeal (2009) 46 Cal.4th 1183, 1188 (McNeal).)
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