It’s quite straightforward how Blood Alcohol Concentration (BAC) is measured from a blood test, but how does a breathalyser or a breath tester do it?
An alcohol breath tester don’t actually measure BAC directly; they measure our Breath Alcohol Concentration, or BrAC. The BrAC reading is then quickly converted to BAC before the breathalyser gives its reading.
BrAC is directly related to blood because alcohol-saturated blood passes through our lungs to provide it with oxygen. Some of the alcohol present in blood after a person has been drinking evaporates as it passes through the air sacs, or alveoli. The alveolar air is expelled through the mouth when the person breathes, and this air is picked up by the breathalyser when a person exhales into it. So in effect, the breath can report the alcohol concentration in the blood.
Learn more about this in our post “What is BAC?”
BAC is usually expressed in grams of alcohol per 100ml of blood or %BAC, whereas BrAC is usually expressed as mg/l.
To convert BrAC to BAC, breathalysers use the golden ratio of 2100:1 based on the blood concentration that evaporates and is expelled in the lungs.
The ratio typically ranges from 2000:1 to 2300:1, but 2100:1 is widely accepted as the standard for most uses.
What the ratio means is that every 2100ml of air in the breath will have the same alcohol content as 1ml of blood in the body.
Once the processor in the breath tester makes the conversion, the BAC is expressed as grams per 100ml of blood, i.e. %BAC.
While BAC is the more widely used term in Australia when referring to our legal drink driving limits, BrAC may be used in many industries or other countries when looking at the results readings on breathalysers.
So while our personal breathalysers are preset in the factory to read in %BAC as it is the most useful for personal use, our industrial or workplace breathalysers can be set to read in both %BAC and in BrAC. Just contact us before you buy one so that we can change the settings for you.
If you’re after a breath alcohol tester, check out the models below: