When a person drinks alcohol, some of that alcohol is breathed out as the body metabolises it. This can be measured using a breathalyser and it gives a pretty accurate indication of how much alcohol is in a person’s blood. The more alcohol in your blood, the more affected you are by it – slower reactions, more difficult to concentrate, and so on.
Breathalysers are used because driving with excess blood alcohol, or ‘drink driving‘ causes significant social harm. They provide the first line of evidence (backed up with an evidential blood test) that a person is driving drunk, so that that person can be prosecuted.
There are three ways that breathalysers work:
- Infrared spectroscopy: Infrared light waves from a broadband infrared beam passes through a sample chamber onto a spinning filter wheel. IR light is partially absorbed by the molecules of alcohol. If you can figure out how much is absorbed, you can work out the blood alcohol content.
- Fuel cell sensors oxidise the alcohol in the sample. As it’s oxidised, acetic acid, protons and electrons are given off. The electrical current from the electrons is measured and that gives the blood alcohol content.
- Semiconductor oxide-based testing uses an ethanol-specific sensor. The sample is bubbled through one of two glass vials of the chemical reagent. The difference in the two samples is then compared to determine the BAC.
How do you use a breathlyser?
A basic test may be conducted with the police officer holding the unit while you speak towards it. This gives a quick indication of zero alcohol, some alcohol (but below the legal limit) and some alcohol (possibly above the legal limit). If you register the latter, you’ll be asked to use a calibrated detection unit which will involve you blowing into a tube until enough breath can be collected.
When is a breathlyser used?
If a police officer stops you and suspects you have been drinking, you may be asked to provide a breath alcohol reading using a breathalyser. If you fail that test you will be required to go to a police station with the officer and take an evidential breath test or blood alcohol test.
If you refuse the test you can be convicted of an offence that is the equivalent of being convicted with a BAC above the legal limit.
What affects a person’s BAC?
A person’s blood alcohol level is affected by several factors:
- How much food they have eaten recently
- What type of food they have eaten recently
- How much liquid other than alcohol they have consumed
- How much alcohol by volume they have consumed
- The amount of body fat they carry
- Their metabolism
- Their gender
- Their genetics
- Whether the alcohol was fizzy or flat (alcohol in fizzy drinks is absorbed quicker)
- Whether the person is menstruating.
How quickly can your blood alcohol concentration become over the limit?
If you drink concentrated alcohol on an empty stomach, you can be over the limit in as little as 10 minutes.