How breathalysers are accurate

[Wai]

I know how a breathalyser works, but someone explain to me how it can give a relatively accurate reading of BAC? Is it the colour coding involved with the chemical reaction?

[Glider]

Yes (see here http://science.howstuffworks.com/breathalyzer6.htm) but they're not that accurate. All they do is provide 'sufficient grounds' for arrest. If you are arrested for being drunk in charge of a vehicle or whatever, the arresting officer will say "I am arresting you on suspicion of...". Further evidence in the form of a blood sample must be taken for prosecution.

[jdurg]

No it doesn't. If the breathalyzer says your BAC is 0.08, which is the legal limit in most states in the US, that's ground for arrest. There is no need for suspicion and they do NOT need a blood sample to prosecute you. There is also no way to "trick" a breathalyzer. The amount of EtOH that comes out of your breath is directly proportional to the amount in your blood. You can't alter that. If you give a result that appears off, then the officer can request that you provide a blood sample to verify it. (And refusal could be grounds for arrest). But if you did something to alter the results (e.g., swish with some mouthwash) the results would be insanely high. The test works on a chemical reaction in which more alcohol in the blood generates more electricity. As much as people, people who deserved to be thrown in jail, have tried to argue, no scientific evidence has come out stating that breathalyzers are incorrect and cannot accurately measure the values of alcohol in a guilty person's blood. The only exceptions they can come up with would require the driver to be in such poor conditions that they'd be in horrible, unable to drive, shape anyway.

[Glider]

I'm not in the US. In the UK, where the BAC limit is the same (80mg/100ml), if a roadside breath test shows you to be above the limit, you will be arrested on suspicion of drink driving. However, you cannot be convicted purely on the evidence of a roadside breath test.

 

Once back at the police station, you would be required to provide a further two samples for the Intoximeter equipment, which is accurately calibrated and is used to provide the evidence of your BAC that is presented in court. The reading that used is the lower of the two samples. At this stage, a refusal to provide a specimen is an offence that is treated in law as the equivalent of being convicted with a BAC above the legal limit.

 

If your breath-alcohol level is between 40 and 49 µg, you will be offered the opportunity to take a blood or urine test as an alternative. If the police fail to offer this alternative they have not applied the procedure correctly and this can be used as a defence in court.