system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How exactly do brethalyzers work?

How realistic are breathalyzers in determining how impaired you are? For example, if I swished around some liquor in my mouth, spit it out, and then blew- would it register that I have alcohol in my system?
gpt
For the ones which count as evidence in a DUI, there are two mechanisms which measure the alcohol.  One is infrared, the other is electrochemical.

One of them falls for the liquor-swishing, the other guards against it.  Some of hand-held ones are electrochemical and will be duped by mouth alcohol. But those aren't valid for a DUI trial. 

That's why DUIs wait 20 minutes before taking it just as an extra measure.  All the alcohol will be gone by 15 minutes, unless you puke up fresh booze from the stomach.

It's also why the breath sample has to be almost impossibly long, so the infrared mechanism gets the air from the longs and not stuff from the mouth.