If you've been out drinking with friends at a restaurant or bar and are afraid of getting a DUI, there are a handful of devices that can be found in drug stores or your local watering hole that purport to calculate your blood alcohol level. They range in size from phone apps, to single-use straws you can carry in your purse or pocket, to free standing "breathalyzers" installed in bars. A driver with a BAC of 0.08% or higher can be arrested for DUI if they are over 21. Drivers under 21 may not have a BAC of over 0.02%.
How do they work?
Overall, self-serve breath tests are a clever idea. But they don't work very well, and I think they encourage irresponsible drinking. How?
When you breathe into a breathalyzer, it estimates your blood alcohol content level for that moment. What if you have another drink? What if you don't drive for another half hour? Your level changes over time-- it may go up as well as down.
Are they accurate?
Do you know who calibrates the machine or device? How can you be confident of the results?
Does the machine give you a false sense of security? If you've had a drink and are worried about being over the limit, just don't drive. Machines can be wrong.
What stops a person from using the machine, seeing that he is over the limit, then driving anyway?
Should you use them?
These types of alcohol readers also encourage "power drinking" in my opinion. I've seen folks blow into one, then head back to the bar for more shots to see how high they can blow. I have come to believe that these types of gizmos are worse than useless. No alcohol detector can prevent someone who's drunk too much from driving, but that doesn't stop inventors from thinking up new ways to prey on our fears about being stopped by the cops on our way home. It's a better practice to think of these devices as novelties and let your designated driver take you home.