Do Ignition Interlock Devices Tell Me My BAC Level?

Yes. Ignition interlock devices measure your BAC (breath alcohol content) level, also known as BrAC, each time you blow into the device. They do not measure your blood alcohol content directly. Only a blood test at the hospital can measure that exactly, by drawing a blood sample. But a breath sample can give an accurate indication of what your blood alcohol level is.

Through the beauty of science and technology, ignition interlock devices tell you your BAC level, each time you blow into the device. They do this based on a scientific formula known as Henry’s Law. In a nutshell, Henry’s Law says that “the concentration of gas (in this case alcohol) dissolved in a liquid (your blood) is proportional to its concentration in the air directly above the liquid.”

In other words, the amount of alcohol that is in your breath is directly proportional to the amount in your blood. So, the more alcohol in your bloodstream, the higher your breath alcohol content.

An ignition interlock device (IID) is highly sensitive to alcohol on your breath, and the ALCOLOCK ignition interlock devices use alcohol fuel cell sensors to provide you with the most accurate results. When you blow into the device, it can detect the smallest amount of alcohol on your breath, and prevent you from starting your vehicle if it detects alcohol. By the same token, if you blow into the device and there is no measurable alcohol on your breath, you can start your car like you normally would.

BAC Laws and Your Ignition Interlock Device

In all 50 states, if you are pulled over and you have a BAC of .08 or more, you will be charged with DUI / DWI. If you are under 21, you will be charged even if your BAC is less than that, because you are under age.

If you’ve been convicted of DUI and ordered to install an ignition interlock device in your vehicle, you should know that with an ignition interlock device, the permissible level of alcohol in your blood is far, far less. In most states, you may only have .02 or .025 alcohol concentration and still pass your ignition interlock test. If you voluntarily install an ignition interlock device in your vehicle, you can use it to reliably tell you if your BAC is high enough that if you were stopped, you would be charged with DUI.

And remember, alcohol can stay in your system for hours after you have stopped drinking. So if you were drinking heavily last night, you could fail your IID breath test this morning when you go to start your car to head to work. One standard drink will generally stay in your bloodstream (and therefore, on your breath) for one hour. So if you were drinking until the wee hours, you can see how alcohol could still be in your system when you go to start your car at 7:45 a.m. Either find another ride to work, or be prepared to fail the ignition interlock device breath test.

But Wait, I Haven’t Had Anything to Drink in 24 Hours and Still Failed the Breath Test

We did say the ignition interlock devices are very sensitive to alcohol. That’s alcohol from any source, not just alcoholic beverages. We recommend swishing your mouth with water before taking the breath test to remove any food particles or residue. We also recommend avoiding eating or drinking several things while you have an ignition interlock device installed in your vehicle. These include:

  • Mouthwash
  • Cough syrup
  • Vanilla extract or run extract
  • Foods with alcohol, such as rum cake or pasta with vodka sauce
  • Energy drinks such as Monster or Red Bull that contain alcohol

If you haven’t been drinking, but you fail the IID breath test, you will want to alert your ALCOLOCK service center, your monitoring agency, and your DUI lawyer.

ALCOLOCK Is On Your Side 24/7

At ALCOLOCK, our goal is to see you succeed in your restricted driving program. We believe the more you know, the better. If you ever have any questions regarding your ignition interlock device, you can check out our FAQs page, or call to speak with one of our technicians. You can also ask your technician any questions you might have at your monthly maintenance visit.

If you need to schedule an ignition interlock device installation, you can call us at 866-700-9300, or visit our locations page and call the service location nearest you. You can also use our contact form. Generally, we can get you back on the road with an IID installed in your vehicle within 48 hours of your call.