If you have been arrested or convicted for DUI, you might be facing an ignition interlock requirement. That means you’ll have device installed in your vehicle which will detect any alcohol on your breath. If the device detects alcohol in an amount that’s above the (quite low) limit, your vehicle won’t start. You’ll then get more chances to test, followed by increasingly longer waiting periods.
As you can see, the only thing you need to worry about is if you’ve had any alcohol to drink if you want to drive. But users worry about other things, and among them: just how accurate are ignition interlocks?
Short answer: very accurate. Ignition interlocks like the LifeSafer FC-200 use state-of-the-art fuel cell alcohol detection (breathalyzer) technology when testing a user’s breath. It’s the proven method used in police breathalyzers, and is considered evidentiary quality.
Longer answer: very accurate. In fuel-cell ignition interlocks, platinum electrodes are oxidized by alcohol, generating a measurable current. The current powers the circuit that either cuts off or permits a vehicle to start. The limit is usually low: for example, in Ontario it’s a breath alcohol concentration (BAC) of .02. The legal limit for drunkenness is .08, and anything over .05 is in the “warn range,” which can result in penalties.
Ignition interlock devices use a formula called Widmark’s Equation* to calculate how much alcohol is in the bloodstream. Widmark was a Swedish scientist who figured out a way to test the rate at which alcohol is absorbed in one’s system, controlling for factors like weight, sex and age. His formula has been revised, but the principle still stands.
The permissible BAC level on an ignition interlock is set at .02 and not zero in order to account for oddities such as fruit juice and pizza dough, which can ferment in the mouth and produce a tiny bit of alcohol.
Calibration: The Key to Accuracy
When you get your ignition interlock installed it is calibrated so that the measurement it takes is accurate. Then, during your regular service appointments the device is tested and re-calibrated, so it is always measuring alcohol correctly.
Ignition interlock providers take their mission seriously: to keep you on the road, legally and safely. Every generation of devices gets more accurate and easier to use, in order for us to fulfil our mission, and for you to stay on the road, safe and sober.
* % BAC = (A x 5.14 / W x r) – .015 x H, if you’re interested.