Widmark BAC Estimator
The fuel-cell reaction in the mouthpiece
Modern interlocks and police units use an electrochemical fuel cell. When you exhale, ethanol molecules (the red-and-white cluster in the 3D scene) land on a platinum electrode and are oxidized to acetic acid, releasing electrons:
More ethanol → more electrons → higher current. The device converts current to breath alcohol concentration, then multiplies by the standard 2100:1 blood-to-breath partition ratio: 2100 mL of deep-lung air carries about the same alcohol as 1 mL of blood. Slide the drinks slider and watch the molecule cloud in the sensor chamber grow — that is literally what the electrode "sees."
Why deep-lung air?
Alcohol in the mouth (mouthwash, a sip seconds ago) reads falsely high. Devices require a long 1.5-second-plus exhale so they sample alveolar air, which equilibrates with blood in the lungs' 300 million alveoli.
Interlock rules
US interlocks typically lock the ignition at 0.02–0.025 g/dL — far below the 0.08 driving limit — and demand random "rolling retests" every 5–30 minutes while driving. All 50 states use them for repeat DUI offenses; 30+ mandate them after a first offense.
Accuracy
Fuel cells are alcohol-specific (unlike older semiconductor sensors that could react to acetone). Calibrated units are accurate to about ±0.005 g/dL, and evidential police units to ±0.002.
The math behind the estimator
The panel above uses the Widmark formula, the standard forensic first approximation:
- A = ounces of pure alcohol (one standard drink = 0.6 fl oz = 14 g).
- W = body weight in pounds; r = distribution ratio (≈0.68 male, 0.55 female — alcohol dissolves in body water, not fat).
- 0.015/hour = average elimination by the liver. It is a zero-order process: roughly one standard drink per hour, no matter how much you drank.
Worked example: 160 lb male, 3 drinks in one hour: BAC = (1.8 × 5.14)/(160 × 0.68) − 0.015 = 9.25/108.8 − 0.015 ≈ 0.070 g/dL — under 0.08 but over the 0.05 limit used in Utah and much of Europe, and far over an interlock's 0.02 lockout. Real BAC varies with food, medications, and genetics; only a calibrated device measures it.
What the numbers feel like
0.02 g/dL — mild relaxation, slight judgment decline. This is where ignition interlocks already refuse to start the car.