Fuel cell breath analyzers work through an electrochemical oxidation reaction. When a breath sample containing ethanol passes over the fuel cell electrode, the ethanol is oxidized at the anode and oxygen is reduced at the cathode.
This reaction produces an electrical current directly proportional to the ethanol concentration in the breath sample. Because the reaction is specific to ethanol, fuel cell analyzers are far more accurate than semiconductor devices.
Inside the cell
A fuel cell is a small sandwich: two porous electrodes coated with a catalyst, separated by an acidic electrolyte layer. Breath is drawn across the anode side. Any ethanol present is oxidised there, releasing electrons; on the cathode side, oxygen from the air takes those electrons up. The electrons travelling between the two is the current the device measures.
Nothing about that reaction is a proxy or an inference. The current exists because ethanol molecules were consumed, and its size reflects how many. That is why the reading is a measurement rather than an estimate.
Why the sample has to come from deep in the lung
Alcohol in the blood passes into the air in the alveoli — the tiny sacs at the bottom of the lungs — at a ratio that stays fairly constant. Air sampled from there is a reliable stand-in for a blood test. Air from the mouth and throat is not: it may carry alcohol that never reached the bloodstream at all.
This is why a proper test asks for a long, steady blow rather than a quick puff, and why the device waits for the flow to hold before it takes its sample. It is also why a fifteen-minute wait matters after a drink, a mouthwash or a medicine — the mouth needs to clear before what is measured means anything.
Warm-up, response and recovery
Three timings decide how a device behaves in use. Warm-up is how long it takes to be ready from cold. Response is how long after the blow the reading settles. Recovery is how long the cell needs before it can take a clean sample again.
Recovery is the one people underestimate. After a strongly positive reading, the cell holds residual alcohol for a short period, and testing the next person too quickly can carry part of the last result into theirs. A device used on a queue needs a recovery time that matches the pace of the queue.
What makes a reading trustworthy
Three things, in order.
The sensor has to be specific, which a fuel cell is. The sample has to come from deep lung air, which the device's flow requirements enforce. And the device has to be in calibration, which nothing about the reading itself will tell you.
Calibration is the one that gets skipped, and it is the one that quietly invalidates everything else. A drifted fuel cell still produces a precise, confident, repeatable number. It is simply the wrong number, and there is nothing in the display to say so — only the calibration record.
In practice
For evidential use, a fuel cell device with a documented calibration history and a printed or logged result is the standard to hold to. The printout matters more than it looks: a timestamped slip signed at the point of testing is far harder to dispute than a figure written down afterwards.
The result: precise, reliable BAC measurements that hold up in court and meet international standards like EN 15964 and NIST requirements.