An Introduction to Fuel Cells

How Ethanol Is Measured by Turning It into Electricity

From - Counterpoint Volume 6: Issue 2 - Article 2 (March 2022)

An article for participants in the myCAMprogram

Jan Semenoff, BA, EMA
​Forensic Criminalist

Article information:

2500 words (approximately 9-12 minutes)
​Video length - 21:26
Fuel cells are at the heart of most roadside breath alcohol testing devices — whether they are called TADs (Transdermal Alcohol Devices), IIDs (Ignition Interlock Devices), or PBTs (Preliminary Breath Testers). These devices measure Breath Alcohol Concentration (BrAC) from a sample of exhaled air or a Transdermal Alcohol Concentration (TAC) from perspiration from your skin. Understanding how a fuel cell works helps explain both the strengths and limitations of these devices.

Video

What is a fuel cell?

A fuel cell is a device that creates electricity through a chemical reaction. As long as the cell has both:
  1. A fresh supply of fuel (in our case, ethanol from the breath sample), and
  2. An active chemical reagent inside the cell,
…it will produce an electrical current until the reagent is depleted.

​Fuel Cells vs. Batteries

While both produce electricity, they work differently:
  • Battery – All the chemicals needed to make electricity are stored inside. Once used up, it must be recharged or replaced.
  • Fuel Cell – The fuel comes from an external source. In a breath tester, that external fuel is ethanol in the breath sample.
As long as ethanol is present and the internal reagent is active, the cell generates electricity.
Picture
Figure 1 - The fuel cell from an older Intoximeter RBT-IV. The cell itself is about the size of a dollar coin.
Picture
Figure 2 - The same fuel cell, disassembled. The fuel cell had a highly acidic solution permeating the ceramic disc.

Fuel cell versus semiconductor

Before fuel cells became common, some devices used semiconductor sensors. These are less accurate and more prone to false readings.
  • Semiconductors measure how ethanol changes the flow of electricity through the sensor. They can be affected by temperature, altitude, and other chemicals.
  • Fuel Cells are more selective for ethanol, give more consistent readings, and have a longer working life.
Cheap “keychain breathalyzers” often use semiconductor technology and can give dangerously inaccurate results. DO NOT rely on them to safeguard your future, your reputation, or your freedom!

​How a Fuel Cell Is Built

​Think of a fuel cell like an Oreo™ cookie:
  • The “filling” is a porous ceramic disc soaked in a highly acidic solution (the reagent).
  • The “cookie wafers” are thin layers of platinum black — a porous platinum coating that acts as electrodes.
Picture
Figure 3 - The anatomy of the fuel cell.
​One side of the cell is connected to a small sample chamber (about 1 millilitre of air), where the breath enters. The other side is open to room air. Wires connect the electrodes to the device’s circuitry to measure the electrical current.
Picture
Figure 4 - The breath sample chamber is attached to one side of the fuel cell, with the other side open to room air. Wires attached to both outer discs (or coatings) complete the electrical circuit.
​Tiny Quantities, Big Numbers:
It takes very little ethanol to produce a measurable reading. For example:
  • A 0.08 g/dL BrAC result from a 1 mL sample represents only 0.00000038 grams of ethanol — less than four 10-millionths of a gram.
  • That’s about the weight of 4 tiny fragments if you cut a US one-dollar bill into 10 million pieces.
Because so little ethanol is needed, even tiny contamination (1–10 millionths of a gram) can cause a false reading.

How a Fuel Cell Works

  1. Breath sample enters the chamber, bringing ethanol into contact with the platinum-coated ceramic core.
  2. Oxidation occurs — ethanol loses electrons and is converted into acetic acid (vinegar).
  3. This reaction produces electron flow between the electrodes — an electrical current.
  4. The size of that current depends on how much ethanol was in the sample.
The device is calibrated so that a certain current equals a specific BrAC value.
Picture
Figure 5 - The ethanol captured in the breath sample is drawn into the fuel cell's ceramic disc where the acidic solution breaks it down into acetic acid (or vinegar).
Picture
Figure 6 - When oxidized, the ethanol produces a weak electrical current which is then read by the device.

Advantages of Fuel Cells

  • Greater specificity for ethanol than cheaper semiconductors.
  • Less affected by environmental conditions.
  • Longer lifespan and less frequent calibration.
  • Direct, linear response to ethanol concentration.

​Limitations of Fuel Cells

Although fuel cells are a vast improvement over older semiconductor designs, they still have some limitations:
  1. Lag Time – It can take 10–20 seconds to reach peak response after capturing a sample.
  2. Carryover – Residual acetic acid from a previous test can affect the next reading if the device is not properly purged.
  3. Temperature Sensitivity – Readings can be slightly higher in warm conditions and slightly lower in cold ones.
  4. Other Alcohols – Methanol and isopropanol can trigger readings, though usually at lower sensitivity.
  5. Calibration Drift – Over time, the acidic reagent inside the cell is consumed, making the cell less responsive. Regular calibration is essential.
I’ve seen monitoring devices that stop working properly because their fuel cell — the part that measures alcohol — has worn out. When this happens, the device can give unpredictable results. It might read too high on one test and too low on the next, even if the same person provides both samples. The problem is, the device doesn’t always flag itself as broken, so you might never know there’s an issue. That’s why regular maintenance is so important — usually every year — and why your device should be calibrated at least once a month, sometimes more. Keeping a calibration log helps track how the device is performing over time and can be vital evidence if your results are ever questioned
Action Tip – Watch for Device Drift:
  •  - If your device starts giving unexpected results — higher or lower than you know they should be — write it down in your personal logbook right away. Use your own Personal Breath Tester to obtain independent results. Videotape yourself providing the breath test.
  •  - Ask your supervision officer or monitoring agency when your device was last calibrated. Calibration should be done at least monthly, and the device should have full maintenance once a year. If possible, request a copy of the calibration log for your device. This record shows how accurate it’s been over time and can help prove your case if there’s ever a dispute.
  •  - Keep your own notes about any unusual readings and what was happening at the time.

Fuel cells in other devices

Picture
Figure 7 - The IID device employs a fuel cell to determine the BrAC of the test subject.
  • Ignition Interlock Devices – Use fuel cells to detect alcohol before starting a vehicle.
  • Ankle Monitors – Measure Transdermal Alcohol Concentration (TAC) through the skin using a fuel cell.
  • Evidentiary Instruments – Some combine fuel cell and infrared (IR) measurements for greater accuracy.

Key Takeaways

  • A fuel cell turns ethanol into an electrical signal, which is then converted into a BrAC reading.
  • These devices can be accurate when maintained and used properly, but they have operational limits.
  • Regular calibration and correct procedure are critical for reliable results.
  • Even small amounts of contamination can affect readings.

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For further study:

  1. Harding, P. and Zetl, J. R.; Chapter 7. "Methods for Breath Alcohol Testing", Garriott’s Medicolegal Aspects of Alcohol, 6th Edition; Lawyers & Judges Publishing Company, Tucson, 2015; pages 229 – 252.
  2. Gullberg, R. G., “Breath Alcohol Measurement Variability Associated with Different Instrumentation and Protocols, Forensic Science International 131 (2003) 30-35.
  3. Jones, A.W. & Rossner, S., False-Positive Breath Alcohol Test After a Ketogenic Diet, International Journal of Obesity, (2007) 31, Pages 559-561.
  4. Jones, A.W. & Summer, R., Detection of Isopropyl Alcohol in a Patient with Diabetic Ketoacidosis, The Journal of Emergency Medicine, Vol. 19, No. 2, 2000, Pages 165-168.
  5. Jones, A.W., Andersson, L., Biotransformation of Acetone to Isopropanol Observed in a Motorist Involved in a Sobriety Check, Journal of Forensic Sciences, JFSVA, Vol. 40, No. 4 July 1995, Pages 686-687