A method and apparatus for measuring user breath alcohol concentration. A flow of an expired breath sample is passed through a fuel cell sensor giving an output signal proportional to the amount of alcohol present in the sample. By measuring pressure, the volume of the sample may be calculated by integrating pressure over expiration time of the sample, whereas breath alcohol concentration is calculated based on the fuel cell output signal. Both sample volume and breath alcohol concentration values are continually updated by integrating measured instantaneous pressure and fuel cell output signal over time, irrespective of breath sample volume. When the user stops blowing, volume compensation is performed to obtain a compensated fuel cell output signal using a stored calibration volume. Hence, an improved method for accurately measuring breath alcohol concentration of a user is achieved, capable of handling varied expired volumes of breath, obviating the need for sampling mechanism.
