The invention relates to method and apparatus for the determination of alcohol concentration of expired air. By visualising current or accumulated measuring value and error in real time during ongoing measurement, interactive control of alcohol determination is enabled with respect to accuracy and time lapse. Preferably the measurement is performed without contact with compensation for the dilution of the breath sample, which is performed by simultaneous measurement of temperature, concentration of water vapour or carbon dioxide. The method is divided into a stepwise procedure based on the dependence of the measuring error on the dilution of the expired air, and accumulated time, both being possible to influence by the test person or operator. Special advantages are being obtained at measurements relative to a concentration limit. The alcohol determination is preferably performed by means of absorption spectroscopy within the infrared wavelength range. The apparatus according to the invention includes sensors for the abovementioned entities, an electronic unit for signal and data processing, a presentation unit, and is embedded in a compact housing for handheld use or integration. The response time of the sensors should not exceed 0.5 seconds in order to fulfil the requirement of real time operation. The housing is controllably openable at measurement, and includes a measuring cell illuminated with collimated infrared radiation and means for active air flow.
