Method for the determination of alcohol concentration in expired air from a breath sample delivered by a test person blowing towards a sensor arrangement in an apparatus for alcohol concentration determination, by contactless sampling of said expired air wherein the breath sample is diluted with ambient air, comprising interaction between the apparatus on the one hand and the test person or an operator on the other, wherein said interaction comprises a presenting unit within the apparatus presenting a measuring value and a measuring error of said determination to said test person or operator in real time during ongoing measurement; said measuring error is dependent on the dilution of said expired air, accumulated time of said determination, and noise, interference or other random variations of said measuring value, said dilution being determined by measurement of temperature, concentration of water vapour, or carbon dioxide, or a combination of these entities, wherein an adaptation of accuracy and time lapse of said determination to current objective or requirement is performed interactively between the apparatus and the test person or operator in that the test person or operator choose to finish the determination or continue the determination, wherein time lapse refers to the time required to perform the determination and wherein the prolonged measuring time and compensation for possible dilution of said expired air, is for the purpose of increasing said accuracy, and wherein the presentation unit provides an indication of the measuring value and the measuring error by means of audiovisual or haptic indication from a presentation unit within said apparatus, the method comprising a stepwise procedure, including a starting step, at which said apparatus is activated, followed by a waiting step at which sensor signals corresponding to said measuring value are controlled with respect to stability, subsequently followed by a step of measuring and computation at which real time alveolar alcohol concentration of alcohol and measuring error are carried out and visualised, wherein the real time alveolar alcohol concentration of alcohol and the measuring error are measured values accumulated during a certain measuring time and a final step following from an accumulated inaccuracy having reached a minimum, the accumulated inaccuracy being a ratio between the accumulated measuring error and the accumulated real time alveolar alcohol concentration of alcohol. said interaction comprises a presenting unit within the apparatus presenting a measuring value and a measuring error of said determination to said test person or operator in real time during ongoing measurement; said measuring error is dependent on the dilution of said expired air, accumulated time of said determination, and noise, interference or other random variations of said measuring value, said dilution being determined by measurement of temperature, concentration of water vapour, or carbon dioxide, or a combination of these entities, wherein an adaptation of accuracy and time lapse of said determination to current objective or requirement is performed interactively between the apparatus and the test person or operator in that the test person or operator choose to finish the determination or continue the determination, wherein time lapse refers to the time required to perform the determination and wherein the prolonged measuring time and compensation for possible dilution of said expired air, is for the purpose of increasing said accuracy, and wherein the presentation unit provides an indication of the measuring value and the measuring error by means of audiovisual or haptic indication from a presentation unit within said apparatus, the method comprising a stepwise procedure, including a starting step, at which said apparatus is activated, followed by a waiting step at which sensor signals corresponding to said measuring value are controlled with respect to stability, subsequently followed by a step of measuring and computation at which real time alveolar alcohol concentration of alcohol and measuring error are carried out and visualised, wherein the real time alveolar alcohol concentration of alcohol and the measuring error are measured values accumulated during a certain measuring time and a final step following from an accumulated inaccuracy having reached a minimum, the accumulated inaccuracy being a ratio between the accumulated measuring error and the accumulated real time alveolar alcohol concentration of alcohol.