In many countries breath alcohol measuring instruments have to be calibrated and verified with mixtures of ethanol in water saturated air. For the preparation of such moist gases mainly a system proposed in OIML R 126 is used. Such a "bubble train", consists of a system of three or more flasks filled with an ethanol-water-solution of well known concentration. If air flows through the flasks, the concentration of ethanol in the resulting gas mixture depends on the solution concentration and the temperature.
Using the so-called Dubowski equation (OIML R 126, 1998) the ethanol concentration in the gas mixture can be calculated. Problematic is here that the used factors are ascertained from several empirically determined values. The uncertainty of these factors cannot be neglected, but for being comparable they are set to zero for this comparison. It should be part of later comparisons to include this influence.
To compare in a first step the technical realization of the preparation process (bubble train) only, it is proposed to use two different types of breath alcohol measuring instruments as monitoring devices sent around. Each of the collaborating laboratories will prepare gas mixtures with concentrations around 0,4 mg/l and 0,25 mg/l with its own equipment. The gas mixtures will be analysed with the monitoring devices and the results of the instruments will be compared at the pilot laboratory. Aim of the comparison will be to identify the critical points of the preparation process and in this way to increase the quality of mixture preparation.
If the monitoring devices have to be checked during the comparison depends on the number of collaborating laboratories. Thus it cannot be decided now. In any case each laboratory will be responsible for the transportation of the instruments to the next partner.
Final Report 2016-11-24
The comparison has been completed and the report can be downloaded here>>