HIGHLY ACCURATE CALIBRATION GAS-MIXTURES BY DYNAMIC-GRAVIMETRIC PREPARATION

Calibration gas mixtures of highest metrological quality have up to no w exclusively been prepared static-gravimetrically, i.e. by weighing t he gas masses of the components into a cylinder. Using the four-compon ent mixture needed for the calibration and verification of vehicle exh aust measuring instruments as an example, we have investigated with wh at accuracy gas mixtures with mole fractions in the range of 10(-2) mo l/mol can be prepared by dynamic-gravimetric means. The dynamic prepar ation has mainly two advantages - no or only small influences by wall effects and the possibility of quickly changing the gas composition ac cording to the requirements. Not the gas masses but the gas mass flows which are measured separately and then mixed are the quantities essen tial for the gas composition in this case. The mass flowrates are meas ured with thermal mass flowmeters, which were calibrated with mass flo wrates directly traceable to the SI units kilogram and second. Using t he molar masses of the mixture components, traceability of the composi tion to the SI base unit mole can be established. The mole fractions o f the components of the calibration gas mixtures prepared dynamic-grav imetrically can be given with extended relative uncertainties (k = 2, corresponding to a confidence level of 95 %) of less than 0,2 % (only for propane 0,65 %). This uncertainty evaluation was confirmed bit com parisons with primary static-gravimetric reference gas mixtures of oth er metrology institutes. As a result, dynamic-gravimetric preparation of gas mixtures is a useful complement to the static-gravimetric prepa ration and opens up an independent way of establishing traceability to the SI units for gas-analytical measurements.