Abstract
The relations between the pressure drop values of filter rods and nozzles obtained by customary gauges and the atmospheric pressure are investigated. Measurements are carried out between 500 and 1.000 mm Hg external pressure. This range was produced artificially. In case of filter rods it is shown, that a variation of 4 % only is received on the full measuring range by operating at constant air circulation (cm3/s) independent of atmospheric pressure. This shift is negligible in view of normally occurring local variations of atmospheric pressure. By operating at constant air circulation (cm3/s) the value of the nozzles' pressure drop varies in considerable degree over the full measuring range. Using reference nozzles for devices of pressure drop determination at different altitudes therefore, either a correction of the reference mark fixed at a standard pressure is necessary, or the values for filter pressure drop are to be corrected. The corrections are indicated.