Abstract
Most analyzers of air pollution must be calibrated by the standard gas which is a mixture of the pollutant gas and the diluent gas with known concentration. Usually the mixed gas is prepared in a gas cylinder and the concentration of the pollutant gas is determined by the mass measuring method as it is fed into the cylinder. But it is known that the concentration changes in the cylinder, especially in the case of low concentration, because the gases are absorbed by a cylinder wall. In such a case, the flow measuring method is used for the preparation.
Before being put to use, the standard gas mixed by the flow measuring method must be confirmed that it is homogeneous. In this experiment, nitric monoxide (NO) is choson as the pollutant gas. The mixed gas is taken by a sampling probe at various positions of the tube and the distribution of NO concentration is measured by the chemiluminescence NO analyzer. It is shown that the distribution and the homogenizing process agree well with the prediction by the diffusion equation. According to the result, a length of about 5m is necessary for the tube to make the variation coefficient of the concentration distribution less than 10-3 when the flow rate is 104cm3·min-1, and the required length dose not depend on the diameter of the tube.
Since the use of such a long tube is not advisable for preventing adsorption, several types of mixers are produced and their effects are experimentally evaluated. As a mixer for the flow measuring method, a higher efficiency of mixing is required, and at the same time, a lower pressure loss and a smaller fluctuation of the flow rate. By the present investigation, “twisting plate type” of mixer is found to have the best characteristics.
