Measurement of diffusion coefficients via diffusion in flowing gas I. Method

Abstract

Diffusion of gases was investigated by a method in which one gas (“admixture gas”) in a tube diffuses against a constant flow of another gas (“main gas”) in such a way that a stationary concentration distribution builds up. The (exponential) upstream decrease in concentration of the admixture gas was measured. This decrease, together with the flow velocity, gives the diffusion coefficient. This so-called back-diffusion method is suitable for use with gases up to high temperatures. The apparatus consisted basically of a straight tube for the main gas flow, with four narrow extraction holes to permit mass spectrometric analysis of the local gas mixture, and a side tube at one end to admit the admixture gas. The tubes, made of fused silica, were in a furnace.

A description is given of the apparatus and the measuring procedure, and it is shown how the diffusion coefficient can be calculated from the measured quantities. At the end of this paper the results of the measurements of helium (<1%) in krypton (>99%) at temperatures from 300 to 1300 K are given in detail. A subsequent paper will deal with other gas combinations.