A method is described for simultaneously measuring the diffusion coefficient, solubility, and permeability for a given gas in a single sample of solid material. The method utilizes solutions of the diffusion equation (for both plane and cylindrical geometry) which make it easily possible to calculate the diffusion parameters from data taken either in the period of steady-state gas flow or in the early part of the transient period. Using ultra-high vacuum techniques, it is possible to make the measurements with extremely small amounts of gas and without errors due to leaks or contamination in the gas handling system. Values for the diffusion parameters have been determined for helium in Pyrex glass (Corning 7740), and comparisons with previous measurements are made. Directly measured values of the diffusion coefficient are given for the first time. The diffusion coefficient and permeability both exhibit a temperature dependence of the form exp (−E/kT) both having the same activation energy (6.4×103 cal/mole) within the experimental error. The solubility exhibits little or no variation with temperature over the range 25°C to 300°C, and is in good agreement with the only previous measurement, carried out at 515°C.
Recently developed ultra-high vacuum techniques, with which it is possible to achieve working pressures of 10−10 mm Hg or less, have made feasible the investigation of a number of physical processes occurring at very low pressures. The Bayard-Alpert ionization gauge has been calibrated over its useful range and some of the important limitations on the production and measurement of ultra-high vacuum have been determined. It has been shown that an ultimate limitation on the achievement of very low pressures in glass systems is caused by the diffusion of atmospheric helium through the walls of the system. A simplified omegatron has been developed for the measurement of the partial pressures of residual gases in a highly evacuated system.
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