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Description
Vacuum materials require precise knowledge of their permeability P and diffusivity D, e.g. for sealing or outgassing. Agreement with the methods to measure P and D, mainly infusion/outgassing or membrane techinques1, is sometimes poor and material dependent2.
The paper reports an experimental apparatus for gas flux measurements suitable for membrane techniques. It allows to measure transient fluxes and consequently simultaneously P and D3. A novel membrane assembly and the insertion of a Spinning Rotor Gauge (SRG) allow accurate results respect to a similar system described in ref. 4. A non-permeable material supports the permeation cell, composed by a freestanding membrane. Freestanding configuration avoids miscalculations of permeation area and unwanted lateral diffusion, artefacts often observed in membrane permeability measurements. A turbo pump maintains high vacuum conditions at the membrane downstream side. The system is equipped with a Residual Gas Analyzer (RGA), an Ion Gauge, a SRG, and a leak of known conductance. Upstream side of the membrane is equipped with diaphragm gauge. The measure of the stationary gas flow Jss generated by a known differential pressure of pure gas across the membrane allows to determine the permeability. The diffusion D is calculated by means of eq. (5) of ref. 4 integrating J(t) with respect to time. Because of fast response time of the RGA and taking into account that it is calibrated against SRG, the relative error on D is < 10%.
1 W. G. Perkins, Permeation and Outgassing of Vacuum Materials, Journal of Vacuum Science and Technology 10, 543 (1973)
2 Standard Test Method for Determining Gas Permeability Characteristics of Plastic Film and Sheeting. ASTM International Designation: D1434-82 (Reapproved 2015)
3 G. Firpo et al, Permeability thickness dependence of polydimethylsiloxane (PDMS) membranes, Journal of Membrane Science 481, 1–8 (2015)
4 P. Tremblay et al Gas permeability, diffusivity and solubility of nitrogen, helium, methane, carbon dioxide and formaldehyde in dense polymeric membranes using a new on-line permeation apparatus, Journal of Membrane Science 282, 245-256 (2006).
