AVS 56th International Symposium & Exhibition
    Vacuum Technology Wednesday Sessions
       Session VT-WeA

Paper VT-WeA4
Accurate Measurements of Low Permeation Flows of Hydrogen

Wednesday, November 11, 2009, 3:00 pm, Room J1

Session: Modeling and Accelerators
Presenter: V. Nemanic, Jozef Stefan Institute, Slovenia
Authors: V. Nemanic, Jozef Stefan Institute, Slovenia
B. Zajec, Jozef Stefan Institute, Slovenia
M. Zumer, Jozef Stefan Institute, Slovenia
Correspondent: Click to Email
Permeation of hydrogen isotopes from the upstream pressure through a membrane into high vacuum at elevated temperatures is a challenging task for vacuum technology when very low flows must be determined. The detection limit and accuracy of results depend on several experimental details. Geometrical and mechanical constraints set an engineering issue since the ultimate tightness of seals at high temperature must be preserved. On the other hand, measurements of the steady permeation flux and its transients require high sensitivity and stability of the gauges. It is also essential that the background represents only a fraction of the signal. We present recent improvements applied on a permeation cell design that results in efficient background suppression. When implemented in an all-metal UHV system, low permeation fluxes down to 10-11 mbar L/(cm2 s) could be measured. We also present an innovative technique to perform measurements at a low upstream pressure capable of detecting changes corresponding to a permeation flux as low as 10-14 mbar L/(cm2 s). The interpretation of data is presented by the surface rate constants rather than by diffusivity and solubility since the permeation regime at low pressures is known to be limited by surface reactions. Better experimental capability is needed today in the hydrogen storage technology and also in the field of nuclear fusion reactors to study the efficiency of permeation barriers and to predict tritium retention in the walls.