Qualifying an XHV System by the Residual Gas Accumulation Method.

Abstract

A major concern in the XHV technique is hydrogen outgassing from the vacuum container walls. It will be demonstrated in the present paper that residual gas accumulation under sealed-off conditions and monitoring the pressure rate by a spinning rotor gauge (SRG) is a simple and adequate method for the determination of extremely low outgassing rates. Q, Opening of the pump valve will cause the system pressure, P, to settle according to the effective pumping speed, S, by the simple relation P= Q/ S. For practical reasons, we use Q= V (dP / dt) 0, with V denoting the system volume, so that P= (dP/ dt) 0τp, where τp= V/ S is the pump-down time constant, and (dP/ dt) 0 is the pressure rate inside the sealed system, i.e., with the pump valve shut off. By using a standard commercial SRG unit, systems can be qualified at the 100 pPa (10-12 mbar) level. No pumping or contamination occurs to the system as with conventional ion gauges.