Outgassing Measurement by Switching Between Two Pumping Paths

The throughput method is widely used for the outgassing measurement of vacuum materials. When the outgassing rate to be measured is very low, there are two major problems that causes large errors in the conventional throughput method. First, the measured outgassing rate includes those of the gauges and quadrupole mass spectrometer (QMS). Secondly, the pressure difference between the up-stream and down-stream of an orifice approaches to the lower limit of the gauges. We propose a simple and accurate measurement method, with the sample chamber separated from the measuring part, by switching between two pumping paths of the outgassing from the sample, which we call the SPP method. Advantages of the developed method are that it not only solves the above problems, but also requires only one gauge and one QMS. The measurement system is constructed with a conventional B-A gauge, a QMS and an orifice, 5.1×10-3 m3s-1. It is demonstrated that the outgassing rate, Q=3×10-12 Pa.m3s-1 (q=7×10-12 Pa ·m · s-1) nitrogen equivalent, and mass spectrum signal of only hydrogen, of an electrolytically polished and prebaked stainless steel chamber could be measured at 28°C using the system. The temperature dependence of the outgassing rate of the sample was also measured without heating the gauge and the QMS.