Development of a New Very Low Helium Gas Flow Generation and Measurement System

Reference helium gas flow has become indispensable for many applications such as vacuum leak calibration, helium leak rate detection and et al. In this work, a new system was developed to generate and measure helium gas flow lower than 10-10 Pa m3 s- 1 at 23 ?. The generation of reference helium gas flow in this system is based on the combination of static expansion vacuum standard and constant conductance method (SE-CC method). Static expansion vacuum standard is applied to generate and maintain a standard upstream helium pressure from 5 Pa to 10-5Pa. Based on the pressure, constant conductance method, which crucial part is a leak element (laser orifice, really a small tube) with a molecular conductance of 1.68 x 10-9 m3 s- 1, can produce reference helium gas flows in the range from 10-9 Pa m3 s- 1 to 10-14 Pa m3 s- 1. The generated reference helium gas flow was measured using cumulative helium leak detection method and the corresponding rates of increase ion current at m/z = 4 from 10-10 A s- 1 to 10-15 A s- 1 were obtained. In this study, measurement points were observed deviating from the calculated line when reference helium gas flow less than 10-11 Pa m3 s- 1. Additionally, a vacuum leak with a leak rate of 10-12 Pa m3 s- 1 was used to examine the reliability of the reference helium gas flow.