Paper VT-MoM3
Comparative Outgassing Study of Identical Vacuum Chambers

Monday, October 21, 2019, 9:00 am, Room A213

We have measured and compared the H₂ and water outgassing rates for 7 identical vacuum chambers constructed of common vacuum materials and heat treatments: 304L, 316L, 316LN-ESR (electro-slag remelt), titanium, aluminum vacuum-fired 316L, and vacuum-fired 316LN-ESR. These chambers are of identical geometry and are from the same manufacturer. Comparison studies of outgassing from a large selection of chambers has the advantage over those of single samples or chambers in that the influence of chamber geometry in minimized. Much of the motivation for NIST to conduct this study is to identify ultralow outgassing materials for UHV and XHV vacuum systems, common candidate materials include aluminum, titanium, and vacuum-fired 316L or 316LN-ESR stainless steel. Obtaining these low pressures usually requires vacuum chambers with outgassing rates much less than 10^-9 Pa L s^-1 cm^-2. In addition, vacuum chambers constructed from materials with ultra-low outgassing rates can help reduce the cost of large vacuum systems by requiring fewer pumps (with the associated cost of operation and maintenance) to obtain the desired ultimate pressure. Other considerations in the selection of vacuum materials include the material cost, strength, machinability, weldability, and chemical resistance. One of our aims of this study is to put outgassing rates into the engineering tool kit. In addition to the above materials, we intend to present data on post process of some of the chambers, including electropolishing and a light air-bake, and on mild steel chambers.