As I mentioned in my last post, I had built four v2.2 attenuators and three of them were free of leaks and have held a vacuum for weeks now but one of the attenuators had a slow but noticeable leak. I have a few tests planned to pinpoint the source of the leak. So far I know that the leak appears to be in the rear aluminum cap of the vacuum chamber. The leak could be in one of three places: 1) the Torr Seal epoxy cable feed through; 2) the threaded hole for the sleeve valve; 3) the Viton seal between the aluminum cap and the glass cylinder. I plan to evaluate each of these potential sources for the leak.

At the same time I am building a version of the vacuum chamber with 9 mm acrylic end plates. I have one acrylic end plate ready to test including the cable feed through with Torr Seal Epoxy. One feature (advantage/disadvantage?) of the acrylic end plate is that I can see the quality of the Torr Seal epoxy fill in the cable through because the acrylic is clear. The conclusion is that while the surface of the epoxy appears to completely fill the cable feed through opening in the acrylic, there are air pockets in the epoxy that are clearly visible; so the cable feed through may not be as solid as I thought previously. It may be that the epoxy fill is a bit hit and miss and the air pockets could be a weak point where a leak could develop.

To tackle this problem I have bought a vacuum degassing chamber so that I can put the Torr Seal epoxy joint under vacuum before the epoxy cures. I call this vacuum preparation of the epoxy. I expect that this will avoid the air pockets in the epoxy cable feed through and reduce the chances of a leak at this point.