18 October 2002

Making Plastic Vacuum Chambers - Follow Up

by Joseph DiVerdi

In a previous Lab Note I described some work building large area, ionizing-radiation detectors, in particular the vacuum chambers necessary to contain those detectors. I would first like to thank all the SAS readers with whom I have been corresponding on this subject over the past weeks. Besides enjoying this correspondence, I have learned a great deal, and even solved a problem or two based on the counsel I've received.

As a reminder, one of the test vacuum chambers used in these experiments is shown in the image below.

Click image to enlarge

It is built out of three pieces of approximately 1 cm thick acrylic plastic, has outside dimensions of approximately 15 x 15 x 3 cm, with a 13 x 13 x 1 cm vacuum space on the inside. It is also fitted with a dial-type vacuum gauge and connector for attachment to a vacuum line. The plastic sections are held together with retaining screws and nuts and the joints between them are sealed with a generous layer of a silicone adhesive. The silicone is applied as a gel and it conforms to the gaps as the nuts are tightened. Unfortunately, it extrudes from the gaps as the assembly is tightened. The extruded material may interfere with components inside the chamber and is always a nuisance to clean up on the outside. For these reasons it is desirable to eliminate it from the construction scheme.

The search for a suitable gasket material to replace this gel was my parting comment in the previous Note and was rewarded by several suggestions. One promising material is PVC (polyvinyl chloride) sheet and can be obtained as a product literally named "Shower Pan Liner" manufactured by the Oatey Company. It can be obtained from retail local home construction stores such as The Home Depot. It is 0.040" thick and comes in four and five foot widths, suitable for very large gaskets. At my local Home Depot store it can be purchased by the linear foot and the cost is around US$1 per square foot. More details on the material can be found at the manufacturer's web site and a distributor's web site.

Click image to enlarge

The material is very flexible, is available in a medium gray color, and is easily marked with a permanent marker. It cuts very easily with a sharp knife or a pair of scissors. Holes can be cut in it similarly or be punched in it using a standard office hole puncher or the business end of a cork borer.

I made a pair of gaskets one for each of the two joints in my test vacuum chamber by cutting six inch squares of the material, removing a five inch square from the center of each and punching out holes for the retaining screws. The gasket can be seen in the image on the right.

In order to "focus" the pressure created by the retaining screws on the most important sealing locations of the gaskets I cut away the gasket material outside of the screw holes as shown in the following image.

Click image to enlarge

Assembling the test chamber with these gaskets was much easier than with the silicon adhesive gel. Actually it wasn't much easier but it was much cleaner and that can be considered easier. The retaining screws were tightened in a pattern which applied increasing pressure around the periphery of the chamber in a symmetric fashion. Although I considered coating the gaskets with high vacuum grease before assembly I wanted to determine if the seal is effective without any grease so none was used for this test.

The completed chamber has been evaluated for a limited time period but appears to hold a vacuum without leakage as well than the previous silicon adhesive gasketed version. Additionally, and in contrast to the previous version, exposure to sub-0°C for several hours had no effect on the vacuum - a marked improvement.

In summary, the scheme described in this series of two Lab Notes offers a very easily built and reliable vacuum chamber system with very low loss and very low cost and requiring relatively few specialty tools. Now to get working on what to put inside the chamber...

Unlike nature Joseph DiVerdi does not abhor a vacuum. He rather enjoys creating them occasionally and can be reached at diverdi@xtrsystems.com.