The Designs of Things

As you work with vacuum systems, you may be called upon to design new parts and/or sections for the system. Your success will depend on your experience with the materials you select, your choice of materials, and your access to experienced technicians. 

If you require metal components on your vacuum system, select metals with low gas permeation, such as 300 series stainless steel. It is nonmagnetic and, like glass, is a poor conductor of heat and electricity. Stainless steel, also like glass, is relatively nonreactive, and therefore is less likely to rust or be affected by chemicals. If welding the stainless steel is required, select 304L stainless steel, which is low in carbon. Otherwise, at welding temperatures the carbon will combine with the chromium (within the stainless steel) to form chromium carbide and the corrosion protection of the chromium will be lost. Type 303 stainless steel should not be used for vacuum work because it contains selenium, which has a high vapor pressure. 

The metals zinc and cadmium should be avoided because of their high vapor pressures. Metals that include zinc and cadmium alloys such as brass (copper and zinc) and some silver solders (cadmium) should also be avoided for the same reasons. It is possible to obtain cadmium-free silver solder and brazing materials that use tin, lead, and indium for vacuum use. Some steel screws are cadmium-coated and also must be avoided. 

Parts are fairly easy to attach and change with metal systems it is mostly a matter of unbolting one unit and replacing it with another. Be sure to follow the bolting techniques described in Sec. 7.6.4 to ensure even cutting on gaskets and equal torque on flange surfaces. 

This book is not an instruction book on glassblowing. Chapter 8 has some tips on gas-oxygen torch use, and Appendix D contains some recommended books for your consideration. The following discussion will also provide some of the 

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