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After the Pinch-off

Various methods have been developed to check the finished device for leakage.

Vacuum insulation time rates and electric resistance measurements can be avoided if a sample tube is carried through the entire process, then subjected to a helium leak test or microscopic examination and used for comparative analysis.

Process procedures should be duplicated precisely, since changes as subtle as bending a piece of copper tubing will change its grain structure and work-harden the piece considerably.

Even more significant changes in grain size, crystal structure and ductility will occur during any thermal processes such as brazing, bake-out, soldering or welding.

The cold-welded stub should be protected with a plastic cap or a Varian Torr Seal* resin after the pinch-off, as it is a delicate seal and razor sharp.