For future vacuum tubes, I will most likely be using Pyrex tubing. Pyrex is available in larger diameters than soda lime glass, and unlike recycled glass it has a standard coefficient of expansion, eliminating compatibility problems. The low coefficient of expansion also reduces the risk of cracking during cooling and reheating, as an added bonus. I made a small order of medium-wall tubing, which recently arrived. However, the diameter of the glass is in imperial units, not metric, so I had to make another adapter to connect the 1/4 inch tubing to my vacuum manifold.

   The coupling is identical to that used in my oxy-propane torch; I have found it to be exceedingly reliable and easy to use. This time however, I simply made it by machining a section of 1/2 inch threaded rod and soldering it to a 3/8 NPT pipe plug, instead of making it from scratch. The tip of the threaded rod has a 60 degree internal taper, made with a center drill. This compresses the O-ring onto the glass tubing. The only other feature is a step near the bottom of the through-hole, which reduces the internal diameter from 1/4 to 3/16 inch. This stops the tubing from falling through the adapter. The adapter works well, and when tightened the tubing is held firmly in place.