There are many types of pipe fittings designed to connect together lengths of tubing for gas or water transfer around a vacuum system. In our experience the best fittings for gas/vacuum leak tightness and reliability are Swagelok® and Cajon®. For gas transfer into high vacuum or UHV conditions we recommend stainless steel Cajon® VCR® fittings. Stainless steel Swagelok® fittings are satisfactory for gas transfer in moderate vacuum conditions. For water transfer either brass or stainless steel Swagelok® may be used but we recommend stainless.
Note: To connect tubes to CF (ConFlat-compatible) flanges, QF flanges or quick disconnect couplings with Cajon VCO, Cajon VCR, or Swagelok fittings, see Swagelok, VCR (male), VCR (female), VCO (male), & VCO (female) fittings.
Threaded male and female nuts are slipped over the tube ends which are then welded or brazed into glands. A copper gasket is placed between the sealing surfaces of the glands and the two nuts mated and tightened. The copper gasket is crushed, forming a seal between the glands (Fig. 1).
VCR® fittings are particularly useful for applications where gas lines must be heated to maintain a volatile component in vapor form or when the tubing must be baked to remove internal contamination. VCR® fittings can be baked to at least 250° C.
A female nut is slipped over each tube, followed by metal back and front ferrules (Fig.2). The tube ends are then seated into a body with male threads and the nuts tightened. The front ferrule is constrained to swage into the external surface of the tube, forming the metal to metal seal bakeable to 100° C.
The major advantage of Swagelok® fittings is that two tubes are connected without prior welding or brazing of glands on the tubes. A slight disadvantage is that the joint, once made, cannot be disassembled and reassembled satisfactorily. The ferrules and the swaged portion of the tube must be cut off and new ferrules used to get the same reliability as the original joint.
Swagelok® fittings are also used to join plastic tubing. Special (non- metallic) ferrules and tubing insert are used.
Stainless Tubing: Small diameter stainless steel tubing is used to transport process gas or cooling water to and through the vacuum wall. Never use plastic tubing to transport pure gases in vacuum applications or to carry a liquid through a vacuum wall. The outgassing rates and permeabilities of all plastic tubes are much too high. Where the cooling water to a component inside a vacuum chamber must be electrically isolated, try to design the insulating section external to the vacuum and use a PTFE tube section between the stainless steel tubes.
Plastic Tubing: The nylon reinforced PVC tubing is intended only for water transfer around a system external to the vacuum wall, for example, transporting the cooling water for chamber walls, vacuum pumps, cooled rotatary feedthroughs. It should never be used under vacuum since its outgassing rate is very high. The reinforcing makes this tubing quite acceptable at city water pressures (50 to 80 psi).
PTFE Tubing Use major use for this product, as described above, is for the electrical isolation of a water supply. Despite its inertness, do not use PTFE tubing to transfer gases into a vacuum system. Its permeability for N2, O2, H2 and particularly He, is much worse than most other polymeric materials considered for, or used in, vacuum applications.