AMATEUR VACUUM TUBE MAKING
CONNECTING TO THE VACUUM SYSTEM, CONTINUED.
Refrigeration copper tubing with flare connections or crimp connections are suitable for all vacuum ranges. The main problem with these connections is that they take considerable torque to tighten them to suitable seal. This can be a problem in tight areas where wrenches will not fit. Another disadvantage is the stiffness of the tubing. The tubing must be bent and formed exactly to fit otherwise a cocked connection can result that will leak.
Standard tapered pipe threaded parts can be successfully used for vacuum work. These must be sealed with teflon tape rather than plumbing sealing compounds that outgas severely. The main disadvantage of these connections is that they do have significant trapped volume that can take very long to out gas. This can appear as a virtual leak in the system that changes according to how long the system is in operation.
O ring sealed flanges are completely satisfacory for pressures down to 10 -6 torr. They are easy to machine and do not take high torque to connect. Simple wing nuts on screws can be used to tighten the flanges since the seal is not dependent on the seal face pressure.
SEALING GLASS TUBING TO THE SYSTEM
Well, just grab a piece of plastic tubing and go for it! While this will work fine for pressures above 50 microns, outgassing of the plastic will stall the system when the pressure drops to the below 20 micron range, depending on how long the piece of tubing is. Thus, using plastic tubing is not satisfactory.
The O ring compression seal is best for temporary connections between the vacuum system and glass tubing.
Here we have a compression seal for 1/4 inch (6 mm) pyrex glass tubing to adapt it onto a KF-40 flange. A short stub flange is used as the base. A piece of brass stock is turned to have the O ring groove to seal to the inner surface of the stub. The adapter does not need any kind of mechanical securement at all. The pressure of the air against the adapter will keep it firmly in place. The compression adapter is threaded into the brass base using common tapered pipe threads sealed with solder on the inside. By not mechanically connecting the adapter to the stub flange, it makes it quick and easy to change adapters for different purposes at minimum cost.
By soldering the threads on the inside, no trapped volume of the threads is exposed to the vacuum so pumpdown is rapid with no virtual leaks. Note the cross bar that is pressed into the opening to keep the glass tube from being sucked down into the system.
The inner diameter of the compression seal is tapered just enough to let the O ring sit on the edge of the taper. The cap when tightened compresses the O ring against the glass tube and against the brass taper to form a vacuum tight seal. Use vacuum grease on the O ring.
To adapt a flare to the flange for leak testing of a pinch seal, a slightly different adapter is made. It is made of brass stock with the bottom O ring groove to match the KF-40 pipe stub as in the tubing adapter above. The top of the adapter has a larger hole that will accomodate the flares most commonly used. This one has a .6 inch (15 mm) opening. A slight lip is put on the inner edge of the hole to create a pressure point on the gasket disk. The flare is pressed into the gasket disk and set onto the adapter. It is sealed with vacuum grease. When the vacuum is applied, air pressure presses the flange down into the gasket forming a perfect seal. No mechanical method is necessary to press the flange down, just the ambient air pressure is fine.
The gasket is made of 1/8 inch (3 mm) thick silicone rubber gasket material. Other kinds of rubber can be used but it should be soft and pliable rather than stiff. Silicone gasket material can be made by using sheets of glass. Spread silicone caulking compound onto the glass. (It is best to oil the glass first or use sheets of Saran wrap to isolate the rubber from sticking to the glass) Press the top glass into place using 1/8 inch (3mm) spacers to set the thickness. Let this set for at least a week to harden. Carefully peel the Saran wrap from the rubber when it is cured. This can be used for all kinds of soft gaskets.
These two types of adapters will handle most all of the tube making applications.
ADMITTING GAS INTO THE VACUUM TUBE
There are certain types of vacuum tubes that will be filled with gas to a certain pressure. Nixie tubes, geiger tubes, and gas detector tubes are examples. To do this, there must be a method of precicely admitting gas to the system while monitoring the pressure. A manifold with needle valves is used for this application.
This is a homeade manifold. It is machined out of a bar of solid aluminum, but you could make one using individual valves and fittings. The valves and fittings must be carefully assembled using teflon tape to make good vacuum tight seals. This manifold has three needle valves, to allow three different types of gas to be used simultaneously. The cylinder on top is the gauge sensor. The gas cylinders are connected to the manifold using copper tubing flare connections for vacuum tightness. Do not use plastic tubing. Copper capillary tubing spiralled is used for the connections. It is soldered to short stubs of copper tubing that have the flare connection. This makes the connection between the cylinder and manifold flexible.
The manifold is connected to the system via a pipe with a machined O ring flange. The square block is the adapter to the vacuum system, which is a hole in the aluminum base plate. This is also sealed with an O ring. The vertical cylinder is a glass tubing adapter like the one above for the KF-40 flange.
The tubing adapter on the cylinder is machined from a solid bar of aluminum. The bottom flange is sealed with an O ring seal. The connector is extended up like this so that the tube will be in the center of the bakeout oven. When you make your adapter keep in mind that you don't want the O ring adapter to be inside the oven. The O ring will melt and the seal will be gone!