A seal-off valve possessing several advantages for high-vacuum work has been devised by R. J. Richards of the National Bureau of Standards. The valve is designed so that its handle and stem can be readily removed and used with any number of seal-off seats, leaving only the closed valve seat on the evacuated apparatus. In this way the vacuum system is protected from the atmosphere at all times yet the bulkiness of a complete permanent valve is avoided. The device is also well adapted to use as a pressure seal or as a combination safety valve and seal-off valve. It is now being used for high-vacuum applications in work sponsored by the Atomic Energy Commission at the NBS-AEC Cryogenic Engineering Laboratory.
Vacuum systems are commonly sealed off either by installing a complete valve or by pinching off annealed metal tubing, with or without the use of solder. Both types of tubing seals require the soldering or welding of a new piece of tubing into the system each time it is evacuated. This not only results in exposure of the vacuum system to the atmosphere but often presents difficult problems of manipulation. A complete valve is objectionable because it is frequently an awkward protuberance on a small piece of apparatus.
The NBS valve provides a permanent pumping tap that can be used many times without soldering or welding. Principal parts are a valve handle and stem, the valve body and seat, and a removable bonnet enclosing the seat assembly and adjacent end of the stem. The movable seat contains six ports through which the vacuum system is pumped out. Attached to this end of the stem is a narrow bar which contains two small pins equidistant from the axis of the stem. When the desired vacuum has been obtained, the pins are made to engage two of the ports in the seat, and the handle is turned until the valve is seated. The screws of the bonnet are then loosened, and all the parts except the movable seat assembly are removed as a unit. A rubber or plastic cap may be used to protect the remaining parts from dirt. When the static vacuum space requires pumping again, the removable parts are replaced, the bonnet screws are tightened, and the valve handle is turned to open the valve.
Because of the very short ports in the valve, the valve body can be made very small without sacrificing pumping speed. The parts can be made of any suitable material, but a neoprene seat is recommended for use on high-vacuum systems.