AMATEUR VACUUM TUBE MAKING
Turbomolecular pumps are a mechanical high vacuum pump. They work by using fast moving blades to "blow" gas molecules from the inlet to the outlet. Imagine a vacuum cleaner designed for high vacuum use!
These pumps are not difficult to use but have two serious disadvantages for an amateur system. The first is the high cost. The pump itself, in good used condition, will cost anywhere from less than a hundred dollars for a pump on ebay from a seller that just wants it out of there, to over a thousand dollars for a working pump from a seller that knows what he has.
The turbo pump also must have a controller or power supply. This controler is a box of electronics that converts ordinary 120 volt line voltage to the special high frequency drive voltages required by the turbo motor. The controller can cost as much or more than the turbo if you are shopping on ebay. It is best to get the turbo and controller WITH CONNECTING CABLE from the same seller. The connecting cable can be a real headache if it is missing. It is almost impossible to get schematics and interconnecting diagrams from the manufacturer of the turbo so it can be very difficult to figure out all the pin connections.
All this problem with cost and cables, and if it is in working condition, is still not the most serious probelm with turbo pumps!
The pump when operating has blades that are rotating at high speed, the tips can reach the speed of sound! Thus, any small particle that drops into the turbo will be impacted by the spinning blades. This can result in anything from no damage at all to complete destruction of the turbo, all in a split second! A piece of sand or glass shard the size of a grain of salt will fall through the protection screen on the turbo. When it is impacted by the turbo rotor blade, it demolishes a small part of the leading edge of the blade. This will leave a small pit in the blade. One or two pits will not hurt the balance of the rotor but if a lot of grit falls into the turbo the rotor will ultimately become unbalanced enough to run rough. It will wear out its bearings in short order!
This hazard is not present in a diffusion pump. Junk can fall into a diffusion pump to the point of it stopping pumping and it will not hurt the pump: you just clean it all out and go back to work.
There are certain processes that cannot tolerate the relatively high vapor pressure of pump oil that is present in diffusion pump systems. Processing laser discharge tubes and making precision optical lens and mirror coatings are examples. For a system to undertake these processes, the turbo pump is ideal because the amount of backstreaming oil is almost nonexistent. The oil from the diffusion pump is no problem at all in vacuum tube making.
So if you are set on getting a turbo pump, what do you look for? Of course your budget will determine a lot what you can do. Ebay is a good source of used turbos but you never know what you are going to get. They may be "used up" turbos that have worn out bearings and are due to be rebuilt. This is a serious problem for the amateur because of the special equipment required. It may simply be impossible to rebuild the turbo in an amateur environment due to special presses and fixtures required. A factory rebuild will cost from high hundreds of dollars to thousands! Not practical for most amateurs.
Other turbos can be easily rebuilt by the amateur with simple to make fixtures. (see a turbo bearing replacement under my youtube ID, "glasslinger" )
So how big of a turbo do you need? They come in all sizes from about 1 inch diameter rotor to huge 12 inch sizes for large production chambers. The most common size on ebay seems to be in the range between 3 inches and 8 inches. The three inch or four inch size is ideal for your tube making system. Larger sizes are very expensive and they also tend to be quite heavy. The pumping speed will not be a factor in tube making since the volume to be pumped will be very small. Any of the small turbos will have good pumping speed for tube making.
more on turbos coming up in a few weeks!