Well, a lot of them. There has been an evolution of this comparatively new way to pump over time, and of course, a lot of what is available inexpensively on the surplus market isn't all that great. At first, turbo pumps were thought of mainly as a diffusion pump replacement, without the oil, pretty much a plug in replacement. This means most early turbos need a very good backing pump, capable of pretty low foreline pressures to do much. And those are what are mostly around, often separated from their controllers or cables, both of which tend to be pretty pricey if you go back to the manufacturer for them. There being little standardization, this means you have to get the matching stuff or you've just bought useless (but pretty) junk.
Turbos don't like grit, shards, and some can't handle some chemical vapors without erosion. So for a "dirty" process, you either need a special spec turbo, or go see the diffusion pump thread here. At least if you "crud up" a diffusion pump, the repair is limited to an oil change and maybe a cleaning. A busted turbo is trash, and quite expensive to have repaired if there's damage to rotors or stators.
Older turbo pumps were just that -- a big turbine, spinning very fast that simply mechanically bashed atoms down into the pump. There's a limit to pressure that a spinning turbo can handle - at short mean free paths, the rotor tips are highly supersonic, and can actually burn up from friction with the gas, so inrush accidents are bad -- and the moan you hear with a even a tiny one will stick with you for awhile if you had to pay for one at new prices. Further, Pfeiffer warns you can twist the mounting bolts right off in such a case - there is one heck of a lot of stored rotational energy in one of these going full speed.
Newer pumps are called turbo-drag. These have a second drag stage that doesn't have rotor blades as such, it works more like an auger with fine threads. This means it can take higher pressures at that point and increase the compression ratio of the overall pump. The nice thing about this is now you don't need such a great forepump, which saves both money and electrical power. I have one small system that uses a diaphragm pump similar to what you might find at an aquarium shop as the forepump -- and it works great as a system.
Your checklist when buying a turbo should go like this, more or less.
- Is it complete? What would buying any missing controllers or cables cost you?
- Is it a turbo-drag, or a simple turbo?
- What's the bearing style? Are you getting a maglev, or one you'll have to be changing bearings and lube on?
- Will it run in the orientation I need? Some pumps don't care, some do.
it's too late and it's yours now. After all, Joe probably got it from Bob, who knew it was bad, but knew Joe couldn't test it, and never said anything about it to him.
So Joe didn't rip you off on purpose, maybe, even though it happened.
Really good things aren't getting into the surplus supply chain as often these days, so it's a definate caveat emptor, and if someone is
guaranteeing it to work, that's worth a price premium for sure.
Most turbo systems will get to a good enough vacuum for most home experimenters with no other tricks needed, other than the usual "make it not leak" and "be able to bake it out" -- the rest of the system, that is. In a CF flanged (metal gasket) system, you should expect to go at least to e-8 millibar, probably a good bit lower. Add a little viton, or a lot of surface area, and you might find yourself stuck around e-8 mbar -- still very very good. In fact (maybe someone here will correct me) unless you are studying pure surface phenomenon, or running things that need a super long mean free path (long beams or ion traps that hold ions for days on end) it's good enough, you're done. For example, even the old cyclotrons rarely beat e-6 millbar, and this is so much better you can have that and be assured of very low (<1%) content of anything in the tank you didn't put in there on purpose.
I feel pretty well set up on vacuum pump systems here. I have two Pfeiffer systems I bought new, used for different things, one running diff pump system, and parts for a better diff pump system should I want to put that together. I will say this -- once you get a turbo, you never ever look back. The diff pump systems are in storage.
One Pfeiffer system is the tiny guy -- 60 l/s, spins at 933 RPS, diaphragm backing pump, their controller and display. I use that in a fairly large viton sealed system, it's lucky to break e-6 on the downside due to all the surface area and gaskets, but it gets there quick. I use it for trying things quick without breaking vacuum on the big guy, for making glow discharge tubes off a tubing coupler that system has, evaporation, sputtering, and general playing around. For example, since it's so small and uses so little power, I will let it pull vacuum on my small deuterium tanks for long periods before I refill them from the master tank to keep my deuterium pure as can be. By getting the reception tank and the transfer plumbing really outgassed, I get better results re purity of the result -- and I don't have to run my bulk deuterium tank on my main rig. That's a safety issue -- if there's a mistake, only the little tank is lost, and I don't put much in there, so things like explosion concerns are much less.
The big system is a 520 l/s turbo with a two stage Xtra dry backing pump. This is what I mainly use, and it really gets the hours of operation because I leave my tank at vacuum between uses unless I'm about to crack it to change something in there. This saves on time and eliminates most questions of purity, and this easily gets the very big tank down to the border between e-8 mbar and e-9 with only very minimal baking. There's some viton in there -- a 6" hinged door, and (right now) 4 viton-sealed tubing couplers for HV feedthroughs and other uses. I can get it lower with other things, like evaporating or sputtering some Ti in there, but I rarely bother, as good enough is good enough.
I run both of these in a nifty mode. Since I am off the grid and on solar power, I care about power usage a lot more than most would. Pfeiffer controllers will let you program an output that will go "true" when the turbo draws more to spin than some power you can adjust. Hook this up to a SS relay, and run the forepump off of that. This means that the forepump rarely runs when in a static "holding" mode. Since both of my forepumps are oil free they are higher maintenance than a good old oily backing pump, so lifetime on them is a good thing to increase as well.
Both backing pumps are two stage affairs. And both have an air bleed between the stages to limit the buildup of condensed water vapor. I almost always just leave those open. I didn't know what they were for until I ran into some problems on the little system (the one that gets opened a lot), and called Pfeiffer and described the funny noise the backing pump was making, and the loss of good pumping. My rep had a good laugh and told me to find the little valve and open it -- problem gone, then and forever. Leaving that open doesn't seem to affect how often they have to run, or attainable vacuum much at all, so I just leave them open now. At some point in a pumpdown, there's really not much in the tank but water clinging to the walls. As this gets compressed to near atmospheric pressure, it goes liquid again, and in a positive displacement pump that's poison, no matter the type. (I suppose one of us should add this tip to the mechanical forepump thread if it's not there already?).
All turbo pumps should probably have a nice screen over the intake. The idea of dropping a bb sized chunk of something (things do sometimes shatter in my tanks) is pretty scary - total loss of the pump. Mine all do, and in the case of the little one, something would have to navigate some turns in the plumbing to find its way in there as well. Not a bad plan.
I'd guess the best word is this: If you can get a turbo-drag pump, do it, you'll be happy. You will be able to try more things in a session, due to the speed of pumping, going both ways -- you can get up to STP quick (most have a vent behind the main turbo section to do this safely) and back down quick.
I am NOT saying you have to have one. I have actually done comparative tests on plasma kinds of things (fusors, sputttering, evaporation) that say that a diff pump, properly operated, will do fine -- no essential difference in results. But my diff pumps are in storage for a reason - they are just more trouble.
Some situations you might worry about the little bit of vibration an older turbo might have. In my case, there is enough even with a new one on the big system to cause some slight concern, so I mounted it on a bellows below the tank, problem gone. I did have to add some springs across the bellows to keep them from collapsing too much and moving the pump around, no big deal.
(note to self, add pictures to this)
) If they are stuck on to the thing -- well, they probably aren't getting into me, they are stuck on the thing. If they come off easy, they come off easy -- and get gone quick. There is a potentially nasty middle ground. Use judgment. With reasonable precautions it seems you are OK with all but the very most vicious things -- say radioactives that are also poison. Mere poisons seem to resolve quickly. This is of course if you don't have to get too up close and personal with "whatever", and as in my case, work with a system that puts the exhausts outdoors. If the thing is poisoned and you have to take it apart to fix, yes, that implies that caution and general careful anti-contamination measures are taken - you are putting your fingers and face right in it, leaving any crud around the shop and so on -- best be real careful.