First thing that needs to be asked is what are your objectives in your chamber? What kind of experiments are you wishing to perform. In many cases a diffusion pump is the best pump to use.
Several different things, with different degrees of vacuum requirements ranging from gassy glow discharges (think plasma ball and neons displays), through surface sputtering work, down to electron tube range. I'm aiming to work my way down the torr negative exponent scale as I learn techniques. I've never done vacuum system work before.
Most of that will be just interesting sidelines once I have the gear running, but the main project, that it's all for is something I won't discuss in detail. Sorry. It's _similar_ in requirements to fusors, but isn't a fusor. Though, I do want to replicate those things just for fun and photos at one point, since the gear is so similar.
One thing I do need is to be able to cycle from open chamber down to working vacuum, and back, fairly quickly and often. Also I hate cleaning oil off stuff. This is why using that big diffusion pump is a last resort. A water-cooled back-stream condenser baffle came with that, but it's not very good and I'd need to make a better one.
Additionally I fear that little mechanical pump may be not be able to keep up with such a large chamber as a backing pump. Under IDEAL circumstances of perfect sealing you should be fine, however you will have a very large virtual leak and gas load and any UHV pump may stall.
No no, that little pump is not for that chamber.
It's for general use, and mainly I'm refurbishing it as practice before attempting any of the much larger backing pumps I have. There are pics of them in here:
http://everist.org/NobLog/20141212_racked_serendipity.htmIn fact the pumping system rack will have two backing pumps - one a large Busch 3-phase unit (that I'm still not sure what form it is inside, possibly scroll?) for pumping the chamber down from atmosphere fast, switching over to a 1/2 HP rotary to maintain. Yes, I know about oil suck-back.
Your two options would be to have a small diffusion booster pump or step up to a larger mechanical pump such as a welch 1402. Generally that pump is considered the standard for this sort of work. I have found it to be the overall best balance of pumping speed, ultimate pressure, long life, and low noise. Depending on there sort of experiment you are going to perform will dictate what sort of pump you should be using. If you are going to have a large gas load then diffusion pump with silicone oil is the best way to go.
The main project will have a large gas load, but requires rapid fine-controlled gas mix and pressure in the chamber. The plan is to have a turbo (or diffusion, sigh), with a servo-controlled iris aperture in the port between the chamber and the turbo.
If you will be doing surface research then you can only use dry pumps. A good option to turbo pumps at relatively low UHV would be a charcoal cryo sublimation pump. They are clean and easy to use provided you have access to liquid nitrogen.
No, won't be. The closest I'd come to that will be attempting to do metal deposition mirrors, as sidelines. But everything I do has to run on a frayed shoestring. So equipment has to be what I already have, or can scrounge for almost nothing. Even regular use of liquid nitrogen will be a cost strain, to avoid if possible.
Also ION pumps are a good option. I have found turbo pumps to be more trouble than they are worth. Also I find I cannot think with the noise they produce.
Hmm. I've never heard one running. I'd have thought the main peak would be up beyond audible? And with mag-lev bearing types, why is there any noise at all?
Anyway, my hearing is shot since a noisy industrial employment in my early 20s. I can't hear anything above about 8 to 10KHz. So maybe it's not a problem? If it is, nothing a sound absorbing enclosure won't fix. Or ear protectors. The main noise problem round here is and always will be neighbor's stupid barking dogs.
About 'more trouble than they are worth' I had the impression from reading that turbo pumps are reliable and low maintenance? Provided you don't allow sudden air admittance while they are running.
Is 'low maintenance' not true? It's a major attraction for me, since running costs need to be as near to zero as possible. No oil, and cheap to run - this wins, for me. Sublimation/ion pumps - I'd always be worrying about the ion source material running out.
I do however find the low RPM whuring sound of a welch pump rather therapeutic. As far as trapping goes, for research a good trap is critical. As far as running liquid nitrogen in one, for me that is debatable. It depends once again on the research. For my use in vacuum tubes we don't even bother. A bend or two in the manifold at room temperature is sufficient to condense back streaming vapor,
Yeah, the plan is to have all the oil-containing roughing pumps in one rack, then the chamber and high vacuum pumping in another stand, preferably with no oil at all in that one. Then I can try different oil vapor traps between the two. As you say, probably just a hose with some bends would be good enough.
and our parts are RF heated to the point where any condensate on the parts we are testing are evaporated back off at operating temp. On my home rig I use a water cooled baffle under my valve. The cooling water for the pump runs through the baffle first so its cooler than what is coming off the pump. Optical opacity in the path is a requirement. You may find that you have to use a non silicone oil if you are performing surface studies. Be ware that any admittance of oxygen will begin precipitating carbon deposits in the form of sludge in the pump if you use a hydrocarbon oil. Silicone oils can be exposed to air for a brief period at working temperature and after re stabilizing the system on subsequent operations can return to good performance without the buildup of crud. The nature of the silicone oil however can form a very thin non conductive layer on internal components and experiments if bad back streaming or over heating of the pump occurs.
Well, I still have a lot of other things to do before I have to commit to bolting on that oil diffusion pump. Still keeping my fingers crossed that a suitable turbo pump can be found before then.
Also that I won't need water cooling. I'm putting in a high airflow extraction duct to my soldering bench, which is right next to the vacuum system. So hopefully I can use that to also provide air cooling for vacuum system components where needed.
Lets start narrowing in by figuring out that you have in mind for this system.
Ha ha, ask a volt or time nut what accuracy they want. My main objective doesn't need very high vacuum or pristine conditions at all. But whatever the system can do, allows for more fun things to try. Exploration!