Fusor Forums

As I've said in a couple of posts, I'm trying to do physical vapor deposition. Now, obviously, with PVD, purity is key. As such, it is best to avoid all source of contamination as possible. The first way is to use a turbo instead of diffusion pump, which is something I'm already pursuing. However, I remember seeing somewhere on this board, a person mentioning that if you have a "turbodrag" pump (I would imagine he meant a hybrid turbomolecular and molecular drag pump) you can use a diaphragm pump for roughing and avoid oil all together. This brings me to a few specific questions.

1. Is this information true? I don't want to go buying equipment based off of an off-statement made by a complete stranger on the internet!

2. Are diaphragms, for this application, the best oil-free roughing pump to use?

3. When buying one (assuming the answer to previous question is yes) what should I look for in a used one? What should I avoid?

4. Lastly, is there anything else I should know about oil-free systems, or any pointers you have? I did comb through the excellent vacuumlab website.

1. Is this information true? I don't want to go buying equipment based off of an off-statement made by a complete stranger on the internet!

Keith, hate to break it to you, but you're still soliciting free advice from "complete strangers on the Internet". Hobbyists are generally happy doing that, being that they can take their time to cull the wheat from the chaff and are responsible only to themselves for the cost of ill-advisement. (On the other hand, If you're doing this project with someone else's money, you're surely expected to consult resources with better provenance.)

Anyway, for what my advice is worth, most turbopumps made today are hybrids. Most have a dozen or so turbine stages and one or more drag stages. In a lot of instrumentation pumps, you may find a reversal of this with a Holweck-type drag pump topped off with one or two turbine stages. Many of the smaller offerings are explicitly designed to be backed by diaphragm pumps with ultimate vacuum above 10 torr. It's common for such a turbopump to be matched with a diaphragm pump and packaged as a "dry" pumping station. This trend is in full swing right now, but has been going on for well over a decade.

Diaphragm pumps are not the only oil-free backing pump out there. Scroll pumps are in widespread use and have superior ultimate vacuum and, generally, speed. They are made in capacities suitable for the largest lab-scale vacuum systems. Roots blowers ("lobe" pumps, as the small ones are sometimes called) are also available; the big ones are widespread in industrial semiconductor fabs. Diaphragm pumps don't last as long as the alternatives because of the need to periodically replace the diaphragms. They choke badly on water or other condensable vapors. And their ultimate pressure is considerably higher than the other options. They're used because they are cheap. If your backing pressure is 10 torr, you're 3-4 orders of magnitude higher than what a good old oil-sealed pump can do, and in some applications--particularly those involving light gases--you're more likely to run up against compression ratio limitations on the high-vac side of the turbopump.

When buying a diaphragm pump, assume you will have to replace the diaphragms. Check into price and availability.

-Carl

Carl Willis wrote:Keith, hate to break it to you, but you're still soliciting free advice from "complete strangers on the Internet". Hobbyists are generally happy doing that, being that they can take their time to cull the wheat from the chaff and are responsible only to themselves for the cost of ill-advisement. (On the other hand, If you're doing this project with someone else's money, you're surely expected to consult resources with better provenance.)

Oh, I know. What I meant was that the post where I saw this, the person had just made the statement off-hand as a side comment. I could have easily misunderstood what he meant, misread it, or whatever.


EDIT:

Also, I forgot to ask, with respect to scroll pumps, I know about them but had avoided them because supposedly they require a lot of regular maintenance (as in more than a turbo) and, most importantly they are expensive. Obviously, as you mentioned, diaphragm pumps need to have the diaphragms replaced, but it was my impression that by maintenance, what was meant was taking it apart and re-lubricating the whole thing. That being said, would it be possible to use a scroll compressor as are commonly found in automotive AC units?