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Registered Member #55102
Joined: Tue May 26 2015, 12:03PM
Location: Johannesburg, South Africa
Posts: 4
First off, I do not mean to go off topic too much and/or hijack your thread Toasty!
@jpsmith123: I do not have any experience with that specific pump. I have played with a different brand of the same kind of build quality and price pump. It missed it's ultimate vacuum rating with approximately an order of magnitude or so. My limited experience points to "you get what you pay for" in most cases.
Which side of the pond are you jpsmith123 ?
PS. Thanks for starting the thread Toasty, without it I might never have decided to partake in discussion :)
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
I had a cheap "Dosivac" AC service pump made in Argentina that - surprisingly - buried my TC gauge. I sold the pump one day because I was frustrated with the poor mechanical design of the intake and exhaust ports (they were so close to the handle it was impossible to make any kind of reliable mechanical connections).
A few years later, the design of their pumps changed (outwardly at least) and so I bought another one on ebay. The spec on it was 15u, IIRC, but it wouldn't go below about 70u, so I immediately returned it.
Since then I also bought a used Alcatel pump on ebay which was advertised as being "tested and guaranteed", but it needed a rebuild and possibly a new motor, so I immediately returned that one too.
As I see it, there's no fundamental reason why these inexpensive AC pumps shouldn't meet their own advertised specifications, but being that many of them don't, it's valuable information to find a certain brand/model that does and helpful to post it in forums like this one.
Nowadays, if I buy another pump e.g., from ebay, I will tell the seller beforehand that I'm going to test it and that if it doesn't meet the advertised performance, it will be returned (and not at my expense). These things are heavy, and it's so costly to ship them back and forth across the country that that alone should be an incentive for sellers/manufacturers to provide an accurate set of specs. But I think they are assuming that the average buyer will not have a way to test them, and that's not a nice way to do business.
Anyway, since the OP stated that he needs "a good vacuum pump" I don't think this discussion about cheap imported pumps is going off-topic or hijacking the thread.
LouisHV wrote ...
First off, I do not mean to go off topic too much and/or hijack your thread Toasty!
@jpsmith123: I do not have any experience with that specific pump. I have played with a different brand of the same kind of build quality and price pump. It missed it's ultimate vacuum rating with approximately an order of magnitude or so. My limited experience points to "you get what you pay for" in most cases.
Which side of the pond are you jpsmith123 ?
PS. Thanks for starting the thread Toasty, without it I might never have decided to partake in discussion :)
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
The older, belt drive, pumps are the easiest and cheapest to rebuild, but they were built to last. They also tend to be a lot cheaper than the newer ones.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
I'll never understand the vacuum pump industry.
It doesn't make sense to me that I can buy a new 3 cfm to 5 cfm pump that'll pull down to the range of, say, 25u to 75u, for $100 to $150, but if I want a new pump that'll go to 10u or better, it's going to cost me $1500 to $2000?
The pumps operate on the same basic principle (rotary vane design) and are generally made from the same materials, yet there's about an order of magnitude difference in price? Just because one pump apparently has a relatively somewhat smaller "dead space"?
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615
economies of scale? Plenty of applications are satisfied with 50u. You may also find going 10u or less needs an extra pump stage. At 10u you're getting into the region where gas flow is by diffusion, rather than molecular collision. At 10uTorr mean free path is around 8cm, this is why you need large area pipes for reasonable flow rates - this would translate to a large swept area in the pump, and this is going to cost money. You'd probably find the oil gets a bit special too - has to have a really low vapor pressure at the pumps operating temperature.
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
Yea there is a lot you're not seeing!
The direct drives use cheaper castings... AND they use castings!
Of course my Welch belt drives use castings for the outer pump housing, but they are finished to +/- .0001 tolerances. The rotors are machined to +/- .0001 or better tolerances, the drive shaft is hardened... hardened!
It's all cast iron or steel, and very few parts of other metals, you're paying for the machining.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
@2Spoons
I neither need nor would I expect any mechanical pump to perform well in the molecular flow regime, but that's not the issue.
My Dosivac pump was a "cheap" (< $300) two stage AC pump, yet it buried my Varian TC gauge, so it was bottoming out well below 10u. That pump was the only cheap AC pump I've yet seen that not only met but significantly exceeded its advertised specs.
After that I bought two other AC type pumps (they were both brand new two stage pumps) and neither one even met its advertised spec., let alone exceed it. And I've heard of other people having the same experience.
I realize that 50u is "good enough" for some applications, but I'd still like to know why a pump that'll go <10u should cost 10x more than a pump that bottoms out in the 25u to 50u range.
@Hazmatt
My 1u Dosivac pump was a direct drive pump, just like my 35u Harbor Freight pump. And I think I have to disagree about the machining being so critical; after all, these pumps are "oil sealed".
Edit:
BTW, one specific pump I was looking at was an "Inficon" "QS5" which is advertised as "being able to pull down to 15u", or something like that. So I contacted the manufacturer, and after establishing a friendly dialogue, I asked the following question: "If I were to take say 10 brand new pumps right off the assembly line, fill them with high quality vacuum pump oil and connect my Varian TC gauge (with type 531 tube) directly to the pump's intake port, what kind of average ultimate vacuum would I measure? They never responded to the question, so I never bought the pump.
Registered Member #56785
Joined: Fri Aug 28 2015, 02:54PM
Location:
Posts: 20
Hey guys, little update here
Made a nice vacuum chamber with a big mason jar and a one way valve, now i wamt to buy a cheap vacuum pump online. However what kind of vacuum pressure should i look for when i want some decent looking plasma?
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
Yea you can do that, but make sure to tape the mason jar with electrical tape in a spider-web pattern. If you don't, and there is a weak spot on the jar it will implode then explode sending glass at like 100 feet per second in every direction.
That's why you buy vacuum rated heavy wall bell jars for this kind of thing, and even the big ones use steel cages for this reason.
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Vacuum for high voltage plasma
