Re: Vacuum for high voltage plasma
woodchuck, Wed Feb 10 2016, 11:34PM

My first vacuum pump was a section of surgical rubber tubing and two print rollers. Sort of a manual peristaltic pump. It helps if you put a tiny amount of a low vapor pressure oil on the inside of the tubing (like the oil used in diffusion pumps).

It's slow, but it is more effective than you might imagine.

Re: Vacuum for high voltage plasma
LouisHV, Thu Feb 11 2016, 02:16PM

Hi Toasty,

I have used a refrigeration compressor from a fridge I found in a dumpster on the street I live in. It doesn't hold a candle to my Edwards RV5 but it got me into "playful" plasma territory really cheap. Be careful of the stuff inside the copper pipes... disposal regulations and not to mention what it could mean for your health. Someone with more experience will hopefully chime in on this subject.

From experimentation I found fridge compressor > tyre compressor.

The bicycle pump... I once tried to pump down a jam jar (metal lid) with a "reversible"bicycle pump. It worked but it leaked through the cheap seals within a minute or so. The type of bicycle pump that has a foot pedal type of actuator works really good, if you can use an average quality one way valve inline.

If you are willing to spend some money...

this: http://www.amazon.com/Tenive-Double-Rotary-Economy-Vacuum/dp/B017AYO82E/ref=sr_1_2?ie=UTF8&qid=1455198581&sr=8-2-spons&keywords=vacuum+pump&psc=1 is not bad at all! I can not vouch for the quality though.

for less than half the price http://www.amazon.com/Zeny%C2%AE-Single-Stage-Rotary-Vacuum-R410a/dp/B012CFTYX4/ref=sr_1_3?ie=UTF8&qid=1455198581&sr=8-3&keywords=vacuum+pump will get you to a claimed 5 Pascals.. probably much higher pressure than that but hey for 35 USD + shipping... Plasma all day long!

If you are willing to spend a 100USD or so a 2nd hand yellow jacket vacuum pump built for HVAC servicing will last long, be serviceable, have parts and oil available and is reputable and known in the industry.

If you are desperate to play with plasma, I once jammed a nail into the tip of a big-ish plastic syringe and pulled back on the plunger, touched the nail to a small flyback used to power a bad quality plasma ball toy (+-5kv maybe) and got some great quick and cheap plasma goodness. Used some hot glue to seal the nail to tip part and if careful enough to not pull the plunger out completely could use it quite a couple of times.

Good luck and peace :)

Louis.


edit: I like woodchuck's method. with an okay one-way valve and nice silicone rubber hose it should work surprisingly well.

Re: Vacuum for high voltage plasma
2Spoons, Thu Feb 11 2016, 11:41PM

If you want a really simple solution you could make a venturi pump, also known as a water aspirator. I made one when I was about 14, out of some bits of copper tubing. Worked a treat for plasma-in-a-jar experiments. Just have to be careful not to let water get sucked back into the jar when turning off the water supply.

How good your vacuum is depends on how cold your water supply is (colder=better).

Re: Vacuum for high voltage plasma
Hazmatt_(The Underdog), Fri Feb 12 2016, 02:28AM

I made an aspirator too, it worked pretty well, but I think buying one for the $14 would be more do-able.

I would recommend to stay away from the really nice pumps on ebay because they always need a rebuild.
I still have yet to successfully rebuild my Welch 1402 because the main seal just listed as "seal" in the drawing has a trick to install it, AND when rebuilding the pump you actually have to repaint it. The painting process serves to seal the pump partially, so they are tricky bastards to rebuild, much more so then what appears on the surface.

A major rebuild kit for a simple Welch is gonna run $250, in addition to the pump running $150, but you're going to need a Hastings gauge as well to see if you rebuilt it right, which is going to run another $250 for the meter and tube.
And if you didn't rebuild it tight enough, you might have to spend $45 on heavier oil to get you where you want to be, half a micron.

So I would stick to the aspirator for now.

P.S. don't even get started on vacuum line! that stuff is really expensive too!

Re: Vacuum for high voltage plasma
Ash Small, Fri Feb 12 2016, 02:35AM

Have you looked on eBay?depending on what side of the pond you're on it's either Edwards or Welch, I think.

Thet are surprisingly cheap, and even when the buyer puts 'untested' or whatever, the chances are they are still perfectly servicable.

There are tutorials online for rebuilding them cheaply if required.

It's sensible to change the oil.

I've two I paid next to nothing for.

Diffusion pumps come up cheaply as well, but you won't need one of those yet

Re: Vacuum for high voltage plasma
2Spoons, Fri Feb 12 2016, 03:01AM

Sometimes I forget how easy it is to just buy stuff from ebay. I made my aspirator before the internet existed. Buying one was just not an option. And frankly, building one and seeing it work was very satisfying.

Re: Vacuum for high voltage plasma
woodchuck, Fri Feb 12 2016, 04:25AM

FWIW, of the many varied water aspirator pumps out there, the best I know of is the polypropylene Thermo Scientific Nalgene pump. It produced the best vacuum and used the least amount of water.

Re: Vacuum for high voltage plasma
jpsmith123, Fri Feb 12 2016, 04:46AM

Hello LouisHV,

I'm wondering, do you have any experience with that "Tenive" vacuum pump?

The reason I ask is because on the amazon web site, the ultimate vacuum spec. is stated to be 0.3 Pa, which is very good for a cheap AC service type vacuum pump.

If that pump will actually go that low I want one (although it would be better if it also had a gas ballast feature).

Re: Vacuum for high voltage plasma
Toasty, Fri Feb 12 2016, 05:57AM

Oh nice! Never heard of it but it looks like a simple way to get a nice vacuum
Thanks

Re: Vacuum for high voltage plasma
LouisHV, Fri Feb 12 2016, 10:02AM

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 :)

Re: Vacuum for high voltage plasma
jpsmith123, Fri Feb 12 2016, 04:12PM

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.

Re: Vacuum for high voltage plasma
Ash Small, Sun Feb 14 2016, 02:14AM

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.

Re: Vacuum for high voltage plasma
jpsmith123, Sun Feb 14 2016, 08:38PM

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"?

Is there something I'm not seeing here?

Re: Vacuum for high voltage plasma
2Spoons, Sun Feb 14 2016, 09:56PM

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.

Re: Vacuum for high voltage plasma
Hazmatt_(The Underdog), Sun Feb 14 2016, 09:57PM

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.

Re: Vacuum for high voltage plasma
Ash Small, Sun Feb 14 2016, 10:38PM

Same with my Edwards belt drives

Re: Vacuum for high voltage plasma
jpsmith123, Mon Feb 15 2016, 02:53AM

@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.

Re: Vacuum for high voltage plasma
Toasty, Tue Feb 16 2016, 03:54PM

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?

Re: Vacuum for high voltage plasma
Hazmatt_(The Underdog), Tue Feb 16 2016, 10:14PM

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.

Re: Vacuum for high voltage plasma
Toasty, Wed Feb 17 2016, 01:41PM


Thanks, its pretty thick glass but i got a plastic sheet in front of it so i should be fine if it implodes.... i hope

Still... what kind of vacuum pressure should i have to get some decent plasma going?

Re: Vacuum for high voltage plasma
jpsmith123, Wed Feb 17 2016, 02:48PM

Generally speaking, the results you get will depend on geometry, the electrodes, the applied voltage (AC, DC, pulsed), the type of gas, etc.
You can even make a plasma at atmospheric pressure; i.e., under certain conditions you wouldn't even need a vacuum pump. (See, e.g., US Patent 8471171).

If you just want to make a glow discharge plasma inside a glass tube, for example, then you probably want a pressure in the range of say, 5 to 20 torr, or something like that, so even a cheap single stage AC type vacuum pump would probably work.

I imagine you would evacuate the container down to the limit of the pump and then open a leak valve (could be just some rubber tubing with a clamp or something like that) and then see at what point a glow discharge starts for your particular set up.

Re: Vacuum for high voltage plasma
woodchuck, Wed Feb 17 2016, 04:35PM

There is a book called X-Rays, by a fellow named Kaye, that is available at archive.org (written sometime around 1917). It is a great read, and by the time you've finished the first few chapters, you'll be something of an authority on the subject of electrical discharge phenomenon at reduced pressure.

Re: Vacuum for high voltage plasma
2Spoons, Thu Feb 18 2016, 02:02AM


Based on the vapor pressure of water at 15C, my water aspirator would not have gone below 13 Torr. I found the air plasma (roughly a 10cm discharge) I had going in a small jar starting to fade if I left the aspirator running. So that shows the vacuum requirements are rather modest.

Re: Vacuum for high voltage plasma
Ash Small, Thu Feb 18 2016, 09:48AM

Most commercial vacuum plasma systems ignite the plasma at a higher pressure than the working pressure. Once the plasma is established the pressure is then reduced to the working pressure.

Paschen curves for various gases should give you some idea.

Re: Vacuum for high voltage plasma
klugesmith, Fri Feb 19 2016, 04:02PM

>> Still... what kind of vacuum pressure should i have to get some decent plasma going?

You can quickly see lots of plasmas at different pressures by poking around on the fusor forum.
Neon signs and fluorescent lights work in the 10 to 100 torr region.

Rotary pumps often quote ultimate pressures on the order of one micron (millitorr,microbar,decipascal) but don't actually get there, even with gauge & nothing else on the pump. Even industrial pumps from reputable manufacturers. One excuse would be testing with a McLeod gauge, that's used to characterize & calibrate electronic gauges. McLeod gauge measures the compression ratio to bring a "vacuum" sample up to atmospheric pressure, so they miss the partial pressure of condensible vapors in the sample.

Rectified HV adds interest. For example, at pressures where electrodes are coated with a layer of plasma, it's the negative electrode.

Re: Vacuum for high voltage plasma
Ash Small, Fri Feb 19 2016, 11:08PM

It may seem like you're not getting direct answers, but the question isn't that direct. You've not specified a gas, not specified the type of coupling (capacitive or inductive, I think we assume capacitive), you've not specified voltage or power.

You've not said 'I want a pump that will reach this pressure with this chamber'.

We can only reply to the questions you ask. Maybe you should read up a bit on Paschen's law and the various types of coupling, etc? With AC, even the frequency can make a difference.

Re: Vacuum for high voltage plasma
Toasty, Sat Feb 20 2016, 01:04PM

I'm learning a lot from all this, i think i will poke around first, see what happens.