Is this a good vacuum pump for making plasma/neon displays? - Page 1
Forum section: electronics / general

author: Ben321
author_rank: Frequent Contributor
timestamp: September 17, 2022, 06:08:23 am
content: I was looking at thishttps://www.amazon.com/CPS-Products-Pro-Set-Vacuum-micron/dp/B00SV8B3NGwhich says it is a 2-stage vacuum pump, that can pull the pressure down to only 10 microns. From my research, a pressure rating in microns means the number of micrometers of mercury that would be indicated on a mercury column barometer. Also I learned that standard atmospheric pressure is 760 millimeters of mercury. Each millimeter is 1000 micrometers, so standard atmospheric pressure is 760 THOUSAND micrometers of mercury. Therefore a 10 micron vacuum, means that 99.999% of the air has been removed! That sounds VERY IMPRESSIVE!However, I would like to know if this is sufficient for making a neon tube, or a plasma globe, or any other type of high voltage plasma display.

author: Whales
author_rank: Super Contributor
timestamp: September 17, 2022, 06:16:11 am
content: What's the purity of the gas you're putting in them?If it's less pure than the vacuum you can pull: you don't need to go that far.If it's more pure than the vacuum you can pull: you can probably let some in and keep pumping to rinse out the gunk.

author: Ben321
author_rank: Frequent Contributor
timestamp: September 17, 2022, 10:33:02 am
content: Quote from: Whales on September 17, 2022, 06:16:11 amGood points, but I'm wondering about the strength of the vacuum itself. Is a 10 micron vacuum sufficient for things like neon lights or plasma globes? Or do I need something even lower pressure?

author: jpanhalt
author_rank: Super Contributor
timestamp: September 17, 2022, 01:17:53 pm
content: 1 micron = 10E-3 torrAccording to Wikipedia, neon lamps are generally at 1 to 20 torr (1 to 20 mmHg). So, that pump, if it really goes to 10 microns, is adequate. If the pressure is too low, you don't get any plasma effect. (Edit: You can get about 15 to 20 mmHg with a simple water aspirator pump.)I would be more worried about oil contamination. I always used at least a dry ice trap and usually a liquid N2 trap between my roughing pump/diffusion pump and the system to prevent contamination. Do you have a pressure gauge for that range of pressure?

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 17, 2022, 09:11:47 pm
content: All you ever wanted to know about vacuum pumps:https://repairfaq.org/sam/lasercva.htm#cvatoc

author: james_s
author_rank: Super Contributor
timestamp: September 18, 2022, 01:16:40 am
content: The problem is not the final pressure but the fact that you typically need to evacuate these things rapidly and get the pressure down low enough while heating the glass to drive moisture out of the glass and suck it out before it can condense again. Typically for neon tube processing you'll have a 2 stage mechanical backing pump, usually with around 1/2 to 3/4 HP backing an oil diffusion pump. You can also use a turbopump but you have to be very careful as neon units sometimes shatter during processing and even a tiny fragment of glass can cause catastrophic failure of a turbopump.Now if you just want to mess around with low pressure discharges and don't care about making high quality sealed gas discharge devices then yes, one of those refrigeration service pumps can do ok. You have to be careful not to let oil vapor back up into the evacuated object though.

author: Wolfram
author_rank: Frequent Contributor
timestamp: September 22, 2022, 12:43:33 pm
content: It depends on a lot of things. High pressure (say 100 torr and above) plasma mixtures with heavy gases can be pretty forgiving, getting a clean helium discharge at 8 torr will take significantly more effort.The residual pressure in your lamp will come mostly from surface outgassing from the glass, mainly trapped moisture. Pumping speed is often also limited by the size and length of your tubulation. So neither the speed nor final pressure of the pump is the most critical factor here, as long as they are not the main bottleneck in the system. Any well-functioning two stage rotary vane pump will allow you to get a lot of nice plasma effects if you have the right gases, and a good system to meter both the evacuation and the filling process.As long as you have a two-stage pump that is working decently, your final gas purity will mainly be determined by how well you clean and bakeout the lamp before filling. Backstreaming of pump oil is also a thing to watch out for with oil-sealed pumps and long pumpdown cycles.At the price of that pump, you can probably get a used Alcatel, Edwards, Pfeiffer or Varian on eBay. Ultimate pressure and pump speed are likely to be marginally better, and serviceability is significantly better. Most professional pumps, even fifty year old ones, still have spare parts and rebuild kits available.No matter which pump you get, I strongly recommend a system to accurately meter the vacuum, both to verify that the pump is working well and to find and help eliminate the inevitable leaks in the system. Surplus Granville 275s from eBay always worked well in my experience, some people like the MKS 901p gauges that are also available surplus regularly. Some surplus baratrons for monitoring fill pressure are also highly recommended.

author: DavidAlfa
author_rank: Super Contributor
timestamp: September 22, 2022, 09:29:24 pm
content: I've done that in the past just with a freezer/aircon compressor from the scrapyard.It made a decent vacuum for cheap, in fact leaving it pumping for too long stopped creating plasma.Perfect for DIY.

author: james_s
author_rank: Super Contributor
timestamp: September 22, 2022, 10:36:54 pm
content: I've used refrigeration compressors too, they were a great source of vacuum when I was a broke teenager. Even the best refrigeration compressors are going to be at least an order of magnitude worse than even a cheap Chinese vacuum pump.

author: Ben321
author_rank: Frequent Contributor
timestamp: October 12, 2022, 09:38:02 am
content: Quote from: james_s on September 18, 2022, 01:16:40 amI think some vacuum pump companies actually make oil filters for the exhaust port to keep oil from spraying back into the room. Would this also help absorb enough oil to keep it from getting into the object being pumped out?

author: jpanhalt
author_rank: Super Contributor
timestamp: October 12, 2022, 10:37:26 am
content: Quote from: Ben321 on October 12, 2022, 09:38:02 amNo. The oil that contaminants an evacuated system comes from the pump.

author: james_s
author_rank: Super Contributor
timestamp: October 12, 2022, 05:28:42 pm
content: Those filters will reduce the amount of oil mist from the pump exhaust, which is in the form of relatively large droplets. They won't do much of anything to prevent microscopic oil vapor from contaminating the system though. High vacuum is tricky, it's easy to get down to a few thousand microns, it's not too hard to get down to a few hundred microns, but going lower than that becomes exponentially harder and more expensive. Many materials that would normally be good at sealing start to leak like a sieve. Stuff starts to outgas, a fingerprint inside your vacuum vessel can outgas for days contaminating your vacuum.You can have a lot of fun with gas discharges and stuff with a pretty basic vacuum rig, but if you want to seal off your creations and have them last a reasonable amount of time you need a vacuum orders of magnitude better. A good high vacuum rig can be put together for a few thousand dollars if you shop carefully for surplus bits but it takes patience.

author: Ben321
author_rank: Frequent Contributor
timestamp: October 13, 2022, 04:34:51 am
content: Quote from: james_s on October 12, 2022, 05:28:42 pmI don't need an absolute vacuum like a CRT monitor tube. I just need very low pressure gas like in neon/fluorescent/plasma type lights and displays.

author: james_s
author_rank: Super Contributor
timestamp: October 13, 2022, 07:24:38 am
content: Quote from: Ben321 on October 13, 2022, 04:34:51 amLike I said, if you want to seal off the tubes you do need a deep vacuum like a CRT, you have to be able to drive the moisture and contaminates out of the glass and metal parts. You can get away with less if you process them in an oven and keep it attached to the pump for several hours with the glass heated to several hundred degrees. You draw a deep vacuum to get all the air and contaminates out, then you backfill to the desired pressure with the fill gas. It takes very little air remaining to poison the tube.

author: Ben321
author_rank: Frequent Contributor
timestamp: October 13, 2022, 07:54:33 am
content: Quote from: james_s on October 13, 2022, 07:24:38 amWell I don't have any expensive equipment like glass working equipment (ovens, torches, etc), so there's no possibility for me to seal it. I'd need to either leave the pump attached, or close a valve after pumping to hold the vacuum. I was thinking of a plan to fill the discharge vessel like this.Pump as hard of a vacuum as the pump will go. Fill the tube with the desired gas until the vacuum is gone (pressure is back at 1 atmosphere). Then pump it down again. Repeat this a few times, and it should get a pretty pure gas fill. After the last gas filling operation, pump it down one last time to the desired pressure (not so low that the discharge would be so faint as to be hard to see, but not so high that a discharge can't form). So during the last pump down step I'd have the high voltage hooked up, and stop running the pump when the discharge looked the way I wanted it to. I might experiment with keeping different amounts of air in (the nitrogen and oxygen in the air could potentially change the texture and color of the discharge). Of course my first experiment would be to just pump down the air pressure as far as I could go, and see what a low-pressure air discharge looked like. That itself might be all I'd need for making my plasma display, if it had the color and texture I was hoping for.That's the way I was thinking of doing this. By the way, my equipment is very simple and for now (and I hope to keep it that way, with the exception of buying the gas bottle, which I may not even need if a low-pressure air discharge looks good enough for me). My discharge vessel is an erlenmeyer flask with a side-port for attaching a vacuum pump. It's a flask similar to this onehttps://cdn.shopify.com/s/files/1/0250/0623/products/dc4nk088x5o6pgyknxz9_354x354.jpg?v=1659977236I have a #7 rubber stopper for capping off the main opening of the flask. I can easily poke a needle through the rubber stopper, and put a ball of aluminum foil on the tip of the needle, to make a discharge point (using the actual tip of the needle without the aluminum ball would likely focus the arc on such a tiny spot that it would melt the tip of the needle, which I don't want).All I need now really (at least at this point in my experiment) is a decent vacuum pump and a way to connect the pump to the side-arm of the flask. I'm guessing that the vacuum hose that is designed to connect to one of those refrigeration maintenance vacuum pumps would NOT fit on the side-arm of the flask. I'd need some kind of hose-size adapter and 2 hoses (one for the side-arm of the flask, and one for the vacuum pump).

author: james_s
author_rank: Super Contributor
timestamp: October 13, 2022, 05:07:41 pm
content: Yes repeated flushing is a usable way of getting contaminates out. Your best bet on a tight budget is probably a used refrigeration service pump, a decent one should go under 100 microns pretty easily, occasionally much lower. The standard fitting used on these pumps is 1/4" SAE flare, and the typical service hoses are made to fit these. You can get adapters or replace the fitting on the pump to something else if you like. Your biggest issue is going to be getting an adequate seal, even tiny leaks can easily overwhelm the pumping capacity at low pressures and that is likely going to be your limiting factor. You need the right kind of hoses and seals, vinyl tubing for example will leak gas right through the walls as if they're made of fabric. Gum rubber is quite good though.

author: Ben321
author_rank: Frequent Contributor
timestamp: October 13, 2022, 09:59:16 pm
content: Quote from: james_s on October 13, 2022, 05:07:41 pmI don't know what type of plastic or rubber the tubing is made out of, but I have the short segment of plastic or rubber tubing that came with a used hand-operated vacuum pump (I've been using that for some vacuum experiments and it seems to pull a decent vacuum (abut 25 inches of mercury) in my 250mL flask. My hand operated vacuum pump has a builtin one-way valve (with manual release to let the air back in) and a vacuum gauge. I've never seen the the needle on the gauge slowly rising after stopping pumping (which would indicate a leak in the system), even after the flask has been pumped down as far as this pump will go. So I think that whatever this tube is made of, it will likely handle an even stronger vacuum that approaches a complete vacuum (a complete vacuum being 30 inches of mercury). However hand pumping is very tiring. That's why I'm looking for a motorized pump that's at least as good as this hand operated pump.

author: james_s
author_rank: Super Contributor
timestamp: October 13, 2022, 10:07:18 pm
content: That clear plastic tubing is going to be vinyl. Inches of mercury doesn't really apply to high vacuum, when I say microns I'm referring to microns of mercury, there are 25400 microns in one inch of mercury and even a worn out single stage pump should easily peg a mechanical gauge at 30 inches, achieving perhaps 1,000 microns. A good 2 stage pump is capable of getting down to under 100 microns, I have one that will reach 1.5 microns after an hour or so once the oil is properly de-gassed. Deeper vacuum gets exponentially more difficult to achieve.Read Sam's vacuum FAQ that somebody else posted, it has all the information you will need.

author: Ben321
author_rank: Frequent Contributor
timestamp: October 14, 2022, 08:49:44 pm
content: Quote from: james_s on October 13, 2022, 10:07:18 pmWhich is the best type of mechanical pump, in terms of its ability to pull a vacuum? I know there's the rotary vane pump, the piston pump, and the diaphragm pump. I think only the rotary vane pump uses oil, as the others are often advertised as oil-less alternatives. However I'm not sure which of those will pull the strongest vacuum. Of the ones I've checked so far, the 2-stage rotary vane pumps seem to pull the strongest vacuum, but maybe top-of-the line variants of some of those other types of pumps could do better (though they may not be commonly sold due to how expensive they would be). Do you know which of those 3 types of pumps (at least for their top-of-the line variants) are capable of pulling the strongest vacuum?

author: james_s
author_rank: Super Contributor
timestamp: October 15, 2022, 04:53:30 am
content: Rotary vane is the only game in town, diaphragm and piston type pumps can't come anywhere close. The better rotary vane pumps are 2 stage, the limiting factor to ultimate vacuum achieved is usually the vapor pressure of the oil which is what forms the seal in the pump.

author: Ben321
author_rank: Frequent Contributor
timestamp: October 15, 2022, 08:33:10 am
content: Quote from: james_s on October 15, 2022, 04:53:30 amI just discovered yet another vacuum pump type. A "dry scroll" vacuum pump. Not sure what that is, but is it any good?

author: Ben321
author_rank: Frequent Contributor
timestamp: October 15, 2022, 08:35:58 am
content: I've also heard there's something called a "diffusion" vacuum pump that can make a very strong vacuum, and "ion" vacuum pumps that can make even a stronger vacuum than diffusion vacuum pumps, but I'm guessing both of those are very expensive.

author: jpanhalt
author_rank: Super Contributor
timestamp: October 15, 2022, 10:45:48 am
content: This describes the dry scroll pump:https://www.buschvacuum.com/us/en/products/vacuum-pumps/scroll/scroll-technology/#:~:text=Scroll%20vacuum%20pumps%20consist%20of,spaces%20between%20the%20two%20scrolls.I have never had one. As for diffusion pumps, I go back far enough to have used mercury diffusion. As the name implies, at very low pressures, air does not move well by mass flow. It is more of a diffusion process. Simple diffusion pumps are not much more than a refluxing distillation in which the condensate is continually pumped on by a "roughing pump." The ones I used early on were single stage (see attachment). As there are no mechanical parts and one can be made by any decent glassblower (even students), they are not particularly expensive. However, to work they must be part of a system and that system plus gauges can be quite expensive. They are not what you need. Nor do you need turbomolecular pumps mentioned earlier.

author: james_s
author_rank: Super Contributor
timestamp: October 15, 2022, 06:18:33 pm
content: Scroll pumps are often used in modern HVAC systems in place of the old style piston compressors. They're smooth, quiet, efficient and reliable but as a vacuum pump they can't touch an oil sealed rotary vane pump, nothing can, trust me on this, 2 stage rotary vane is the absolute best mechanical pump you can get for high vacuum, they are the workhorse of industry and are used as backing or roughing pumps.A diffusion pump is used to attain very high vacuum, they were ubiquitous in vacuum tube and CRT production and rebuilding and they are the standard pump for neon sign production. The issue with them is they are a secondary pump, they require a good quality mechanical backing pump behind them in order to work. IIRC you have to get the pressure down under 100 microns before a diffusion pump will work at all, and lower is better. Turbomolecular pumps are much cleaner and less finicky but those too need a backing pump, and they rotate at extremely high RPM so they are fragile and cost several thousand dollars to rebuild. Both diffusion and turbopumps require a good 2 stage rotary vane pump to work.I don't think you have read Sam's vacuum FAQ yet because most of your questions have been covered in detail there.

author: Ben321
author_rank: Frequent Contributor
timestamp: October 22, 2022, 10:09:41 am
content: Quote from: james_s on October 15, 2022, 06:18:33 pmWhat is the difference between a turbo pump and a turbo molecular pump?Also how does an ion pump compare to a diffusion pump in terms of pump speed and ultimate vacuum? Are ion pumps commonly used for making CRTs and neon tubes?

author: Spirit532
author_rank: Frequent Contributor
timestamp: October 22, 2022, 02:12:55 pm
content: Chiming in as someone who has a lot of experience with vacuum systems and gas handling(I have ~$70k of gases sitting in my closet, next to a closet full of turbos/vacuum stuff):To manufacture plasma displays of any kind, youdoneed to get to ultrahigh(1e-7 mbar), or at least high vacuum(1e-5), with a bake prior to filling, otherwise you'll end up with contamination and lifespan issues.Here's an excellent, reasonably accurate video explaining what you need for making vacuum tubes:On top of that, you would need a VCR(swagelok) high purity gas manifold(plenty on ebay), gases(neon is very expensive, especially now), and a 100 torr/mbar baratron(absolute capacitance manometer gauge, 100 is enough to cover all cases but low enough to be accurate) to start making gas discharge indicators, as you need a precise, controlled gas mixafterevacuating to high vacuum.

author: james_s
author_rank: Super Contributor
timestamp: October 22, 2022, 05:41:19 pm
content: Quote from: Ben321 on October 22, 2022, 10:09:41 amYou still haven't read the FAQ have you?Nothing, turbopump is a shorter name for a turbomolecular pump, they're the same thing.Ion pumps are used in addition to a high vacuum pump like a diffusion or turbopump to attain the next level of purity.

author: jonpaul
author_rank: Super Contributor
timestamp: October 22, 2022, 05:51:09 pm
content: Most Modern lamp , physics, valve, vacuum systems use a twin stage rotary backing pump and a turbomolecularDiffusion pumps have mercury or pump oil which require traps, water cooling, dry ice, LN, etc.AFIK the ion pumps are not as popular as turbo.The books on Vacuum Engineering detail the many tradeoffs in pump speed, ultimate vacuum, and ancilliary equipment for each type.Jon

author: KE5FX
author_rank: Super Contributor
timestamp: October 22, 2022, 06:01:35 pm
content: Ion pumps are used in situations where a vacuum must be actively maintained in the field over long periods of time -- masers, cesium-beam tubes, accelerators, that sort of thing. They're 'popular' enough in that context but not that common as part of a workbench-level system.If you (the OP) ever get around to reading Sam Goldwasser's FAQ, as recommended several times in the thread, you will kick yourself for not having done it sooner.