Next Steps?

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Blake Resnick
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Real name: Blake Resnick

Next Steps?

Post by Blake Resnick »

Hello,
Over the last couple of months, I have been altering my reactor to improve its performance. However, I have hit a bit of a roadblock. It seems that tinkering can no longer improve my system. Specifically when it comes to the vacuum levels it can achieve. With the help of a copious amount of epoxy resin and a bit of research into the proper tightening techniques of high pressure systems, I have reached roughly 230 microns (quite the jump from the 325 before). At this point I think the fundamental flaws of the system began overpowering the pump's abilities. So my question is, "Whats next?" I do want to continue improving my reactor, but I am not sure how exactly to go about it. Below are several pictures that should shed light on the specifics of the reactor.
If you have any questions, please feel free to ask me.
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Thanks,
-Blake
prestonbarrows
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Re: Next Steps?

Post by prestonbarrows »

There is only so far you can go with a pump that fits in the palm of your hand and rubber hose clamps.

Your ultimate pressure will come down to two things, your pumping speed curve versus your leak rate. You don't need physical holes in the system to have leaks. All of your rubber hardware will be outgassing various hydrocarbons as well as permeating atmosphere directly through its bulk material. You will also have a significant amount of water vapor coming off everything since you can not bake your glass. Such 'virtual leaks', combined with any actual physical leaks, will input some finite mass flow into your chamber. Your ultimate pressure is when your pumping speed matches this input load.

If you want to reach lower pressures will you need to either increase your pumping speed or decrease the gas load. It is usually more helpful to reduce leaks first. This would require some proper vacuum hardware made of stainless steel or aluminum with either KF, conflat, or ISO flanges appropriately sealed. Use copper or viton gaskets to eliminate/reduce gas permeation. Copper gaskets with conflat flanges will give the best performance but are generally the most expensive. KF is generally the cheapest and easiest for smaller things.

A decent dual stage rotary vane pump (Welch) will get you down into the 1E-3 Torr ranges. Adding a diffusion or turbo pump should easily get you down into the 1E-5 to 1E-7 Torr ranges depending on the specifics. Diff pumps are much simpler and cheaper but generally messier. Turbo pumps are insanely expensive new and require specialized control electronics; used units can be affordable occasionally with luck on eBay. Both require some knowledge and following a specific procedure to avoid destroying them.

In parallel, making sure all oils etc. are removed from interior surfaces is crucial. Use powder-free gloves and lint-free cloth with methanol or similar to wipe everything down. Avoid anything with lots of surface area as this will adsorb a lot of crud.

On a properly sealed system, most of the gas load will be from water outgassing from surfaces. Heating the chamber walls to as hot as your o-rings can handle will greatly speed the removal of water (this is another reason copper gaskets are so nice). Plasma discharges will also help. Otherwise, actively pumping the system for days or months will continuously improve your vacuum level as water is removed. Avoid opening the chamber to atmosphere as this will revert all your progress. If it is necessary to shut off the pumps, seal the system with an isolation valve and backfill the chamber with dry nitrogen from a sealed bottle to avoid introducing water back into the system.

Check out
http://www.ebay.com/
for used hardware

or
http://www.lesker.com/newweb/index.cfm
or
http://www.n-c.com/Default.aspx
or
http://www.order.duniway.com/
or
http://vacuumshopper.stores.yahoo.net/
or
http://www.mdcvacuum.com/Displayproduct ... aspx?d=MDC
for new vacuum hardware.
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Dennis P Brown
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Re: Next Steps?

Post by Dennis P Brown »

prestonbarrows has a lot of good information. If your pump can get much better than 230 microns (going from 325 microns to 225 micron is not a significant vacuum improvement), than you are most likely dealing with leaks and that is an issue that can be addressed - adding a kf or similar exhaust flange/port system with a real vacuum hose (steel or vacuum grade large diameter tubing - 25 mm min) would be a good start. If the best your pump does is similar to what you are reading, than it is the pump - replace it as prestonbarrows says with a two stage unit. If both problems are at issue, then get a new pump and fix the leaks.
prestonbarrows
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Re: Next Steps?

Post by prestonbarrows »

What is the rated ultimate pressure of your pump according to the data sheet? You should be able to find a PDF copy of this online for the specific make and model.

What is the ultimate pressure of the pump when it is deadheaded? (the gauge directly attached to the pump with proper vacuum hardware and nothing else)

If you can not come within an order of magnitude or so of the rated value when deadheaded, there might be something wrong with your pump. If the ultimate pressure of the full system is many orders of magnitude higher than the deadheaded pump, your system is full of real physical leaks.

Making the above measurements should be standard practice for any new pump and/or vacuum chamber.

As Denis said, an improvement of ~30% is not that significant in the context of vacuum leaks. Eliminating physical leaks will generally give one or two or more orders of magnitude improvement in the ultimate pressure.
Blake Resnick
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Re: Next Steps?

Post by Blake Resnick »

Here is a link to my vacuum pump: http://www.harborfreight.com/3-cfm-two- ... 60805.html
The data sheet claims 22.5 microns, however when deadheaded my gauge shows about 45 microns. With that being said, I doubt there are many more things I can do to stop leaks in the system. All of the bolts are tightened evenly, gaskets are placed between the glass and steel flanges, everything has teflon tape on it and what doesn't, is sealed with epoxy. Plus, when I test the system for leaks I can not find any. Any thoughts?
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Rich Feldman
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Re: Next Steps?

Post by Rich Feldman »

As others have said, you reach an equilibrium between pumping speed and the rate of gas leakage and/or evolution in the chamber.

Right now your chamber pressure is almost an order of magnitude higher than the deadheaded pump pressure.
And looking at your vacuum hose, its conductance (in units of pumping speed, such as CFM)
is much less than that of your pump. (look up vacuum conductance formulas).
So the effective pumping speed is set by the hose.
Even if you replaced the pump with a magical perfect vacuum reservoir,
it would not (IMHO) make much of a difference in your chamber pressure.

Conclusion: Investing in a much fatter hose will have more benefit than any investment in a better pump.
All models are wrong; some models are useful. -- George Box
prestonbarrows
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Re: Next Steps?

Post by prestonbarrows »

That sounds about right. You will never reach a pump's rated pressure when deadheaded; less than an order of magnitude difference and the pump is in good shape.

Even with a proper vacuum chamber, you should expect the ultimate pressure to be one or three orders of magnitude above the pump's rating, depending on the specifics.

You are pushing nearing the molecular flow regime (~1E-3 Torr) where gas stops acting like a normal fluid and gas conductance becomes much more important. Short and wide tubes will work better than long and thin ones. Rough pumps can not really get much further below this pressure since they do not work very well with molecular flow; this is why turbo and diffusion pumps were invented.

At this point, you have squeezed just about the most you can out of this system. If you want to get deeper vacuum, you will need better hardware as outlined in my first post in this thread.
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Richard Hull
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Re: Next Steps?

Post by Richard Hull »

A good, two stage, refrigeration pump working dead head can almost instantly drop to well below 30 microns and over a short period of time go below 20 microns. Using a very short hose of 3/4 inch or larger diameter on an extremely well sealed system with few if any elastomer gaskets or seals should have the afore mentioned pump hit the 30 micron range in a just few minutes.

It sounds like your pump, even if new, is not up to snuff if 45 microns is your bottom at deadheading a gauge. I recently posted that I have an old, well used 5 CFM yellow jacket refrigeration pump that deadheads to 20-25 microns in about 1 minute.

If fusion is your goal and not a demo fusor then you will need at least a diff pump with your current pump. Forepump to diff pump connections are rarely over 4-6 inches long in high end systems. Your pump will work with a diff pump, but it will be a struggle.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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