Question About Design Trade-offs for Backing Pumps

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
Post Reply
Michael Bretti
Posts: 175
Joined: Tue Aug 01, 2017 12:58 pm
Real name: Michael Bretti

Question About Design Trade-offs for Backing Pumps

Post by Michael Bretti »

I would just like to ask a quick question to the community to see what some of the experienced member's recommendations would be for my current dilemma. I have recently purchased a Robinair 15800 2 Stage 8CFM pump to use as a backing pump to my system. My diffusion pump is an Edwards EO4, with a backing requirement of at least 4CFM. However, I somehow failed to realize that the Robinair pump has a factory rated final vacuum of 35 microns. I don't know why I thought it was lower, but unfortunately I missed this point. Going through all of the forum posts related to backing pumps for diffusion pumps, this ultimate rated pressure makes me a bit uneasy now. I know it is acceptable, and can be used, but is certainly not ideal, especially if I want to achieve lower ultimate vacuum in my system. The pump and oil that I ordered are both brand new, however I know that it is still not guaranteed that I will reach the rated vacuum, and I have to assume that my system will perform worse than expected upon initial startup. I also should assume that performance could degrade over time as well, which could raise the minimum ultimate pressure. My roughing line is all KF25 hardware, and is extremely short, connecting to the diff pump backing port through a 90 degree isolation valve, KF25 tee, very short bellows, and to a KF25 90 degree foreline trap. You can see the layout at the bottom of this thread that I just recently posted in: viewtopic.php?f=18&t=12011.

Here is my dilemma: would you recommend to use the 8CFM pump capable of 35 micron, or go with a 6CFM pump capable of 15 micron? Is this pump currently sufficient, or is it safer to go with a lower micron capable pump, though at slightly reduced CFM, which also leads to the question if the extra 2CFM is enough overhead, or should I spend a lot more extra to go with an 8CFM pump capable of 15 micron? A large issue is cost here. I can return the pump I bought (still currently in its shipping package unopened) and proceed to purchasing an upgraded pump, though at a loss. However I am on an extremely tight budget, which I have been carefully planning out and researching parts to maximize my cost savings for months. Getting the new pump will undoubtedly require a sacrifice of high vacuum components that will make me most likely fall short of my goal for this year with this system, which is ultimately to pump down past the 10^-6 torr range, and go even lower with the aid of an ion pump on my chamber, and very well baked out, prepped, and outgassed for future experiments. However, if this current pump will greatly struggle to back the system to achieve down into the 10^-6 range, then it wouldn't really be worth using it anyway. The plus side to upgrading the pump is it gives me more overhead for ultimate backing pressure, and may just as well be better in the long run. I am more interested in ion beam and beam on target systems than a traditional fusor, so I will need the higher vacuum for my projects.

How big of a change would one expect in performance between 35 microns and say 15 microns at the inlet, assuming all other variables equal and that the system is reasonably well sealed and prepped?

Any thoughts or recommendations would be tremendously appreciated.
User avatar
Richard Hull
Moderator
Posts: 14976
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Question About Design Trade-offs for Backing Pumps

Post by Richard Hull »

if you can reach 35 microns, fine. However the rule is go as low as you can go on the foreline pump. A diff pump goes lower if you can go below 10 microns on the foreline at the inlet. As we have in the FAQs Find out what your pump can do with a known good TC gauge at the inlet. That is what the diff pump will see. (assuming you don't have 10 feet of pipe between the fore pump and the diff pump.)

I would return the pump. I am rather stuned that an 8CFM, 2 stage, Robinair won't go below 35 microns. I worry that first line U.S. companies go out of business and sell their name to the ChiComs, allowing a famous name to become crap. Worse, still, is a first line U.S. company stop manufacturing in their plants and import a ChiCom product with their name on it.

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
Michael Bretti
Posts: 175
Joined: Tue Aug 01, 2017 12:58 pm
Real name: Michael Bretti

Re: Question About Design Trade-offs for Backing Pumps

Post by Michael Bretti »

Richard Hull,

Thank you very much for your fast reply. Note that I can't actually test the pump since that would require me to open the box, fill it with oil, i.e. essentially use it, in which the seller would not accept returns. The 35 microns is just the rating from the datasheet, and it always seems that vacuum systems generally do worse than expected due to a whole host of variables in the system. I was kind of leaning towards returning it now diving more into the subject, but I didn't want to do so if it would work fine. However, it appears that I may need to get a different pump.

That leaves me with a couple of replacement options. I can go with something like a Robinair Cool Vac 15600, which is 2 stage rated down to 15 microns @ 6CFM, which would not be that much more than the price I got my current pump. Or, I could go with something like a Yellow Jacket 93580, which is also rated to 15 microns or lower @ 8CFM, though at a significantly higher cost. While I can get used pumps off ebay that are supposedly much better ultimate vacuum rating (more scientific grade backing pumps), I'd rather not take any risks with potentially non-working used pumps since I gotta get all this right from the get-go otherwise I'll be out of funds until next year. I won't be getting to beam or plasma systems, but I want to spend this year prepping my vacuum system and taking measurements, as well as completing my calculations, simulations, and designs for the things I would like to test with it. Unfortunately, the higher the cost, the less I will be able to do with my current system. I still need at least one high vacuum gauge, and while I would like the ion pump, I may have to forego that depending on the cost of the roughing pump for now.

If you or anyone else has any recommendations or experience with either the above mentioned pumps I am looking at or other alternatives in the similar ebay-price range, I would be very grateful.
Michael Bretti
Posts: 175
Joined: Tue Aug 01, 2017 12:58 pm
Real name: Michael Bretti

Re: Question About Design Trade-offs for Backing Pumps

Post by Michael Bretti »

Richard Hull,

I also see going through previous forum discussions that you have mentioned the yellow jacket brand pump several times that you seem very happy with its performance. Would it be safe to assume that a yellow jacket 6CFM pump would be a good replacement based on your experience with it? It costs more than the Robinair, but not as much as the Yellow Jacket 8CFM that I mentioned looking at before, so it is probably a good trade-off between quality and price for what I need. I also like that it includes a built in gauge, which could be very useful for quick troubleshooting too.
User avatar
Richard Hull
Moderator
Posts: 14976
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Question About Design Trade-offs for Backing Pumps

Post by Richard Hull »

The 5 CFM Yellow jacket that I had was 15 years old and was 100% U.S. made. In spite of it having been used in HVAC work for God knows how long, it still pulled to 10 microns in the bell jar of fusor II. I can't speak for current product. The gauge is worthless and is in inches of vacuum. Only a good TC gauge should ever be used to test a pump.

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
Jerry Biehler
Posts: 975
Joined: Tue Nov 24, 2009 3:08 am
Real name:
Location: Beaverton, OR

Re: Question About Design Trade-offs for Backing Pumps

Post by Jerry Biehler »

Anything under 50 mtorr will be fine while running. Your chamber is so small you could get away with a pretty tiny roughing pump. You will need a good gauge to watch it, I am sticking convenctrons on the new control I am building for my big system.
Michael Bretti
Posts: 175
Joined: Tue Aug 01, 2017 12:58 pm
Real name: Michael Bretti

Re: Question About Design Trade-offs for Backing Pumps

Post by Michael Bretti »

Thank you both for your replies. I have decided to return the pump, and just sent it out this morning. I figure that even though I may be technically ok with it, better safe than sorry and give myself a little extra room to work with on the micron range for the roughing side. A pump rated to 35 microns will probably struggle more to stay under 50 than something rated for 15. Apparently the pump I got is also in the Robinair economy line, and is made in China, so it may not be the best quality. The yellow jacket pumps still seem to be made in the US, and unlike the Robinairs, they do offer a complete list of replacement parts for everything in the pump. Again, since it is technically rated to 15 microns it gives me some extra theoretical overhead to work with hopefully.

I also think that for this chamber, the 8CFM rating may be unnecessarily large. The diff pump I have requires a min of 4CFM to achieve its maximum throughput, 600L/s. However, the pipeline conductance and effective pumping speed of the system is so grossly under that, that I won't need even close to the full throughput. For my system, 600L/s or 100L/s, really does not change the final numbers too much anyway, so I could go with a smaller CFM pump.

I may end up going with the 6CFM yellow jacket. Its probably a good upgrade from the robinair, and may allow me to get my chamber ion pumped as well if I can negotiate prices on ebay low enough. At least the gauge, even though very inaccurate, would be a good rough indicator - if it is not completely bottomed out, then I know I got a bad leak somewhere in the line. I also like that it has a gas ballast, which the current pump I got does not include. I will be using a new thermocouple gauge in the line as well, which has been set into a KF25 adapter and helium leak checked by LDS Vacuum, so I'm not worried about leaks or if the gauge working.

I also just found out last night to a bit of good surprise that the foreline trap that I purchased is actually a molecular seive style trap. I originally thought it was a metal filter one, but it came completely intact with the basket filled with adsorbent material. I will probably need to refill it since I don't know how old it is, but I can get new zeolite for these traps from KJL for $25.00, which is cheaper than the metal screen stuff, and has the benefit of reducing oil backstreaming while also absorbing water, protecting both the diff pump and the roughing pump oil from contamination.
User avatar
Dennis P Brown
Posts: 3147
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Question About Design Trade-offs for Backing Pumps

Post by Dennis P Brown »

If you are hoping to use a diffusion pump without a real cold trap your system might not get below 10^-6 torr. Worse, the oil vapor back flow from the DP will contaminate the system even if you gate the DP off after reaching low enough for the ion pump to start. You might need a dry ice/alcohol or similar temp trap on the diffusion pump but liquid nitrogen is better; however, if you have access to liquid nitrogen, a cryo-sorption pump is far better (using an absorber like a molecular sieve) than a mechanical pump and is ideal for for ultra clean systems that use an ion pump. While a turbo would also solve the issue, I guess that cost is prohibitive.
Michael Bretti
Posts: 175
Joined: Tue Aug 01, 2017 12:58 pm
Real name: Michael Bretti

Re: Question About Design Trade-offs for Backing Pumps

Post by Michael Bretti »

Dennis P Brown,

Thank you for your input regarding the diffusion pump. From what I have seen though, it appears that DP oils seem to be rated for much higher than just 10^-6 untrapped. I plan on using DC-705 or equivalent oils. From what I have read, it is suitable for systems in the range of 10^-9 to 10^-10 untrapped. The sources also say that traps or refrigeration are not necessary for some systems with this oil due to its vapor pressure and very low backstreaming rates:

https://www.tungsten.com/products/diffu ... fXEALw_wcB

https://inlandvacuum.com/portfolio/dow- ... ump-fluid/

Maybe I am missing something crucial, but I was originally going off the assumption that given if my system is built to close to ideal high vacuum sealing, minus the ultimate limit set by any viton (at 10^-8), I shouldn't really have much trouble getting over 10^-6 given I properly bake out, prep, and pump my system sufficiently well. If I can hit 10^-6 at this point I would be extremely happy. Long term my goal is higher, but for now 10^-6 is my first short term goal.

What I want for the ion pump is not to run anything when it is being ion pumped, but just sealing off the main chamber and keeping it pumped down at high vacuum levels so I can condition it and keep it clean and baked out for very long periods of time, so I don't need to run the diff and backing pumps all the time. So effectively the ion pump would be on only when I am not actively using the system for experiments. Ion pumps can be started as low as 10^-4, though really not ideal and is quite a struggle, so if I can hit 10^-5 or even 10^-6 at the end of an experiment I can switch it on, seal it off, turn off the diff pump, and keep my system clean and ready until next time.
Michael Bretti
Posts: 175
Joined: Tue Aug 01, 2017 12:58 pm
Real name: Michael Bretti

Re: Question About Design Trade-offs for Backing Pumps

Post by Michael Bretti »

A bit of a correction to the above statement. While I generally will not be running with the ion pump except for keeping the system under vacuum when everything else is off and already pumped down, I would like to try and run it if I am doing tests with either electron guns or miniature thrusters.

Thinking more about it now though, since I won't really have the funds to get these systems built until next year (and after I run simulations and CAD everything out first), I might just put the ion pump on hold for now and focus on getting both my roughing side and high vacuum side rock solid and well running before adding more capabilities. I do have a spare 2.75" conflat with some small ceramic insulated feed-throughs, so what might be more economical short term instead is to use that to do either some plasma cleaning or build a simple titanium sublimation pump with that and a spare nipple section I have and play around with that for now.
Jerry Biehler
Posts: 975
Joined: Tue Nov 24, 2009 3:08 am
Real name:
Location: Beaverton, OR

Re: Question About Design Trade-offs for Backing Pumps

Post by Jerry Biehler »

10^-6 is doable but you are going to get some pretty good backstreaming without at least a water cooled baffle. That's why i am converting all my DP based systems to turbos. I dont want to deal with backstreaming.

I had one system with a 18" x 18" bell jar on it and a 6" diff pump on it and left it running for a couple days and had a nice film coating the inside of the jar. On the JEOL FESEMs I have worked on they have a bypass line to pump the ion pumps down to a low enough pressure to get them to start. On the DP pumped systems you can only leave this bypass open for about 20 minutes before you have to worry about DP oil contaminating the column from back streaming.

You will probably want to make sure the ion pump is not in line of sight with the DP or it could get contaminated by oil. You could just use an elbow or something.
Michael Bretti
Posts: 175
Joined: Tue Aug 01, 2017 12:58 pm
Real name: Michael Bretti

Re: Question About Design Trade-offs for Backing Pumps

Post by Michael Bretti »

Jerry Biehler,

Thank you for your input on your experience with diffusion pumps. Reading more into it, while the oils themselves may be rated for much lower torr, I failed to account for the fact the diffusion pumps can experience considerable back-streaming during pump-down, especially when the foreline pressure is at a certain level, mainly at the level that any backing pump would be able to achieve that I could afford at this point. Right now I wouldn't be able to afford a pre-built water cooled or refrigerated baffle for my system, and at this point I'd rather not try to integrate it into my current design with a homemade approach going from atmosphere to vacuum with water line connections and risk introducing massive leaks due to poor construction. I do have another idea though that may help my problem.

Currently, the ion pump is directly in the line of sight of the diffusion pump, since the pipeline and chamber is completely straight through and very short. This arrangement has allowed me to double my molecular flow conductance from my initial design which didn't have a straight line of sight to the top of the chamber. However contamination would be an issue without a baffle as you mentioned. I do have an idea though that would cost nothing to implement and probably be helpful in reducing this back-streaming without effecting my conductance with the system as is. I am thinking about making a very simple baffle mounted underneath my adapter plate above the diffusion pump inlet. This way I could suspend the baffle over the pump without having it in direct contact with any potential hot parts (such as if I were to rest it on top of the diff pump stack.) Since the throat of the diff pump is getting choked effectively from 5" down to 1.5", this should also help reduce back streaming somewhat as well. Eventually what I could even do is integrate cooling on the adapter plate itself (for example, having an array of peltier coolers mounted on the plate around the adapter inlet) so that the entire plate could act effectively as a cooled plane. If I design the baffle correctly with enough thermal transfer from the plate, I could also use this to help indirectly cool the baffle as well. I could very easily run thermal simulations with my model in fusion360 to verify if this would work. Whenever I get around to this I will post pictures of my design concept and the thermal modelling results. The good part about this approach (besides its low cost and ease of fabrication) is that it won't really affect my final conductance and speeds. Even if the baffle reduces the main pump speed by a significant amount, since the pumping speed is still so much greater than the molecular flow conductance, the resulting effective speed would essentially stay the same.

Also, doing some very preliminary calculations for system pump-down, I can definitely get a feel now for how much of a struggle it will be to cross past the 10^-7 torr range. Assuming no back-streaming or leaks, the out-gassing load alone is enough from preventing me from reaching 10^-7 torr, even though my system is quite small, under normal pumping. It appears that I really will only be able to achieve this baking the system and pumping for periods longer than 24 hours. This is obviously common sense for high vacuum systems, but it is nice to have some numbers now to back this up, and specifically for my system as well. I'm also in the process of figuring out my pump-down times from atmosphere to high vacuum as well, so hopefully it will give me some rough orders of magnitude what to expect for the various stages of pumping.
Michael Bretti
Posts: 175
Joined: Tue Aug 01, 2017 12:58 pm
Real name: Michael Bretti

Re: Question About Design Trade-offs for Backing Pumps

Post by Michael Bretti »

So I went back to my diffusion pump just to check out where the stack ends at the top to try and gauge how I could implement my baffle idea. It turns out the stack is secured at the top with a cap that has two adjustment screws to clamp it to the wall:

20180111_211314.jpg

20180111_211333.jpg

Looking at it from the side and taking a quick measurement, it appears that the cap wider than the inlet hole of the adapter plate, which sits directly on top of the diffusion pump, and is a zero-clearance transition from 5" to 1.5". The cap is a bit bigger, about 1.6875" wide, and would sit only 0.25" from the entrance of hole. In effect, is it possible for the entire combination of the larger diameter cap sitting so close to the smaller diameter hole in a massively reduced zero-clearance throat create kind of a pseudo "self-baffle"? If the inlet to the chamber was much bigger, on the order of several inches or even closer to the throat diameter of 5" of the diffusion pump, then I would most definitely need some kind of baffle, since there is a very clear and unobstructed line of sight. However, in this case, its not as open or direct. If not, I could probably cut out some disks and stack them on the top cap clamp in a way to effectively make an optically dense baffle, though I am not sure if it would still be effective since it is not water cooled.
Jerry Biehler
Posts: 975
Joined: Tue Nov 24, 2009 3:08 am
Real name:
Location: Beaverton, OR

Re: Question About Design Trade-offs for Backing Pumps

Post by Jerry Biehler »

What the dimensions of the inlet on the diff pump? Outer o-ring diameter? I might be able to scrounge up a water cooled baffle that will fit that pump.
Michael Bretti
Posts: 175
Joined: Tue Aug 01, 2017 12:58 pm
Real name: Michael Bretti

Re: Question About Design Trade-offs for Backing Pumps

Post by Michael Bretti »

Jerry Biehler,

Thank you a ton for your response and help looking into this, I greatly appreciate your efforts, as well as everyone's help in their responses so far. I made up a quick drawing sheet of the top flange of the diffusion pump from my model:

Edwards EO4 Diffusion Pump Drawing - TOP.pdf
(167.02 KiB) Downloaded 394 times

To summerize the dimensions:
Inlet Diameter: 5"
Flange Outer Diameter: 9"
O-ring Inner Diameter: 5.25"
O-ring Outer Diameter: 5.75"
Mounting Holes Diameter: 0.75"
Radius of Mounting Holes to Center: 3.75"
Number of Mounting Holes: 8
Jerry Biehler
Posts: 975
Joined: Tue Nov 24, 2009 3:08 am
Real name:
Location: Beaverton, OR

Re: Question About Design Trade-offs for Backing Pumps

Post by Jerry Biehler »

OK, let me look around. I think I know of one you though you might have to make an adapter plate. it just clamps between flanges.
Jerry Biehler
Posts: 975
Joined: Tue Nov 24, 2009 3:08 am
Real name:
Location: Beaverton, OR

Re: Question About Design Trade-offs for Backing Pumps

Post by Jerry Biehler »

Here's what I found, You will have to make a plate plate that goes between this and the DP since both have o-ring grooves.

https://flic.kr/s/aHsmckNkwf

It's $20 plus shipping.
Michael Bretti
Posts: 175
Joined: Tue Aug 01, 2017 12:58 pm
Real name: Michael Bretti

Re: Question About Design Trade-offs for Backing Pumps

Post by Michael Bretti »

Jerry Biehler,

Thank you so much for your help on this, I truly appreciate it. I have just been CADing up some quick models based on your pictures and dimensions to see how I could fit the baffle to my current system with a simple ring adapter, but I think it could work. The overall dimensions should also let me use my original plate adapter on the top, and allow for me to clamp down the baffle and ring adapter for the bottom of it to the diffusion pump all together with just some longer bolts. Does the orientation of the baffle matter? I think the simplest way to adapt it to my current design would be to mount it so that the o-ring groove on the baffle is facing up. This would seal against the bottom of my original plate adapter which would go to the 2.75" conflat hardware on top. The ring adapter I would need machined would have an o-ring groove on the top face, so it seals against the flat un-grooved plane of the baffle. I think this would be the simplest and cheapest way for me to put this whole system together with the baffle. I have attached a PDF below with a quick cross-sectional view of how the whole system would stack together. You can see all of the major plates and components, with the appropriate o-ring grooves. Let me know what you think:


If you think that my above arrangement is acceptable and would work, then I think I would definitely be interested in purchasing this baffle from you. Should I contact you through your gmail so we can arrange the details?
Jerry Biehler
Posts: 975
Joined: Tue Nov 24, 2009 3:08 am
Real name:
Location: Beaverton, OR

Re: Question About Design Trade-offs for Backing Pumps

Post by Jerry Biehler »

Yeah, send me a message and we will get this thing going.
Jerry Biehler
Posts: 975
Joined: Tue Nov 24, 2009 3:08 am
Real name:
Location: Beaverton, OR

Re: Question About Design Trade-offs for Backing Pumps

Post by Jerry Biehler »

Oh, and yes, your adapter should work fine.
Michael Bretti
Posts: 175
Joined: Tue Aug 01, 2017 12:58 pm
Real name: Michael Bretti

Re: Question About Design Trade-offs for Backing Pumps

Post by Michael Bretti »

Thank you very much. I will send you an email to your gmail that was displayed on the baffle pictures in your link above. Looking forward to it!
Post Reply

Return to “Vacuum Technology (& FAQs)”