Double-Checking Diffusion Pump before Buying

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.
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Josh Smith
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Double-Checking Diffusion Pump before Buying

Post by Josh Smith » Tue Jan 16, 2018 3:44 pm

Hey everyone,
I'm very tight for money right now so I want to double-check before I make a costly mistake. I've been looking at the following diffusion pump:
https://www.ebay.com/itm/Diffusion-Pump ... 0#shpCntId
I understand that it's water-cooled, but I'm willing to set up a system to do so (I built a custom water-cooled pc for a friend, don't know if it's similar). I haven't bought a chamber yet, as I want to figure out the size of the diffusion pump so I can fit the chamber to it's size. This diffusion pump seems to come with everything, the jet slack, a power cable (my mechanical pump didn't come with one), etc. It's been listed for about half a year as well, so I should be able to negotiate the price down to an acceptable price. The only issue is the inlet flange, as I haven't seen any like it, but reading through many other posts you just need to make an adapter for it, so it should be fine. Do you guys think it's an acceptable purchase?

Thanks -Josh

John Futter
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Re: Double-Checking Diffusion Pump before Buying

Post by John Futter » Tue Jan 16, 2018 6:03 pm

about 40 years old or older
at least the christmas tree appears to be all there
Check the heater when you get it if not working return it

Michael Bretti
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Re: Double-Checking Diffusion Pump before Buying

Post by Michael Bretti » Wed Jan 17, 2018 12:36 am

Water cooled is standard for diffusion pumps, and shouldn't take too much to cool them, especially one of that size that is not too large. For comparison, my 850W EO6 diffusion pump with an inlet diameter of 5" requires 75 L/h or .27 gallons/min cooling water flow at 20C, which is very easy to find a pump with that flow rate for cheap and cool at those requirements. A simple heat exchanger can even be made with some lengths of 8020 with water flowing down the bore holes and externally cooled by fans. Obviously if you have built cooling supplies for liquid cooled computers the concept should be fairly similar, and it never hurts to do a quick thermal load calculation to validate your setup (or simulation, another reason why I strongly advocate learning Fusion360.) If you are worried about cooling, you can also easily monitor the water temperature with the appropriate sensor and read it out with something like an Arduino, and even interlock the diffusion pump power if the temperature climbs above a certain set point.

The inlet flange looks fairly standard. A lot of diffusion pumps inlet flanges are ISO or ASA compatible - my flange is an ISO160 compatible flange. I would measure the outer diameter of the flange when you get it as well (I assume you already purchased it since the link goes to an ebay listing that has already been bought.) For a 3" tube ID, that correlates to an ISO 80, but you should always measure yourself and double-check (including the center to center diameter between mounting holes.) It also looks like it could be an ASA flange however - for that size, you generally would have 4 mounting holes, as does the one you have. Again, you have to measure everything to determine the compatible standard.

That being said, an adapter plate would be very easy to make, and can be quite simple depending on your chamber. For my system, I am using a 1"x9"x12" plate of ATP-5 aluminum plate to go from my 5" inlet to my 2.75" conflat based chamber. Just mounting holes for the parts, no grooves needed in my design. The center hole is 1.5" through, and the 6 holes around it are blind-tapped to bolt the conflat part to. Everything else is through-hole for mounting the diff pump and to 8020 supports on the side:

ISO160 Diffusion Pump to CF-2,75in w Mounting Holes V2.jpg

ISO160 Diffusion Pump to CF 2,75in w Mounting Holes V2 - TOP.jpg

The diffusion pump has an o-ring can seal to the bottom of the plate, and I will be using a flat cross-section viton o-ring on the top between the plate and the 2.75" chamber. My current design will have a water-cooled baffle, so I will need an additional adapter, but they all stack and work identically. You can see how I stack my design here:


From a NASA paper I found regarding design criteria for elastomer o-rings for high vacuum chambers used in space simulation, the surface roughness of the o-ring gland and mating surfaces should be 32 micro-inch or better. ATP-5 aluminum has a maximum surface roughness of 25 micro-inch, so the plate will work as is without need for further surface machining, and is not as expensive as some of the other higher-tolerance tool-plating.

I have designed similar plates for chambers and flanges ranging from 2.75"CF all the way to 10"CF, and various other very large sized ISO and ASA flanges between my pump and other components. I have not used them, just planning out CAD models and designs, but I will be having the above plate machined soon and ready for when I start assembling my system in the next coming months.

A few things upon receiving the pump if you bought it. First thing, as mentioned above, check the heater. You can get replacement heaters, but its better to just return the pump if it is in poor shape and not working well, especially if you have no info on the pump. If you decide to keep it, you will definitely want to clean the stack - it looks like there is black gunk all over it which is not good, especially if the jets are clogged. If there is that stuff all on the inside you will want to clean that as well. Also, the forepump port seems less than desirable - usually there is some sort of mating flange such as a KF flange, which may make things challenging if you want a leak-free seal, and it is not threaded or looks like there is much of any attachment besides a tube. You can however look into possibly using a compression fitting to KF adapter like here: http://www.idealvac.com/subcatagoryb.as ... on-fitting. That may be a very simple and easy vacuum tight seal with a KF attachment point, though it depends on the tube OD and if it is compatible with any of the standard sizes.

Personally for me, I would hesitate to purchase the pump without knowing as much as possible about it. I also have a very restricted budget I am working with, which necessitates me to make sure that each part I buy has the highest probability of functioning as expected, especially since most of the stuff is purchased used off ebay for myself as well. That means many many hours of research and planning ahead of time. The fact that there is no part number or manufacturer would make me hesitate - I would want to at-least find a datasheet if possible, since I heavily calculate the vacuum parameters of my system and I would need to know at-least the pumping speed of the pump prior, in addition to the power consumption and cooling rate required. Also, the pump seems a bit gunked up and dirty on the inside - easily cleanable but possibly a sign of misuse. For my vacuum system that I am currently designing, I have taken maybe a rather extreme engineering approach, which includes months of budget planning and optimization, based on all available parts and costs, followed by several design iterations, and finally some backing calculations to make sure everything is in order or I at least know as much details as possible about my system before I make any purchases. It really depends on what you want out of your system though - for simple brute forcing neutrons with a fusor, surprisingly little is needed, and you can make it work with minimal components and next to no calculations. Hopefully you were able to at-least negotiate the price down - given the condition, I myself would offer less than $100 for it, especially since the shipping is so high as well. Again, for my goals I may not get the same pump, but for you it could work perfectly (assuming the heater is ok).

Michael Bretti
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Re: Double-Checking Diffusion Pump before Buying

Post by Michael Bretti » Wed Jan 17, 2018 1:52 am

If you are looking to make your own adapter plate, here are some great cheap resources of aluminum plate:

General 6061 Aluminum Plate Stock:
http://stores.ebay.com/Stoners-Tools-and-Raw-Materials

Custom Cut Aluminum Plate Stock Including ATP-5:
https://www.midweststeelsupply.com/store/

Jerry Biehler
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Re: Double-Checking Diffusion Pump before Buying

Post by Jerry Biehler » Wed Jan 17, 2018 3:32 am

It's just a standard 3" ASA, no big deal.

As long as it is complete and the heater works you will be good to go.

I get my cast aluminum tooling plate from these guys. great prices.

http://www.sandsmachine.com/alumweb.htm

Josh Smith
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Re: Double-Checking Diffusion Pump before Buying

Post by Josh Smith » Thu Jan 18, 2018 3:45 pm

Unfortunately, something happened to the pump a few hours after I posted the pump on here, either someone bought it or the seller took it down :(.
However, you guys gave me a lot of good advice I can use shopping for one in the future. Thank you so much for helping me out. Big thanks to you too, Michael. You really simplified the adaption process for me that would've been a lot harder otherwise. Your breakdown of the pump is especially useful, too. I'll be sure to look through it and keep your points in mind when looking for a new pump.

Big thanks for the help, I'll update this thread when I find a reasonable pump similar to what you guys said -Josh

Michael Bretti
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Re: Double-Checking Diffusion Pump before Buying

Post by Michael Bretti » Thu Jan 18, 2018 10:12 pm

Josh,

No problem, glad to provide some help and assistance! You will find this forum has such an incredible wealth of knowledge that can provide you with an extraordinary amount of information from all of the cumulative expertise of other members here. If you have done as much research as you can and read through, it never hurts to ask - I have found it already immensely helpful to ask questions here since someone will most likely know the answer.

Unfortunately, having to rely on eBay makes this type of hobby both extremely do-able for the average hobbyist but also challenging as well - the perfect, critical component at a great price can pop up and disappear unexpectedly. It also forces you to take some rather unconventional approaches sometimes to make things fit together that normally might not, and necessitates resourcefulness and diligence as well. That is one reason why I spent so much time budgeting and planning, to build in contingencies based on what is available and most probable to attain at the lowest prices. Even with all this planning, I am now on my fourth design iteration for my chamber, and my margin of safety for cost is slowly evaporating. It's another reason why I decided to spend my free time doing calculations and cad for my system, as well as fully immersing myself in the study of high vacuum engineering - looking and waiting for parts can take a long time, and doing the calculations and cad allows you to essentially work on your system and understand how it should behave before you build it and have them physically. It can also save money in the long run.

Definitely keep your progress updated and posted on your efforts, and don't hesitate to ask more questions!

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