Mechanical Pump Hose and Hose Fittings for high-vacuum

[AndrewBourhis]

Hi,
I am trying to create a high-vacuum system using a diffusion pump and a mechanical roughing pump. I need to replace my mech. pump's rubber hose with something that will not leak into the vacuum. Any suggestions on what hose and hose fitting would work best to weld permanently to my base plate?

Thanks

 

[Danger]

The only thing that comes to mind is braided stainless such as is used in aircraft and high-performance or show cars. It's a neoprene hose covered with a braid of stainless steel "fabric". The usual attachment method is with AN fittings. Unless maybe by filling the hose with water, I don't know how you could weld the braid without melting the rubber. You shouldn't need to, though. (Or did you mean to weld the fitting to the plate? That could be tricky depending upon the plate. The fittings are anodized aluminum.)

 

[berkeman]

But since the neoprene is *inside* where the vacuum is, it will outgas as well.

@Andrew -- this doesn't need to be flexible, does it? Can you just bend some aluminum tubing to the right shape?

 

[Danger]

But since the neoprene is *inside* where the vacuum is, it will outgas as well.

I see; I misunderstood the question. When he said "leak into the vacuum", I took it to mean that the rubber degraded and released particles into the chamber. Sorry.

 

[AndrewBourhis]

It would be nice to have a flexible hose, but I suppose it is not totally necessary. Perhaps it would be best to weld an aluminum pipe to the base plate, then feed that to a connector piece on the mech. pump. What do you guys suggest would be the best type of fitting on the mech pump?

EDIT: @ Danger, Yes I meant welding the fitting to the plate, not the hose. And my base plate is anodized too, it isn't too hard to shave off the layer.

 

[Danger]

EDIT: @ Danger, Yes I meant welding the fitting to the plate, not the hose. And my base plate is anodized too, it isn't too hard to shave off the layer.

Yeah, that shouldn't be a problem. I was thinking of your plate being steel, which is a bit difficult to weld to aluminum. I'm going to bail out of contributing to this thread, since it is obviously far beyond my level of knowledge... but I'm going to continue reading it to increase that knowledge. I hope that it works out for you.

 

[ChaseRLewis]

Most high vacuums use aluminum or metal fittings, but a heavy polymer with metal braiding should work fine especially if you treat it by heating it up under vacuum for awhile (24 hrs or so). Silicone doesn't outgas significantly and especially after the above treatment.

http://outgassing.nasa.gov/cgi/uncgi/sectionc/sectionc.sh

here is a list from NASA of materials they certify for high vacuum environments. What degree of vacuum, and out of curiosity what type of mechanical pump are you using?

 

[moogull]

I did some experiments using a high vacuum system (diff pump with roughing pump setup) a year ago, and I think I recall that you want your plumbing to have as few bends as possible with short, straight, and wide pipes. This will help your pumping speed. IIRC, the vacuum system I used had relatively large diamater stainless steel tubing from the diff pump to the roughing pump, but the rest of the plumping I think was Tygon vacuum tubing. If you can, look at this book, it is awesome. Expensive, but awesome: https://www.amazon.com/dp/0444880100/?tag=pfamazon01-20

 

[AndrewBourhis]

I did some experiments using a high vacuum system (diff pump with roughing pump setup) a year ago, and I think I recall that you want your plumbing to have as few bends as possible with short, straight, and wide pipes. This will help your pumping speed. IIRC, the vacuum system I used had relatively large diamater stainless steel tubing from the diff pump to the roughing pump, but the rest of the plumping I think was Tygon vacuum tubing. If you can, look at this book, it is awesome. Expensive, but awesome: https://www.amazon.com/dp/0444880100/?tag=pfamazon01-20

Doesn't tygon outgas at anything above 3*10^-2 torr? I need to reach at least 10^-5 torr in my system.

 

[moogull]

I don't know, I'm thinking about it more and I believe there were two roughing pumps, one for using a low vacuum for the whole system (when the diff pump was not needed), and the other to clear out the diff pump when it was on. Of course you need to get the system to at least 10^-2 Torr or so before turning on the diff pump, and I do think we had some thick tubing that was like tygon, maybe it wasn't tygon, attached to at least one of the roughing pumps. I got my system down to less than 2*10^-6.

 

[AndrewBourhis]

I just found out it is a possibility to use different fittings in my system, so do any of you suggest a special type of vacuum hose that does not outgas, but is flexible?

 

[ChaseRLewis]

I just found out it is a possibility to use different fittings in my system, so do any of you suggest a special type of vacuum hose that does not outgas, but is flexible?

oops realized my link was messed up. Here is again go to low gassing->rubber and elastomers

http://outgassing.nasa.gov/

Remember it isn't only about material, but also material prep. You WILL need to heat the material in a good vacuum to prepare it. It recommends Silicone Rubber. If you look at Silicone even the initial treatments require treatments at pressures between 1E-4 and 1E-6 to outgas it. I'd trust NASA with vacuum tech. It has fittings, gaskets, etc. also recommends how to outgas it initially to prepare it.

 

[AndrewBourhis]

I can use metal hoses too. Do braided hoses have plastic liner?

 

[AndrewBourhis]

hey so if a hose is rated at a max working pressure of 1800, how does that translate to how low of a pressure it can handle without collapsing?

 

[Danger]

I can use metal hoses too. Do braided hoses have plastic liner?

Yes. The plastic (or rubber, or whatever) is the functional part. The braid is to protect it from abrasion, flexation, and heat. (And to look cool, in the case of cars.)