High-Vacuum Connectors: Best Way to Vent and Attach

[AndrewBourhis]

Hi all,
I was hoping to get some insight on vacuum systems. I am currently trying to set up a high-vacuum via a roughing pump (mechanical) and a diffusion pump. I have a LN2 cold trap and a water baffle in between the diff. pump and base plate, and the mech. pump will most likely be connected somewhere to the diff. pump.

My question is, how might I best vent my system once the vacuum has been made? Specifically, what piece of hardware would work the best in theory here, and why? Also, if anyone knows what might be the best way to attach a mechanical pump (via a hose) to a chamber, that would be helpful.

Thanks so much.

 

[fedaykin]

I'm sorry I can't be more specific since, but hopefully some of this helps.We use Tygon plastic tubing to connect our mechanical pump to our vacuum chamber at my work's lab, but we're only seeking to achieve 10^-5 torr.

As for venting your chamber, if you need to rapidly cycled your vacuum (such as in coating for electron microscopy) you'll need separate valves four you backing and diffusion pumps. We use a very large stainless steel valve for our diffusion pump and aluminum brass bellows valves to control our backing pump. I think Denton vacuum makes our sliding valves. Our venting valve is just a brass screw valve.

Make sure to keep connections short, fat, and straight if you can. Also, wear gloves when handling materials that might go in your chamber. A greasy fingerprint can sometimes take a while to offgas.

 

[AndrewBourhis]

For my SEM, I need to achieve 10^-7 torr in total, but the roughing pump should get down to like 100 millitorr probably. Is there any difference between the type of valve used? I was even thinking of not using valves at all, and just having the pumps run throughout the operation of the SEM.

 

[nsaspook]

For my SEM, I need to achieve 10^-7 torr in total, but the roughing pump should get down to like 100 millitorr probably. Is there any difference between the type of valve used? I was even thinking of not using valves at all, and just having the pumps run throughout the operation of the SEM.

You could pump the entire system from the diffusion pumps fore-line without valves but you have to be very careful not to back-stream oil into the chamber when venting without a hi-vacuum isolation valve on top of a warm diff pump.

This is a good valving system, anything else limits the system usability. http://privatewww.essex.ac.uk/~bola...system_files/vacuum_system_diffusion_pump.gif

 

[AndrewBourhis]

Is there a way to use this system without the hi-vac valve? I have seen this schematic before and was planning to use it, but the hi-vac valve is an expensive part that I don't think will be totally necessary. Money is an issue for me here because I am a high school student funding it myself.

Thanks

 

[nsaspook]

Is there a way to use this system without the hi-vac valve? I have seen this schematic before and was planning to use it, but the hi-vac valve is an expensive part that I don't think will be totally necessary. Money is an issue for me here because I am a high school student funding it myself.

Thanks

Without the hi-vac valve you are stuck with having to completely cool-down the diff pump before venting the chamber. It's been years (1990s fabs) since I've worked with diff pumps but it's not a quick process to get it up to operating temperature and then cool it back down.

http://www.physics.rutgers.edu/ugrad/387/388s06/film_deposition/operation_evaporation_chamber.pdf
http://people.rit.edu/vwlsps/LabTech/Pumps.pdf

 

[fedaykin]

For a typical diffusion pump on an electron microscope, you can expect about a 20-30 minute warmup with a 40 minute cool down time with water cooling. We have a hi-vac valve on top of the diffusion pump though, so we can keep the diffusion pump under vacuum as it cools. That may shorten our cool down time. If you vent your system while the diffusion oil is warm, it will ruin the oil.

If you run your system with air cooling, be careful that your oil and pump will tolerate the higher temperature. Some diffusion pumps are designed to work with a water jacket, and running them on air will decrease their effectiveness and possibly damage the oil.

Another tip that your probably don't need:

If you leave your system at atmosphere for any extended period of time, you will probably need to degas your rotary pump's oil. You can do this by turning on the pump and letting it warm, closing your roughing valve, then opening a small port near the exhaust called the gas ballast valve. It's typically black. I run mine for about 2 hours or so whenever we contaminate the chamber or as needed.

Just out of curiosity, are you building your own SEM?

 

[AndrewBourhis]

Just out of curiosity, are you building your own SEM?

Yes I am building it with another student.

I am still a bit unclear with the order of operations here to vent my system. Without a hi-vac valve, in order to test my electron column, the roughing pump and diffusion pump will both continue to run throughout the entire operation. When I wish to shut down the system, I will need to turn off the diffusion pump first, let it cool, then turn the roughing pump off? Since I do not have any valves, will it vent automatically through the roughing pump? Or do I need to add a vent valve somewhere.

Thanks for the insight.

 

[nsaspook]

In general you will want to vent the system in the high vacuum chamber to keep it clean. A simple needle valve threaded to your vacuum plate will work.
http://www.swagelok.com/search/find...NA2&item=42e66d92-14ca-497c-9246-d6922ca2a4c3
Use a vacuum seal on the threads.
http://www.lesker.com/newweb/fluids/sealants_epoxy.cfm?pgid=torrseal

 

[fedaykin]

Yes I am building it with another student.

I am still a bit unclear with the order of operations here to vent my system. Without a hi-vac valve, in order to test my electron column, the roughing pump and diffusion pump will both continue to run throughout the entire operation. When I wish to shut down the system, I will need to turn off the diffusion pump first, let it cool, then turn the roughing pump off? Since I do not have any valves, will it vent automatically through the roughing pump? Or do I need to add a vent valve somewhere.

Thanks for the insight.

Your roughing pump shouldn't let air back into the system (at least not very quickly), so you'll at least need a venting valve. NSAspook's recommendation ought to work great. Just remember, try not to get fingerprints on hard to clean areas when you build it.

What are the makes and models of of your pumps?


Good luck!

 

[AndrewBourhis]

How does the size of the valve affect the venting? I have a 1/2" needle valve already which is why I ask.

 

[nsaspook]

How does the size of the valve affect the venting? I have a 1/2" needle valve already which is why I ask.

The quality of the valve is that's important. A 1/2" valve that's rated for hi-vacuum service would work fine, the same size valve from a big box store might not at the vacuum levels you need. What do you have to measure the high vacuum levels?

 

[AndrewBourhis]

I am using a penning gauge and a thermocoupled gauge. Where might be the best place to place these gauges?

 

[nsaspook]

I am using a penning gauge and a thermocoupled gauge. Where might be the best place to place these gauges?

If you have only one thermocouple it would normally be on the diff pump foreline because that's a critical vacuum level. I would use single hi-vac port on the plate with tee connections for gauges, venting and (optionally) rough pumping the chamber.

https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcTFX1XBTI-WKsh3vKEMsUz5aRkZIXFxOHITLH6DokQFk843zTM2LQ