Diagnosing pressure rise in bell jar

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Arun Luthra
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Diagnosing pressure rise in bell jar

Post by Arun Luthra » Tue May 11, 2021 10:39 pm

I thought I would post this pressure curve here in case there was some obvious suspects that people could identify.

I have a new vacuum chamber setup, it is a steel baseplate with a glass bell jar on top. The gasket is 1/8" thick orange silicone rubber. Currently the plumbing is all KF16 and KF25 piping (and metal bellows hose), connected with 9 KF clamps.

2-stage rotary vane pump -> Valve -> pirani gauge on tee -> chamber.

This is what the pressure vs time looks like when I close the valve:
pressure-log.png
pressure-linear.png
The fact that the linear-scale chart shows some curvature suggests to me that at least some of the pressure rise is due to outgassing, and the outgassing material is slowly getting depleted over time.

If there are any obvious improvements I could make that jump out at you, let me know. For example, I have a foreline trap that I can put between the valve and pump. Would this help a lot?

2-stage rotary vane pump -> foreline trap -> Valve -> pirani gauge on tee -> chamber.

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Mark Rowley
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Re: Diagnosing pressure rise in bell jar

Post by Mark Rowley » Tue May 11, 2021 11:44 pm

Is the pressure rise while the pump is running or is this something occurring after it's turned off?

Any new system takes awhile to stabilize and remove the moisture. However, looking at the list of material you posted, I'd bet your problem is with the orange rubber gasket. Bell jars and rubber gaskets are famous for being leaky. The best bell jar arrangement has no gasket at all. Just a flat ground glass mating surface and silicone vacuum grease.

Mark Rowley

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Richard Hull
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Re: Diagnosing pressure rise in bell jar

Post by Richard Hull » Wed May 12, 2021 2:38 am

I am sure Mark nailed it. Outgassing of components in the freshly sealed system as opposed to leaks. The very lightest coating of the gasket with vacuum grease might help. However all of this assumes you are sealed well all the way down the line from the valve to the chamber.

On great test is to warm the steel base plate while pumping....(DO NOT over heat or the gasket may get involved) If the pressure rises while pumping, the plate has occluded gases. Give the system time and keep running the tests after pump down. Taking the bell jar on and off will ruin all your good deeds obtained from long pump downs. So, seal the system for real use and go from there.

Richard Hull
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Arun Luthra
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Re: Diagnosing pressure rise in bell jar

Post by Arun Luthra » Wed May 12, 2021 4:52 am

Just for fun I fit the data to a leakage + outgassing model. I made the simple assumption that outgassing is proportional to the remaining mass of some volatile substance. It fits the data well. Leakage rate is a constant (should be valid for low pressure).

The outgassing decreases by 1 e-folding every 483 seconds.

I'm probably going to get some vacuum grease...

Richard, your point about keeping it sealed is well taken. I got a similar curve in 3 separate pump-downs which means the H2O/etc keeps resetting (letting in air and removing the jar each time).
pressuremodel.png

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Re: Diagnosing pressure rise in bell jar

Post by Arun Luthra » Wed May 12, 2021 5:16 am

Mark Rowley wrote:
Tue May 11, 2021 11:44 pm
Is the pressure rise while the pump is running or is this something occurring after it's turned off?

Any new system takes awhile to stabilize and remove the moisture. However, looking at the list of material you posted, I'd bet your problem is with the orange rubber gasket. Bell jars and rubber gaskets are famous for being leaky. The best bell jar arrangement has no gasket at all. Just a flat ground glass mating surface and silicone vacuum grease.

Mark Rowley
The pressure rise is while the valve is closed and the pump is off.

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Re: Diagnosing pressure rise in bell jar

Post by Mark Rowley » Wed May 12, 2021 6:20 am

Looks like this was discussed pretty thoroughly here:
viewtopic.php?f=6&t=13086&p=85013#p85013

A bell jar on top of a 1/8" o-ring sounds really problematic. Maybe a picture would help make it more clear. Still though, refer to the above thread. A wide ground glass mating surface with silicone grease directly in contact with the base plate is a sure win. No troublesome o-rings required.

In the off chance you don’t know, practically every operational fusor requires continual engagement of the pumping system. Granted, a demo fusor could do its thing for awhile with no active pumping but it will eventually leak in air as well as outgas from a hot grid. You’ll notice changes in plasma formation as well as the slow demise of “star mode”.

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Re: Diagnosing pressure rise in bell jar

Post by Arun Luthra » Wed May 12, 2021 3:40 pm

I went with an 1/8" thick o-ring because a thicker one would have had greater lateral forces, and if there was no o-ring it would require more extreme flatness requirements, and more grease. I have read bad things about vacuum grease getting all over the system.
Last edited by Arun Luthra on Wed May 12, 2021 3:51 pm, edited 1 time in total.

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Re: Diagnosing pressure rise in bell jar

Post by sinan olcun » Wed May 12, 2021 3:41 pm

I have a similar set-up and I agree that the silicone gasket is probably the issue, I was able to get down to and hold vacuum at around 45 microns once I replaced the silicone gasket with a Viton one that I baked according to the instructions in the link below. Though having two ground surfaces is probably the best bet, I had no way of getting my metal plate ground so Viton it is. You can get Viton gaskets from Mcmaster-carr or any specialty providers.

https://www.duniway.com/images/_pg/vito ... orings.pdf

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Re: Diagnosing pressure rise in bell jar

Post by Arun Luthra » Wed May 12, 2021 11:47 pm

sinan olcun wrote:
Wed May 12, 2021 3:41 pm
I have a similar set-up and I agree that the silicone gasket is probably the issue, I was able to get down to and hold vacuum at around 45 microns once I replaced the silicone gasket with a Viton one that I baked according to the instructions in the link below. Though having two ground surfaces is probably the best bet, I had no way of getting my metal plate ground so Viton it is. You can get Viton gaskets from Mcmaster-carr or any specialty providers.

https://www.duniway.com/images/_pg/vito ... orings.pdf
Did it hold 45 microns with the pump turned off?

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Re: Diagnosing pressure rise in bell jar

Post by John Futter » Thu May 13, 2021 8:56 am

Arun /Sinan
Arun please do not repeat with block repeats --We are all pretty clever here and can read what the thread is saying--a few years ago this was costing extra money with another provider not now, but we ask that people do not block repeat to keep things succinct.

Sinan
I see this is your first post
Your first post should have been in the "please introduce Yourself" forum as the rules you signed up to dictate-- please amend!!!

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