Vacuum Stability for Longer Period. I Have a Difficulty.

Thank u very much for this valuable information. Can you tell something about degasing due to presence of Titanium traces in the SS304 material . Iam thinking because its porous charactristic.

( C K ManojKumar )

We did not perform outgassing tests on any metals that we put into orbit. We used lots of "stainless" steels, titanium and aluminum, and never tested them. There was no reason to.

The initial pump-down should have removed any traces of volatile materials, with the possible exception of some trapped gasses as redfred has indicated.

I will defer to him on that subject, but I can tell you that that was never a concern when building orbital spacecraft/satellites.

It sounds to me like you have unexpected trapped cavities that are leaking back into your chamber. Remember every blind screw hole inside your chamber will hold a small pocket of air that will very slowly leak past the screw threads.

Let me say thank u for giving this insight.

Yes, it is possible. But iam doing a periodic heating of the chamber to about 100 degree celsious during the roughing vacuum process - before going for the final high vacuum. Iam doing this through a nitrogen purging process. Iam doing this to remove all suspected moisture and to remove presence of any degasing agent that is possibly traped in.

Almost no metals can have that vacuum maintained for so long.

The best choice is red copper that has been heated afterwards. That is why glass is used for long time vacuum.

There once was even a glass bulb used for incandescent lights and that was not only to let the light pass through.

The molecular structure of most alloys is too rough to not permeate.

You mention heating to 100C. That might be effective for water, but there are other items that become volatiles when in low vacuum. Maybe a higher heating temperature to evaporate those while in the roughing process would also assist.

(My background in this was vacuum metalizing where holding times were only process cycle dependant, but the comment on blind screw holes and the other non metalic components were exactly supported in our observations in that process also.)

NASA outgassing tests are run at 125° at 10-6 torr.

The condensable materials are collected on a 25°C "cold" plate. But condensables are not the issue here.

How thorough are your clean room procedures?

A single greasy fingerprint can 'outgas' for weeks.

Dear SWB123,

Yes , my clean procedure is little bit rude. I must improve it. Iam just on the process of getting a budjet approved from our management for a clean room. Thank u very much.

I would re-vacuum and collect the exhaust for analysis if possible this might give you a clue what is gassing or leaking. Periodic revacuum is going to be routine for this vessel, as LYN mentioned. I'd like to know if this is a new process or if the vessel has maintained vacuum better in the past. If it is new then it looks like you will have to come up with something more robust to last 5 years max with no repumping, and I doubt you can achieve that reasonably.

Dear LongintheTooth,

Yes it is a challange. It is actualy a cryo cylinder with super insulation. The cylinder cap is about 250 ltr.