High Vacuum Systems

Vacuum - Evacuation Time

Listen to the old timers.

Dang! VGA! Your experience in this area is quite evident. That, along with your detailed reply, is the essence of what we do here. Well done.

Good comments from Redfred. We spend a lot of time trying to avoid trapped volumes in vacuum assemblies.

I've been told that leaving a finger print on a component in a vacuum chamber will increase the pumping time by 1 hour. I have no way of knowing how true this is. We pump down to 1x10-10 torr so our pumping cycle is quite long.

I don't know what you are doing, but, this:

Outgassing Data for Selecting Spacecraft Materials System

was my bible for 20 years when working for a major builder of military/spy satellites.

Cheers.

Hi,

some basics:

Glass is best, but often not allowed. Surface can be very clean and is usually very smooth.

Stainless steel, other non-corrosion alloys as based on Nickel, Titanium are very good if the surface is smooth and if there are no included voids (as mentioned above below screws there are these trapped gas leaks from machining of surfaces and defects in materials.

Avoid any brass: the Zinc will do no good, allow copper or copper-alloys only where necessary the oxide will absorb big amounts of gas.

If working with aluminum then no 7xxx-alloys (Zn) but 2xxx or 6xxx are fine. Do the final milling without any lubricant or with isopropyl (fire and explosion hazard). Do not allow any waxy or fatty stuff ever touch the aluminum as these may migrate in and through aluminum. Add a very thin (much below 1 µm) very special anodising layer to preserve initial quality. Or make a thermal treatment in 5%-Oxygen-Argon mixture.

Use only the shortest screws that will do the fastening and drill lengthwise as thick a hole as possible.

Any non-porous ceramic is a very good choice but any porous ceramics - magnets for example are often porous - may be constant sources of gas for years. I once measured ball-bearing-oils, magnet-rings, insulating materials for copper-wirings and more and the magnets stayed at high outgassing rates for months and never came down to acceptable levels.

Any plastics to be avoided as far as possible, choice according to the above-mentioned NASA or newer ESA specifications, your own out outgassing procedure a necessity (24 hours at highest allowed temperature or minimum 30 °C above highest service temperature). Never use any poly-amides (Nylon).

Any glues should be hot-curing and carefully mixed to ensure total reaction - ask the chemical specialists of the glue manufacturer if the 2 components of the glue will totally react with each other or if the excess by non-exact dosing will remain non-cured and slowly comes out outgassing into your system, may be cracked by any ions and then be able to quickly migrate to everywhere.

Inspect and rework your systems flanges to achieve and maintain a good polishing finish if using VITON-O-rings. Use double Viton-seals with in-between pumping if one is not good enough or use cooled Viton-seals.

I don't like the copper (CONFLAT) seal rings as these are expensive and are requiring expensive flanges. I like better self-made seal-rings from silver (Ag 999 to AgCu20 is fine), with some training you can easily braze these with a gas-torch. To be put either between flat flanges or one side flat the other side with a V-groove. To be compressed to roughly 70% of original diameter of wire depending on quality of the seal surfaces.

All this done and never working without cotton-gloves you will pump down a chamber of 10 liter volume and 0.5m² surface and 2m total length of Viton-seals by a 200l/s turbo-pump backed by a 16m³/h roughing pump within 1 hour to 10^-6 mbar.

The Viton-seals were the biggest leaks in this chamber, these rings leak and store gas!

If you change the Viton to Silver then you will reduce pumping time to below 10 minutes. The tiniest scratch on a flange will make everything much worse!

Be patient with your system and have success

RHABE

Although others have provided significant detail, it goes without saying that you must eliminate all leaks. Not only must direct leak paths be eliminated, indirect paths, called virtual leaks must be dealt with. An example of a virtual leak is a drilled and tapped bottoming hole with a screw in it. The gas volume under the screw may not leak to atmosphere, but represents a gas source that will leak into the pumped chamber for a long time. The solution is to drill a vent hole so this trapped gas can be pumped directly.

This is tough question. I work routinely at vacuum stability of better than E-9. It is the surface desorption behavior, along with grease, dead areas and helium which we use to test the vacuum stability absorption properties with the wall of stainless steel used to generate vacuum. One leak part and will take 24 hours before we start test again. this is to complicated topic to put in one para .

Careful with measurement and stability of your vacuum tester along with the material system and design