Guides / Quality

Outgassing, RGA Testing and What a Buyer Can Actually Specify

September 21, 2026

A vacuum system pumps down and then sits at a base pressure higher than it should. Nothing leaks. The cause is often a new part releasing water, solvent or hydrocarbons from its surface, slowly, for days.

Where the gas comes from

Adsorbed water on every surface, more on a rough surface than a smooth one
Machining fluid trapped in blind holes, cross drills, threads and porous welds
Handling residue, mostly skin oils, which is why gloves and packing matter
The material itself, especially polymers, elastomers and some coatings
Trapped volumes behind press fits, under screw heads and inside unvented pockets

The first four are process. The fifth is design, and it cannot be cleaned out later. Vented screws and relieved pockets exist for exactly this reason.

What RGA testing tells you

Residual gas analysis puts the part in a chamber, pumps down and measures which species come off and at what partial pressure. The output is a spectrum: water, hydrocarbons, solvents and anything else the part carries.

It answers a narrow question well: will this part contaminate this system. It is a qualification and troubleshooting tool, not a receiving inspection you run on every piece. Bake out conditions, chamber history and timing all change the result, so a number without its method is not comparable to anyone else’s number.

What a buyer can put on a drawing

A useful requirement has four parts:

The cleaning process class, referenced to your own standard or stated in words
The surface finish on vacuum facing surfaces, with the surfaces named
Whether vacuum compatible materials are required for every component, including fasteners, seals and any adhesive
The test, if any: method, conditions and acceptance, applied to a qualification lot

What does not work is the word “vacuum compatible” alone. It has no test behind it, so every shop reads it differently.

Design choices that lower outgassing before any cleaning

Avoid blind holes where a through hole works, and vent the ones you keep
Avoid trapped volumes behind fits and under heads
Reduce unnecessary surface area and internal texture
Choose a finish appropriate to the surface, following how to specify surface finish
Keep polymers out of the vacuum boundary unless the application requires them

Cleaning is the lever that actually moves the number

For most parts the difference between a good result and a bad one is the cleaning and packing chain: degrease, rinse, dry, pack and never touch the part bare handed after the last rinse. Details are in what cleanroom machining actually requires. A supplier that can describe that chain step by step will outperform one that owns better machines but hands parts over in a cardboard box.

If you have vacuum hardware to quote and want it matched to shops with a written cleaning process, send the RFQ and state the cleaning and test requirement in it.

Frequently asked questions

Is outgassing a material problem or a process problem?

Both and usually process. Material choice sets the floor. Cutting fluid in a blind hole, a fingerprint, a rough surface or a trapped volume sets what you actually get and all four come from how the part was made and handled.

Should I require an RGA test on every part?

Rarely. It is normal on qualification parts, on parts entering a sensitive process and after a process change at the supplier. On repeat hardware, a cleaning process that is audited and recorded gives more value per dollar.

Does a smoother surface really outgas less?

Yes, because there is less true surface area holding adsorbed water and hydrocarbons. That is a large part of why vacuum facing surfaces are specified smooth and gas wetted surfaces smoother still.

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