Outgassing is the release of adsorbed or dissolved gases and volatile compounds after pressure is reduced. In a thermal vacuum chamber it contributes to the gas load and can transport condensable material to colder surfaces. Material screening, controlled curing and cleaning, bake-out, quartz crystal microbalance measurements and residual-gas analysis address different parts of the problem. ECSS-Q-ST-70-02C and the NASA outgassing database provide established screening methods and reference data; the project contamination-control plan remains binding.
The physics: desorption, diffusion, permeation
Three mechanisms feed gas into a vacuum chamber. Desorption releases the molecular layers – above all water – that every surface adsorbs in air; it dominates the first hours of pumping. Diffusion moves dissolved volatiles such as residual solvents, plasticisers and absorbed moisture from the bulk of polymers to the surface, where they evaporate; because the supply sits deep in the material, this source decays only slowly over days. Permeation lets gas migrate through elastomer seals from the outside – small, but permanent.
All three mechanisms are temperature dependent, but the increase is material-specific and should not be reduced to one universal rule. Bake-out uses this dependence deliberately by raising the hardware temperature within qualified limits while the released species are pumped away or captured.
Why outgassing matters in a TVAC test
Under high vacuum the chamber operates in molecular flow: outgassed molecules travel in straight lines until they hit a surface, and they preferentially stay where it is cold. Two consequences follow – a pressure problem and a contamination problem.
The consequences
- Pressure load: outgassing is a continuous gas source the pumps must remove; a strongly outgassing test item can hold the chamber orders of magnitude above its clean base pressure.
- Condensation on cold surfaces: volatiles migrate to the coldest spots – in a TVAC test often exactly the optics, detectors and radiator samples you care about.
- Changed surface properties: even thin condensed films shift absorptivity and emissivity, quietly skewing thermal balance results.
- Limited reversibility: some deposits do not simply evaporate again on venting; cleaning contaminated optics afterwards ranges from expensive to impossible.
Material screening: TML and CVCM per ECSS-Q-ST-70-02C
Material screening reduces the risk before integration. In the widely used vacuum outgassing test, samples are exposed at elevated temperature under vacuum and condensable material is collected on a cooler surface. The NASA reference method uses 125 °C for 24 hours with a 25 °C collector; the applicable ECSS procedure and project requirements should be checked for the campaign.
Common screening criteria are TML below 1.0 percent and CVCM below 0.10 percent. These values are screening thresholds, not a universal cleanliness guarantee. Optical or otherwise contamination-sensitive programmes may impose lower project limits, additional witness measurements or restrictions on particular material families.
Typical offenders in real test items
Most contamination findings trace back to a familiar cast of materials – and to the places nobody looked.
Watch out for
- Plastics and 3D-printed parts: absorbed moisture plus residual monomers and plasticisers, often with large porous surfaces.
- Adhesives and potting compounds: under-cured resins outgas dramatically more than fully cured ones – curing discipline is contamination control.
- Cable insulation and heat-shrink sleeves: unremarkable per centimetre, but harnesses add up to square metres of polymer surface.
- Tapes, foams, labels and packaging remnants: the classic forgotten items that dominate an otherwise clean setup.
- Lubricants and greases: only vacuum-rated products belong in the chamber; standard greases creep and migrate.
Mitigation: selection, cleanliness, bake-out
The order of countermeasures matches the order of leverage. First, select screened, low-outgassing materials where you can. Second, keep hardware clean: fingerprints and machining residues outgas too, so degreasing, lint-free wipes and gloves are part of vacuum practice, not lab theatre. Third, bake out: holding hardware warm under vacuum drives volatiles out under controlled conditions – before they can settle on flight optics or falsify a test.
Bake-out temperature and duration are limited by the qualified hardware configuration. Campaigns may use temperatures from moderate levels to around 125 °C, but no generic range replaces a material and component review. Completion should be tied to agreed measurements, such as pressure behaviour, RGA composition or QCM deposition rate, rather than elapsed time alone.
Monitoring: QCM and RGA
A quartz crystal microbalance (QCM) measures deposition directly: a cooled quartz crystal sits where contamination would land, and its resonant frequency drops in proportion to the mass condensing on it. That turns an invisible film into a live rate you can log, trend and write acceptance criteria against – a widely used instrument for bake-out completion and witness monitoring during critical tests.
A residual gas analyser (RGA) answers the complementary question: which gas is it? Its mass spectrum separates water from solvents, hydrocarbons and air, so you can tell ordinary drying from a real contamination source or a leak. Together, QCM and RGA turn a vague pressure reading into a diagnosis – which is why both are common options on TVAC chambers.
Takeaway
Outgassing can be managed when material screening, cleaning, cure status, bake-out and measurement are treated as one contamination-control process. TML and CVCM support material selection, while QCM and RGA provide campaign-specific evidence.
Frequently Asked Questions
Outgassing is the release of gas and volatile molecules from materials once pressure is removed: water desorbs from every surface, solvents and plasticisers diffuse out of polymers, and gas permeates through seals. It slows pump-down, limits the achievable pressure and can condense as contaminating films on cold surfaces such as optics and thermal control coatings.
TML is the total mass loss measured in a standardised vacuum exposure; CVCM is the fraction collected as condensable material on a cooler surface. Frequently used screening limits are TML below 1.0 percent and CVCM below 0.10 percent, but the project may require stricter limits or additional contamination evidence.
A bake-out holds hardware at an approved elevated temperature under vacuum so volatile material is removed before a sensitive integration or test phase. The temperature, duration and completion criterion must follow component limits and the contamination control plan; they are not universal values.
Three instruments cover it: the pressure trend gives a coarse overall picture, a QCM measures the actual deposition rate on a cooled crystal at a representative position, and an RGA identifies the species – separating harmless residual water from solvents, hydrocarbons or a genuine leak. Bake-out completion is typically defined as a QCM rate falling below an agreed threshold.
Technical references
Primary standards, agency material and current Deepvac product pages used to verify the technical statements in this article.