--- title: "Outgassing in UHV Systems" canonicalUrl: "https://mpfpi.com/blog/outgassing-uhv-systems/" excerpt: "Direct Answer Outgassing is the release of gases from the surfaces and bulk of materials inside a vacuum system. After bakeout, hydrogen diffusing out of the bulk of stainless steel and other metals typically becomes the dominant residual gas and the practical limit on base pressure. In ultra-high vacuum (UHV) systems, outgassing is frequently the […]" metaDescription: "What causes outgassing in UHV systems, how it affects base pressure and pumpdown time, and practical steps to reduce it through material and process choices." datePublished: "2026-09-24T15:49:27-04:00" dateModified: "2026-09-24T15:49:27-04:00" --- - Outgassing in UHV Systems ** # Outgassing in UHV Systems ## Direct Answer Outgassing is the release of gases from the surfaces and bulk of materials inside a vacuum system. After bakeout, hydrogen diffusing out of the bulk of stainless steel and other metals typically becomes the dominant residual gas and the practical limit on base pressure. In ultra-high vacuum (UHV) systems, outgassing is frequently the dominant factor limiting how low a base pressure can be reached and how long pumpdown takes, since even small quantities of adsorbed water, hydrocarbons, and other volatile species can add significant gas load relative to the very low pressures a UHV system is targeting. Managing outgassing involves selecting vacuum-compatible materials, controlling cleaning and handling practices, and using a properly controlled bakeout to accelerate desorption before the system reaches its operating condition. ## What Contributes to Outgassing Outgassing sources generally fall into a few categories. Adsorbed water vapor on interior surfaces is often the largest single contributor in a system that has been exposed to atmosphere, because water adsorbs readily onto most surfaces and desorbs relatively slowly at room temperature. Hydrocarbons from handling, fingerprints, cutting fluids, and other manufacturing residues can also contribute meaningfully if parts are not properly cleaned before installation. Polymers and elastomers used in gaskets, cable insulation, or other components typically have higher intrinsic outgassing rates than metals and ceramics and can be significant contributors even in small quantities. Dissolved gases within bulk materials, most notably hydrogen in stainless steel, can diffuse to the surface and desorb over extended periods, contributing to a slowly declining background gas load. This source is minor compared with water early in pumpdown, but once a system has been baked, hydrogen typically becomes the dominant residual gas and the main limit on UHV base pressure. Outgassing source****Typical behavior****Common mitigation**Adsorbed water vaporOften the dominant early-pumpdown gas load; declines significantly with bakeoutMinimize atmosphere exposure time before pumpdown; bakeoutHydrocarbon and handling residuePersistent gas load if not removed before installationVacuum-compatible cleaning and handling proceduresPolymers and elastomersHigher intrinsic outgassing rate than metals or ceramicsSelect vacuum-compatible materials; minimize quantity and surface area in the vacuum volumeDissolved gas in bulk material (mainly hydrogen)Slow, extended desorption; typically the dominant gas load in a baked UHV systemMaterial selection and, where applicable, vacuum-fired or hydrogen-degassed stock ## How Outgassing Affects Base Pressure Chamber pressure at any point is a function of total gas load, pumping speed, and conductance. Outgassing adds continuously to the gas load, and because a pump can only remove gas at a finite rate determined by pumping speed and system conductance, a system with a high outgassing rate reaches equilibrium at a higher pressure than an identical system with lower outgassing, even with the same pump. This is why reducing outgassing, through material selection, cleaning, and bakeout, is often more effective for reaching UHV pressures than increasing pumping speed alone. ## Distinguishing Outgassing From a Real Leak Because both outgassing and a real leak can produce a slow pumpdown or an elevated base pressure, they are frequently confused during troubleshooting. Outgassing generally declines over time as the readily available surface inventory of gas is depleted, and it typically responds strongly to bakeout. A real leak generally persists at a roughly constant magnitude regardless of bakeout, since it is fed by a continuous external source rather than a depleting internal inventory. A rate-of-rise test, in which the chamber is isolated from the pump and the pressure rise is tracked over time, is a practical way to distinguish a declining rate (more consistent with outgassing) from a roughly constant rate (more consistent with a real leak). ## Reducing Outgassing in Practice Select materials with outgassing rates appropriate for the target base pressure, favoring metals and ceramics over polymers and elastomers wherever the application allows, and minimizing the surface area and quantity of higher-outgassing materials in the vacuum volume. - Clean components using vacuum-compatible cleaning procedures before installation, removing cutting fluids, fingerprints, and other handling residues. - Minimize the time components spend exposed to atmosphere between cleaning and installation, since surfaces begin re-adsorbing water and contaminants as soon as they are exposed. - Use gloves and appropriate handling practices during assembly to avoid reintroducing contamination after cleaning. - Apply a controlled bakeout profile, with an appropriate peak temperature, ramp rate, and hold time, to accelerate desorption of water and other volatile species while the system is under active pumping. - Where extremely low outgassing is required, consider vacuum-fired or specially processed materials that have had trapped and dissolved gases – particularly hydrogen – removed before final assembly. - Avoid unvented trapped volumes, such as blind holes or overlapping interfaces, since these can behave like a slow-release gas source that resembles ongoing outgassing even after the surrounding surfaces have been fully baked. ## Outgassing and Feedthrough Selection Feedthroughs contribute to a system’s total outgassing load through their materials and construction, including any polymers, adhesives, or elastomeric components used in connectors, and through the surface area and geometry of the feedthrough body itself. In outgassing-sensitive applications, such as semiconductor process chambers, accelerator beamlines, and low-noise research systems, feedthrough material selection and cleanliness should be evaluated as part of the overall gas-load budget rather than assumed to be negligible because the feedthrough is a small component relative to the chamber. MPF Products’ UHV bakeout and outgassing guidance addresses the relationship between bakeout practice and gas-load reduction, and it should be used alongside application-specific material and cleanliness requirements. - External technical reference: [https://mpfpi.com/resources/uhv-bakeout-outgassing-guide/](https://mpfpi.com/resources/uhv-bakeout-outgassing-guide/) ## Related Reading - [UHV Bakeout and Feedthrough Reliability](https://mpfpi.com/blog/uhv-bakeout-feedthrough-reliability/) - [How to Tell a Real Vacuum Leak From Outgassing or a Virtual Leak](https://mpfpi.com/blog/real-vacuum-leak-vs-outgassing-virtual-leak/) - [Vacuum Feedthrough Failure: Causes, Diagnosis, Prevention, and Selection for UHV Systems](https://mpfpi.com/blog/vacuum-feedthrough-failure/) - [How to Helium Leak Test a Vacuum Feedthrough](https://mpfpi.com/blog/helium-leak-test-vacuum-feedthrough/) ## Frequently Asked Questions **What is the biggest single source of outgassing in a newly assembled UHV system?** Adsorbed water vapor on interior surfaces is frequently the largest contributor in a system that has recently been exposed to atmosphere, which is why bakeout is such an effective tool for reducing gas load relatively quickly. **Does bakeout eliminate outgassing entirely?** No. Bakeout substantially reduces outgassing by accelerating desorption of readily released species, but it does not eliminate gas release entirely. Hydrogen diffusing from the bulk of stainless steel and other metals continues well after bakeout is complete and typically becomes the dominant residual gas in a baked UHV system. **Why do elastomer seals typically outgas more than metal seals?** Polymers and elastomers generally have a higher intrinsic outgassing rate and greater gas permeability than metals and ceramics, which is why all-metal or metal-gasket sealing interfaces are commonly favored in UHV applications where bakeout capability and low outgassing are priorities. **Can a component be a significant outgassing source even if it is small?** Yes, particularly if it uses higher-outgassing materials such as polymers or has a large internal surface area or trapped volume relative to its size. A small component’s contribution should be evaluated as part of the system’s total gas-load budget rather than assumed to be negligible. **How can outgassing be distinguished from a virtual leak during troubleshooting?** Outgassing typically shows a rate-of-rise that decreases over the isolation period and responds strongly to bakeout, while a virtual leak often produces a slower, more extended improvement that persists longer than expected and tends to repeat consistently after venting and re-pumping, since the trapped volume itself does not change. ## Next Steps When evaluating a system’s gas-load budget, account for material selection, cleaning practices, bakeout profile, and any feedthroughs or components with significant surface area or trapped volume. Contact MPF Products with the target base pressure and application environment to evaluate feedthrough materials and configurations suited to low-outgassing requirements. - [MPF Products vacuum feedthrough technical resources](https://mpfpi.com/resources/vacuum-feedthrough-failure-prevention/) - [UHV Feedthrough Documentation and Qualification](https://mpfpi.com/blog/uhv-feedthrough-documentation-qualification/) Share ** LinkedIn ** Twitter ** Facebook ** Email