Outgassing load estimation and base pressure

Outgassing is the release of gas from surfaces and bulk materials into vacuum. In practice, outgassing rates can span several orders of magnitude depending on material, cleaning, bake history, and exposure to air. For engineering estimates, it is convenient to use an area-based outgassing rate q_A with units mbar·L/(s·cm²).

If the internal surface area of a chamber (including walls, shields, fixtures, elastomer seals, cables) is A (cm²), the total outgassing load can be approximated as Q_out = q_A · A. Representative values for unbaked stainless steel might be 10⁻⁷–10⁻⁸ mbar·L/(s·cm²) shortly after pumpdown, decreasing towards 10⁻¹⁰ mbar·L/(s·cm²) after long pumping and conditioning. Elastomer materials such as Viton O-rings can be an order of magnitude or more higher.

Given an effective pumping speed S_eff at the chamber, the outgassing-limited base pressure can be estimated from the steady-state balance S_eff·p ≈ Q_out, resulting in p ≈ Q_out / S_eff. This simple relation is widely used as a back-of-the-envelope check when a chamber's base pressure seems worse than expected.

Our Outgassing Load Estimator lets you choose representative q_A presets (unbaked steel, baked/cleaned surfaces, Viton seals, etc.), enter an approximate area, and optionally specify S_eff to obtain Q_out and a rough base-pressure estimate.