Outgassing Load Estimator

Estimate outgassing load Q_out from internal surface area and typical area-based outgassing rates, then approximate the resulting base pressure for a given effective pumping speed.

Calculator

Estimate outgassing load from surface area and typical area-based outgassing rates qA. Results are order-of-magnitude only and depend strongly on surface history, cleaning, and bake conditions.

After extended pumping; surface water partially removed.

qA ≈ 1.0e-9 mbar·L/(s·cm²)

Use approximate internal area of metal walls, shields, fixtures, and polymer surfaces that are exposed to vacuum.

If provided, the tool estimates a rough base pressure using p ≈ Q_out / S_eff.

Q_out (mbar·L/s)

5.000e-6

Q_out (Torr·L/s)

3.750e-6

Q_out (Pa·m³/s)

5.000e-7

Provide S_eff to estimate a rough base pressure using p ≈ Q_out / S_eff. This estimate assumes outgassing is the dominant load and that S_eff is constant over the pressure range.

Warning: Outgassing rates can vary by orders of magnitude with cleaning, bake, and exposure history. Use this tool for back-of-the-envelope estimates, not for tight specifications.

Technical Explanation

Area-Based Outgassing Rate

Outgassing describes gas released from surfaces and materials under vacuum. It is often parameterized as an area-based rate q_A with units mbar·L/(s·cm²). For a surface area A (cm²), the corresponding gas load is:

Q_out = q_A · A

Typical values for unbaked stainless steel span roughly 10⁻⁷–10⁻¹⁰ mbar·L/(s·cm²) depending on cleaning and pump time, while elastomers and polymers can be orders of magnitude higher. Because of this large spread, outgassing estimates are always approximate.

Time Dependence and Surface History

Water and other adsorbed species dominate early pumpdown. Their outgassing rate typically decreases with pump time (often roughly proportional to 1/t on a log–log scale). Baked, well-cleaned metal surfaces can reach very low outgassing rates (<10⁻¹¹ mbar·L/(s·cm²)), while unbaked or contaminated surfaces may remain much higher.

This tool uses representative q_A values for a few common cases (unbaked stainless at ~1 h and ~10 h, baked/cleaned metal, elastomer seals) to provide order-of-magnitude estimates.

From Outgassing Load to Base Pressure

For a system with effective pumping speed S_eff, the approximate equilibrium pressure set by a constant gas load Q is:

p ≈ Q_out / S_eff

If outgassing dominates and S_eff is known, this relationship offers a quick estimate of achievable base pressure. In practice, leaks, virtual leaks, and additional outgassing sources (cables, gauges, heaters) often contribute as well.

References & Disclaimer

Outgassing data is drawn from typical ranges reported in vacuum engineering literature (e.g., CERN tutorials, vendor handbooks). Actual rates depend strongly on cleaning, bake history, materials, and exposure to air. Use these calculations for back-of-the-envelope estimates and always validate with measurements where possible.

Engineering Guide

Why this calculation matters

Base pressure floors are often set by Q_out from walls, shields, and elastomers—not pump catalog speed. Order-of-magnitude Q_out estimates explain why a chamber stalls above spec after PM or vent.

In semiconductor equipment work: Base pressure budgeting, PM root-cause when pump tests OK, comparing baked vs unbaked chamber expectations, Viton seal contribution estimates.

Input parameters

q_A preset or custom
Area-based outgassing rate mbar·L/(s·cm²).
Internal area A
Total wetted area including fixtures (cm² or m²).
S_eff (optional)
Effective pumping speed for p ≈ Q/S estimate.

Output interpretation

Q_out
Total outgassing load mbar·L/s.
Base pressure estimate
p ≈ Q_out / S_eff when S_eff provided.

If measured base is much worse than estimate, check leaks, virtual leaks, or underestimated polymer area. If better than estimate, recent bake or long conditioning may have reduced q_A.

Assumptions

  • Uniform q_A over each material class; steady-state after sufficient pump time.
  • Preset q_A values are order-of-magnitude from literature ranges.
  • No separate leak term — add leak Q separately if known.

Limitations

  • q_A varies with humidity exposure, bake history, and contamination — spans orders of magnitude.
  • Elastomer area often under-counted (feedthrough boots, door grooves).
  • Does not model time-dependent outgassing decay during pumpdown.

Worked example

Chamber 15,500 cm² steel + 200 cm² Viton, S_eff = 120 L/s

  1. Steel: q_A = 1e-9 (10 h pump preset) → Q ≈ 1.55e-5 mbar·L/s.
  2. Viton: q_A = 1e-7 → Q ≈ 2e-5 mbar·L/s.
  3. Total Q ≈ 3.55e-5 mbar·L/s → p ≈ 3e-7 mbar with S_eff = 120 L/s.