Calculator Inputs
Example Data Table
| Input | Example | Notes |
|---|---|---|
| Total area A | 2000 cm² | Internal surface estimate |
| qref | 1×10⁻⁹ Torr·L/s·cm² | Typical order-of-magnitude |
| tref, n | 3600 s, 0.8 | Time‑decay approximation |
| Volume V | 20 L | Chamber free volume |
| Pumping S | 50 L/s | Effective speed at chamber |
| Temperature | 293.15 K | Room temperature baseline |
Formula Used
The gas load from outgassing is computed from surface area and a specific rate.
If a pump with speed S is connected, the pump-limited pressure contribution is:
If there is no pumping, the pressure rise rate in a volume V is:
Optional time‑decay and temperature scaling:
q(T) = qref · exp[ −Ea/k · (1/T − 1/Tref) ]
The calculator converts all inputs to SI units internally, then reports helpful alternate units for vacuum work.
How to Use This Calculator
- Select a material preset or keep custom values.
- Enter total internal surface area and choose its units.
- Enter the specific outgassing rate and select its units.
- Provide chamber volume and pumping speed with correct units.
- Enable temperature correction if you know Ea and temperatures.
- Enable time‑decay to estimate pumpdown behavior over time.
- Set start, end, and step times, then press Calculate.
- Download CSV or PDF for reporting and archiving.
Professional Notes on Outgassing Calculations
1) What outgassing rate represents
Outgassing is the release of adsorbed or absorbed gas from a surface into a vacuum volume. The specific outgassing rate q is commonly reported per unit area and depends on material history, cleaning, humidity exposure, and temperature. This calculator converts inputs to consistent units and computes the resulting gas load Q for your surface area.
2) Typical magnitudes used in design
For many unbaked metal chambers at room temperature, order‑of‑magnitude values around 10−9 Torr·L/s·cm² are often used as a first estimate, while well‑baked systems can be several orders lower. Elastomers and polymers can be much higher, so treat presets as starting points, not guarantees.
3) From specific rate to gas load
The total gas load is computed as Q(t)=q(t)·A. If you double surface area, Q doubles; if you halve q, Q halves. Include internal fixtures and hardware because they may rival the chamber wall. The exported table reports q(t) and Q(t) side by side.
4) Pump‑limited pressure estimate
With an effective pumping speed S at the chamber, the steady contribution from outgassing is approximated by P(t)=Q(t)/S. Always use the effective speed at the chamber, accounting for conductance limits.
5) Pressure rise without pumping
If S is set to zero, the calculator reports dP/dt=Q/V, showing how quickly pressure rises in an isolated volume. Larger volumes rise more slowly for the same gas load. This is useful for leak‑check planning, blank‑off tests, and estimating how fast contamination builds when valves are closed.
6) Time‑decay model for pumpdown
Many systems show decreasing outgassing over time as adsorbed water is removed. The optional model uses q(t)=qref(t/tref)−n. For example, with n=0.8 and t increasing by a factor of 10, q decreases by about 10−0.8≈0.16, which is a useful planning heuristic.
7) Temperature correction and activation energy
Temperature can strongly affect desorption. The calculator applies an Arrhenius‑style scaling using Ea (eV or kJ/mol) between T and Tref. Even modest heating can raise q temporarily, but baking can reduce long‑term q by removing water and volatile residues. Use realistic temperatures for your procedure.
8) Reporting, units, and practical limits
Results are computed in SI (Pa·m³/s), then summarized in common vacuum units (Torr·L/s, Torr, and mbar). The time series stores up to 5000 rows for performance, while the on‑page table may be down‑sampled for readability. Use CSV for analysis and the PDF summary for quick documentation. Record assumptions for traceable calculations.
FAQs
1) Which units should I use for q?
Use the units you have from a datasheet or lab notes. The calculator accepts Torr·L/s·cm², mbar·L/s·cm², and Pa·m³/s·m², then converts internally to SI.
2) Are the presets “accurate” for my setup?
They are typical starting points only. Real outgassing varies with cleaning, bake history, handling, surface finish, and humidity exposure. For design margins, bracket q over one to three decades and compare outputs.
3) What does “effective pumping speed” mean?
It is the speed seen at the chamber, not the pump’s nameplate value. Duct conductance, valves, and baffles reduce effective speed, which directly raises the predicted pressure P=Q/S.
4) Why does pressure sometimes increase with temperature?
Heating can increase desorption and diffusion rates, raising q in the short term. Baking can still help overall by removing water and volatiles, reducing long‑term q after cooldown and re‑pump.
5) How do I pick the decay exponent n?
Use lab pumpdown data if available. As a rule of thumb, n between 0.5 and 1.0 is often used for water‑dominated regimes, while very clean baked systems may show weaker time dependence.
6) Why does the table show fewer rows than my time range suggests?
To keep the interface fast, the display may down‑sample long series. Exports preserve stored results up to 5000 rows. If you need finer resolution, reduce the time range or increase the step cautiously.
7) Does this include real leaks or permeation?
No. The calculator focuses on outgassing‑driven gas load. Real systems can also have leaks, virtual leaks, permeation through elastomers, and backstreaming. Add those contributions separately when building a full gas‑load budget.
Accurate outgassing estimates help design cleaner vacuum experiments today.