Pumpdown log analysis for S/V and gas load

The pumpdown of a vacuum chamber obeys the rate equation V·dp/dt = Q_total − S_eff·p, where V is the chamber volume, S_eff the effective pumping speed at the chamber, p(t) the measured pressure, and Q_total(t) the total gas load (leaks, permeation, outgassing and process gas).

In the early stages of pumpdown, the gas load is often dominated by removal of free gas and Q_total is small compared with S_eff·p. In that regime, the equation simplifies to V·dp/dt ≈ −S_eff·p, giving p(t) ≈ p₀ exp(−(S_eff/V)t). Taking the natural logarithm, ln p(t) ≈ ln p₀ − (S_eff/V)·t, so a plot of ln(p) vs time should be approximately linear. The slope in that regime directly yields −S_eff/V.

If the chamber volume is known, S_eff can be estimated from the fitted slope: S_eff ≈ −(d(ln p)/dt)·V. This allows one to detect changes in effective pumping speed over time (for example, caused by partially closed valves, clogged traps, or deteriorated pumps) even without explicit knowledge of the pump curve and conductances.

For later times, when outgassing, leaks, or process gas dominate, the full rate equation can be rearranged to estimate the instantaneous total gas load: Q_total(t) ≈ V·dp/dt + S_eff·p. By computing dp/dt from finite differences in the measured log and using an S_eff estimate, one can obtain a rough Q_total(t) and distinguish outgassing-dominated tails from true leaks.

Our Pumpdown Log Analyzer implements both pieces: it fits ln(p) vs t in a user-selected time window to obtain S/V and S_eff, and then uses V·dp/dt + S_eff·p in another window to estimate Q_total, helping you interpret why a pumpdown curve has slowed or developed a tail.