Pumpdown Log Analyzer

Paste time–pressure logs from your vacuum gauge to estimate S/V or effective pumping speed from the early exponential region, and to approximate gas load Q_total in later regions.

Calculator

Paste measured pumpdown data (time vs pressure), then fit the early exponential decay to estimate S/V or S_eff and approximate gas load Q_total in a chosen time window.

Two columns per line: time and pressure. Separator can be comma, tab, or space. Header line is ignored automatically.

Pressure unit:
Time unit:

If volume is unknown, the tool reports only S/V from the log (slope of ln(p) vs t).

Leave blank to use all points. Use an early region where free-gas removal dominates and the curve looks exponential.

If left blank, the tool uses S_eff derived from ln(p(t)) fit when volume is known.

Leave blank to use the whole log for Q_total. Choose a later region to analyze tails.

Paste a pumpdown log to start analysis. Use the Advanced Pump Down Time Predictor tool to simulate the system with the extracted parameters.

Technical Explanation

Pumpdown ODE and Free-Gas Regime

The pressure evolution in a pumped chamber can be written as V·dp/dt = Q_total − S_eff·p, where V is volume, p pressure, S_eff the effective pumping speed, and Q_total the total gas load (leaks, permeation, outgassing, etc.). When Q_total ≈ 0, the early pumpdown is dominated by removal of free gas and the solution becomes:

p(t) ≈ p₀ · exp [ −(S_eff / V) · t ]

Taking the natural logarithm gives ln p(t) ≈ ln p₀ − (S_eff / V)·t. On a ln(p) vs t plot, the slope in the free-gas region is therefore −S_eff/V.

Estimating S/V and S_eff from Data

This tool performs a linear regression of ln(p) versus time over a user-specified window. The fitted slope d(ln p)/dt gives an estimate of −S/V in that region. If chamber volume V is known, S_eff can be obtained from S_eff ≈ −(d(ln p)/dt)·V. If V is unknown, the result still provides S/V, which is useful for comparing different tools or monitoring changes over time.

Approximating Gas Load Q_total

For a chosen time window, the instantaneous total gas load can be approximated as Q_total(t) ≈ V·dp/dt + S_eff·p. Using finite differences for dp/dt and an estimate of S_eff, the tool reports an average Q_total over the selected region. A region near the tail of the pumpdown curve often reflects leakage and outgassing rather than free-gas removal.

Practical Use & Limitations

Fitted S/V and Q_total values depend strongly on which part of the curve is selected and on measurement noise. Early-time data is best for estimating S/V, while late-time segments are better for characterizing leaks and outgassing. Always interpret results together with knowledge of the system (pump curve, conductance, materials) and repeat measurements where possible.