Vacuum Pumpdown Time Calculator

Calculate vessel pumpdown duration from atmosphere to target vacuum level, determine volumetric gas displacement, and evaluate chamber outgassing.

Vessel Internal Net Volume

Pump Rated Speed

Speed Units

Initial Starting Pressure

Target Ultimate Pressure

Chamber Outgassing & Wall Condition

Estimated Chamber Pumpdown Time

4 min 52 sec

292 seconds total

Total Gas Displaced

1.07M Torr·L

Volume: 1,416 Liters

Average Throughput

3.67 Torr·L/s

6.63 time constants (τ)

Pumping Speed Metric: 144.4 m³/hour

Recommended Foreline Flange: KF50 / ISO63 (Min 2.0" ID)

Flow Regime Evolution: Viscous to Knudsen Transition

Calculation accounts for initial viscous evacuation and surface water desorption delays.

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Vacuum Chamber Evacuation Dynamics & The Logarithmic Equation

Evacuating a vacuum vessel from atmospheric pressure involves two distinct thermodynamic phenomena. In the initial roughing stage (from 760 Torr down to approximately 1 Torr), gas molecules are in continuous collisional contact (viscous laminar/turbulent flow), allowing mechanical displacement pumps to remove gas at an essentially constant volumetric displacement rate.

Evacuation Time Mathematical Formulation

Theoretical Evacuation Time:
t = (V / S) × ln(P_initial / P_target)
Where V is vessel volume, S is effective pumping speed at the chamber inlet, and ln(P_initial / P_target) represents the number of system time constants (τ).
Outgassing Correction Factor:
Below 1 Torr, water molecules adsorbed onto stainless steel walls, elastomer O-rings, and process fixtures begin desorbing into vapor. The outgassing correction multiplier accounts for this surface desorption load.
Gas Load Throughput (Q):
Q (Torr·L/s) = [ V_liters × ΔP (Torr) ] / Time (seconds)

Frequently Asked Questions

What is the standard formula for vacuum pumpdown time?

The basic pumpdown equation is t = (V / S) * ln(P_start / P_end), where V is chamber volume, S is effective pumping speed, and P represents absolute pressures.

Why does chamber pumpdown slow down significantly below 1 Torr?

Below 1 Torr, viscous bulk gas flow ceases and surface outgassing (chiefly water vapor adsorbed onto chamber walls and elastomer O-rings) becomes the dominant gas load.

What is effective pumping speed vs rated pump speed?

Rated speed is the displacement at the pump inlet flange. Effective pumping speed at the chamber is lower due to the vacuum conductance (resistance) of connecting forelines, valves, and traps.