Vacuum pump-down time and conductance calculator
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KEY POINTS
Roughing time is about t = V/Seff × ln(p₀/p₁), and the effective pumping speed 1/Seff = 1/S + 1/C is limited by pipe conductance. Conductance falls in proportion to pressure in viscous flow and scales with the cube of the diameter in molecular flow, so narrow, long pipes are the main reason a pump's capacity goes unused at low pressure.
Inputs
Near the target pressure, pipe conductance is the bottleneck and less than half of the pump's capacity is used. A larger pipe diameter helps most (in molecular flow, conductance scales with the cube of the diameter).
Pump speed is assumed constant with pressure. Real dry pumps lose speed below about 10 Pa and turbomolecular pumps above about 1 Pa, so correct with each pump's speed curve. The pipe is a straight round tube at 20 °C; bends and valves are not included.
Pump-down curve
Pipe conductance by pressure (L/s)
| Mean pressure (Pa) | Viscous Cv | Molecular Cm | Combined C | Effective speed Seff |
|---|---|---|---|---|
| 10,000 | 17,400 | 3.88 | 17,400 | 165 |
| 1,000 | 1,740 | 3.88 | 1,740 | 152 |
| 100 | 174 | 3.88 | 177 | 85.9 |
| 10 | 17.4 | 3.88 | 21.2 | 18.8 |
| 1 | 1.74 | 3.88 | 5.61 | 5.43 |
| 0.1 | 0.174 | 3.88 | 4.05 | 3.96 |
| 0.01 | 0.0174 | 3.88 | 3.9 | 3.81 |
What sets pump-down time
During roughing from atmosphere, the pump removes the gas filling the chamber, and the time is roughly volume ÷ effective pumping speed × the natural log of the pressure ratio. Pumping a 100 L chamber from atmosphere to 10 Pa with a 600 m³/h pump takes about 5.5 seconds if the pipe is wide enough. As pressure falls, however, pipe conductance in the viscous regime drops in proportion to pressure, and narrow, long pipes cannot use the pump's full capacity.
At still lower pressures, outgassing from the walls, mainly water, dominates and the pump-down curve flattens. In tools where load lock pump-down limits throughput, the remedies are smaller volumes, wider and shorter pipes, and venting with dry nitrogen so water does not adsorb on the walls.
FAQ
Why does pump-down slow at low pressure?
Pipe conductance shrinks as pressure falls in viscous flow, and outgassing from the walls, mainly water, becomes the dominant gas load.
What is the most effective way to speed up pump-down?
Use a wider, shorter pipe. In molecular flow, conductance scales with the cube of the inner diameter.
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Last updated: October 23, 2026