| Regime | Knudsen (Kn) | Pressure Range* | Gas Behaviour | Everyday Analogy |
|---|---|---|---|---|
| Viscous | Kn < 0.01 | > ~2.5 mbar (25 mm tube) |
Molecules collide with each other far more often than with walls. Gas behaves like a fluid. Flow is driven by pressure gradient. | A crowd pushing through a wide corridor — people bump into each other, forming a stream. |
| Transition | 0.01 < Kn < 1 | ~0.03 – ~2.5 mbar (25 mm tube) |
Mix of molecule-molecule and molecule-wall collisions. Neither viscous nor fully molecular. Hardest regime to model. | A moderately busy corridor — sometimes you bump others, sometimes you bounce off the walls. |
| Molecular | Kn > 1 | < ~0.03 mbar (25 mm tube) |
Molecules travel wall-to-wall without hitting each other. Each molecule acts independently. Gas has no viscosity. | An empty gymnasium — each person bounces between walls without ever meeting another person. |
*Pressure boundaries depend on tube diameter. Values shown are for a 25 mm (KF25) tube with air at 20 °C. Kn = (mean free path) / (tube diameter).
Elements in series: total conductance is less than the smallest individual conductance. The narrowest pipe dominates.
Elements in parallel: conductances add together. Multiple paths always improve total conductance.
Conductance scales with 4th power of diameter. Doubling the pipe diameter gives 16× the conductance. Keep pipes short and wide.
Conductance scales with 3rd power of diameter. Doubling the pipe diameter gives 8× the conductance. Still severe undersizing penalty.
The effective pumping speed at the chamber is always less than the pump’s rated speed. A restrictive pipe wastes pump capacity.
Design guideline: total conductance of the foreline should be at least 3× the pump speed to deliver ≥ 75% of rated speed at the chamber.
Rapid pressure drop as bulk gas is removed. Pump is at full throughput. Pressure falls exponentially.
950 → ~10 mbarFlow changes from viscous to transition. Pump speed may reduce. Desorbed water becomes noticeable.
~10 → ~0.1 mbarPressure drop slows dramatically. Surface outgassing is now the dominant gas load. Pumpdown curve “bends.”
~0.1 → ~0.01 mbarSystem reaches equilibrium: pump speed matches total gas load. Further improvement requires bakeout or better seals.
< 0.01 mbarPressures shown are typical for a clean, small industrial system with a rotary vane pump starting from Selkirk atmospheric (~950 mbar).