Vacuum conductance and effective pumping speed
How piping conductance and pump speed combine to determine the effective pumping speed seen by the chamber.
In high vacuum, gas flow often operates in the molecular-flow regime, where molecules collide more frequently with the walls than with each other. In this regime, conductance depends on geometry, gas species, and temperature, and is independent of pressure.
For a long circular tube (diameter D, length L in cm) carrying air at room temperature, a widely used approximation is C_tube,air [L/s] ≈ 12.1 · D³/L. For a thin orifice of area A (cm²), C_hole,air [L/s] ≈ 11.6 · A. These formulas show the strong D³ dependence: slightly increasing diameter can dramatically improve conductance.
Multiple components combine like electrical conductances: in parallel the conductances add directly, and in series the reciprocals add (1/C_total = Σ 1/C_i). When a pump of nominal speed S is connected through a total conductance C_total, the effective speed at the chamber flange becomes 1/S_eff = 1/S + 1/C_total. Even a large pump can behave like a much smaller one if connected through restrictive piping or partially open valves.
Our Vacuum Conductance & Effective Pumping Speed tool implements these relations for tubes and orifices, including simple √(T/M) scaling for different gases and temperatures.