Conductance is the characteristic of a vacuum component or system to readily allow the flow of gas and can be thought of as the inverse of resistance to flow. It must be closely considered when designing a vacuum system and selecting the pump and other components, otherwise, your vacuum chamber will take too long to reach the pressure required.
Conductance has units of volumetric flow rate divided by pressure drop, expressed as liters per second. The conductance between two points is defined as the gas flow rate flowing through a device divided by the pressure drop that is driving it. Gas flowing through piping produces a pressure differential Δp at the ends of the piping. The following equation applies:
C=L/W=QpV/Δp
The conductance of pipes and pipe bends will differ in the various flow regimes. In viscous flow they are proportional to the mean pressure p and in molecular flow they are independent of pressure. Knudsen flow represents a transition between the two types of flow, and the conductivities vary with the Knudsen number.
The total system conductance CT for components and pipes connected in series is calculated as follows, where the inverse of the system conductance is equal to the sum of the inverse of each the individual conductance values:
1/CT=1/C1 + 1/C2 + 1/C3 +...+1/Cn
The total system conductance CT for components and pipes connected in parallel is calculated as follows, where the system conductance is equal to the sum of the individual conductance values:
CT=C1 + C2 + C3 +...+Cn