For a vacuum pump system, it is important to take into account the system conductance. and Why Does it Matter? 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.
“Definition of Conductance: 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. Therefore, the dimensions that describe conductance are the same as pumping speed (volume per unit time) but conductance is a phenomenon usually used to describe piping and openings, not the vacuum pump.”
Source: Vacuum Science World