Resistance to gas flow through components that make up a vacuum system has a considerable effect on the pumping speed and pressure obtainable within the system. Any pipe or component that gas has to flow through is a hindrance to the flow of that gas, i.e. it offers resistance to the flow. It occurs in the roughing line, the foreline and the high vacuum piping and affects the amount of time taken to evacuate a vacuum chamber to its required base or process pressure.
For example, if a large vacuum chamber is connected to its vacuum pump using a long small bore pipe the gas flow down the pipe will be difficult and the gas will be removed from the chamber very slowly. There is a high resistance to gas flow and the conductance of that small bore pipe is low. Let’s first look at a simple vacuum system, using a single mechanical vacuum pump, the vacuum chamber if often mounted right above the inlet to the vacuum pump.