Pumping speed is the volume of gas removed per unit time (m³/h or L/s) and determines how fast the chamber is evacuated. Ultimate pressure is the lowest pressure the pump can reach (mbar or Pa) and determines the final vacuum level. The two must be read together: near ultimate pressure, actual pumping speed drops sharply.
Select against the speed at your working pressure and allow margin for piping, valves and filters. A common rule is to size for 1.2–1.5 times the required speed.
