Due to their low compression ratios, Roots pumps must always be operated as pump combinations for vacuum generation. Their achievable final pressures will be a function of the ultimate pressure of the selected backing pumps. Due to gas transport through adsorption, it is no longer practical to use Roots pumps in the range below 10-4 hPa. The behavior of the pumping speed and ultimate pressure of pumping stations with various backing pumps is shown in Figure 4.19. The curves clearly show that the pumping speed of this kind of pump combination rises by a factor of 8 and its ultimate pressure reduces by a factor of 15 relative to the backing pump.