The roots pump is used to generate medium vacuum. Its special benefit is that it works without any oil, in contrast to therotary vane pump. Accordingly, it can be used for delivery of aggressive media. However, roots pumps cannot work effectively against atmospheric pressure and therefore require a backing pump such as a rotary vane pump to generate a low vacuum on the outlet side. Two-stage roots pumps, on the other hand, can dispense with the backing pump and also generate pressures up to the high vacuum range (10-4 bar).