Roots vacuum pumps RUVAC, which are also called Roots blowers, are rotary plunger type pumps where two symmetrically shaped impellors rotate in opposite directions inside the pump housing. Roots pumps are used where great volumes have to be pumped. In combination with backing pumps, which compress against the atmosphere, these pumps are capable of improving the attainable ultimate pressure of a pump system by a factor of 10. With two Roots vacuum pump stages and a corresponding backing pump it is possible to attain pressures in the range down to 10^-5 mbar (0.75•10^-5 Torr).