Although a metal system is somewhat more difficult and expensive to construct, it can be placed in an oven to bake out volatile compounds adsorbed on the walls. The glass used for the seal must have a thermal expansion coefficient close to that of the metal. Then a graded seal , which consists of glasses with successively increasing expansion coefficients, connects to the main Pyrex tubing. All of the these require some lubricating grease, which may contain trapped air or organic compounds, and which cannot be subjected to the bake-out temperatures possible with metal systems. The selection of pump type and capacity depends on the system requirements.
Gas is pulled into the chamber and moved to the exhaust as the parts rotate.
The moving parts are immersed in oil, which should have negligible vapor pressure. For proper pump operation, it is imperative that organic vapors be prevented from entering the pump by a suitable cold trap. There is also danger of oil suckback from the pump into the vacuum system when the pump is turned off. It is, therefore, advisable to admit air into the inlet hose as part of the shutdown procedure. The hot vapors rise through a chimney and are ejected downward by a nozzle.