Roughing pumps

Modem UHV chambers are constmcted from stainless steel. The principal seals are metal-on-metal, thus the use of greases is avoided. A combination of pumps is nomially used, including ion pumps, turbomolecular pumps, cryopumps and mechanical (roughing) pumps. The entire system is generally heatable to 500 K. This bakeout for a period of... 

The roots pump may be switched on only after the roughing pump has evacuated the vacuum vessel down to the cut-in pressure. 

Permissible cut-in pressure will depend on the reduction ratios of the roots pump as against the roughing pump and is calculated by dividing the permissible pressure differential Ap, g by the compression ratio, reduced by the value of 1 ... 

Theoretical pumping speed for the roots pump Nominal pumping speed for the roughing pump... 

If the pump is protected using a diaphragm-type pressure switch, then the pump will be switched on automatically. If a combination of roots pump and roughing pump is to convey highly volatile substances such as liquids with a low boiling point, then it is advisable to use a roots pump which is equipped with an integral bypass line and a valve which will respond to a pre-set pressure. Example Roots vacuum pumps RUVAC WAU / WSU. 

The UHV pumping systems are mounted similarly in an open glove box frame and connected with the roughing pumps by vacuum lines containing absolute filters to keep back potential radioactive contaminants. 

Almost all the high vacuum systems with which we will be concerned have at one end a rough pump, usually a rotary oil pump, capable of attaining ca. 10 Torr. This is followed by a high vacuum pump which can attain ca. 10 Torr, which is followed by a cold trap, the purpose of which is to condense out any volatile matter to prevent it entering, and possibly... 

The cold finger is hollow except for the last 10 cm., which are solid. Thermal insulation is provided by a vacuum chamber. After initial rough pumping, operating pressures between 10 7 and 10 8 mm. of mercury are maintained by an 8-liter Varian ion pump. The vacuum is not only necessary for thermal insulation but also is required to minimize contamination of the surfaces of the Ge(Li) detector. 

Many vacuum pumps are supplied with a gas ballast valve which is designed to prevent the condensation of moisture and similar condensable materials. This valve bleeds a small amount of air into the pump and as a result the ultimate vacuum and pumping speed of the pump suffer. There is no occasion to use this feature on a pump on a typical chemical high-vacuum line, and it rarely, if ever, is used in typical laboratory rough-pumping systems. 

Fortunately, vacuum systems in most laboratories are small enough that the fore pump can double as the roughing pump with no problems. For the few times that one needs to go from atmospheric to vacuum conditions, all that is necessary is to close off... 

Operations that require you to cycle your vacuum line back and forth from atmospheric to low pressure can put significant strain on the fore pump and its oil, so a roughing pump should again be considered. 

If the line you are working with is small and does not require a roughing pump, proceed as follows ... 

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