Vacuum system from contamination protecting

Figure 4. Two techniques for protecting vacuum system from contamination...

A dewar type trap made of borosilicate glass, such as that illustrated in Fig. 5, shall be placed between the top of the distillation bead and the vacuum sensor. It shall be kept filled with crushed dry ice at all times during the distillation to protect the vacuum system from contamination with residual vapors. 

Almost any spectrometer can be used in the vacuum ultraviolet (about 1100-2000 A) by properly designing the enclosure and utilizing a vacuum pumping system. Vacuum pumping systems usually include a mechanical pump with an oil diffusion pump. A cold trap provides protection of the grating and the internal optics from long-term oil contamination. A vacuum as low as 10 Torr frequently is used. 

Distillations or concentration operations that involve significant quantities of volatile substances should normally he paformed with the use of a facility vacuum system, a water aspirator, or a steam aspirator-each system protected hy a suitable trapping device-rather than a mechanical vacuum pump. However, the distillation of less-volatile substances, removal of final traces of solvents, and some other opa-ations that require pressures lower than those obtainable with a water aspirator are normally performed with a mechanical vacuum pump. The suction line from the system to the vacuum punp should be fitted with a cold trap to collect volatile substances from the system and to minimize the amount of material that enters the vacuum pump and dissolves in the pump oil. A cold trap should also be used with a water aspirator to minimize contamination of discharged water. The possibOity that mercury wiU be swept into the pump as a result of a sudden loss of vacuum can be minimized by placing... 

A vacuum pump should be scrupulously protected from corrosive vapors and materials which will be absorbed in the pump oil or condense in the pump. For most laboratory operations a low-temperature trap is employed for this purpose, and in the case of fluorine handling systems a soda-lime trap is used to neutralize the corrosive gases. Despite these precautions, the pump oil does eventually break down and become contaminated. Regular oil changes should be scheduled for a pump at about yearly intervals for a well-protected pump and more often for pumps which are not well protected. 

Balzers TPU 170 and the Welch 3134 are good-quality turbomolecular pumps. They have an ultimate vacuum limit of 10 Torr and pumping speeds of 300 L s from 10 to 10 Torr. Other pumps with speeds up to 10,000 L s are available, although these rates are reduced somewhat for light gases such as H2 and He, which have high molecular velocities. The turbomolecular pump is a very clean pump, requires no trap between itself and the system, and is not sensitive to contamination (although in some cases the forepump should be protected by a cold trap). It is not widely used in routine laboratory applications however, as it is quite expensive and demands careful maintenance. 

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