Diffusion pumps glass

The adsorption isotherms were measured with a Micromeritics system, model 2200 A, at 77.35 K. A conventional diffusion pumped glass system was utilized to obtain the water sorption isotherms at 20°C, the H O pressure being measured by means of silicone McLeod. 

The so-called hydro-vac pump, shown in Fig. 11, 22, 2 (the upper half of the mercury reservoir and the column above it are insulated by a layer of asbestos), is an inexpensive, all-glass, mercury diffusion pump, which can be used in series either with an oil pmnp or with a water Alter pmnp (compare Fig. 11,21, 1) capable of producing a vacuum of at least 2 mm. It is accordingly of particular value in the organic laboratory for vacuum distillations, fractionations, sublimations and pyrolyses as well as for molecular distillations (see Section 11,26). The hydro-vac... 

Mercury diffusion pumps are standard pieces of laboratory equipment and, for most cases, quite adequate ones can be constructed from glass. In general, Pyrex glass (or similar glass) is necessary because hot mercury vapour circulates and differential strain is... 

Very Simple Glass Mercury Diffusion Pump... 

Fig. 4. Schematic vacuum system for metal atom reactions. X represents the stopcock or Teflon-in-glass valve. Satisfactory components (for a general discussion of vacuum line design see References 1 and 4) forepump, 25 L/min free air capacity diffusion pump, 2 L/sec main trap is removable and measures about 300 mm deep main manifold has a diameter of about 25 mm, stopcock or valve in manifold should be at least 10 mm substrate container is removable container with 1-2 mm Teflon-in-glass needle valve connected to bottom of container. Connection between this needle valve and the reactor may be 1/8 in. od. Teflon tubing is used. Alternatively, the substrate may be added as shown in Fig. 3.

Mercury diffusion pumps are normally constructed from quartz or heat-resistant glass and are therefore a possible source of hazard should they Sreak, especially whilst they are hot. However, during over 40 years of working with such pumps, the author has neither experienced nor heard of such an accident. The major real disadvantage of mercury pumps is the relatively high vapour pressure of mercury at room temperature (ca. 10 Torr), which makes its necesssary to ensure that the cold traps prevent efficiently the mercury vapour from diffusing forward into the line. 

This main bulb was connected to a second bulb containing a modern ion gauge which measures or records gas pressures in a fraction of a second. It can measure pressures from about 10" ° to 10" mm. The system was connected to a liquid air trap and mercury diffusion pump through a movable glass plate with a small hole. The effective area of this hole was 0.010 cm. , and the pump speed was 0.12 liter/sec. This pump speed could be increased more than tenfold by removing the glass plate. 

The subject of vacuum-line technique is approached here from the perspective of the novice user who may wish to construct a system in collaboration with a glassblower. The literature on vacuum technology [3-11] and glassblowing [12-17] is vast and there are many commercial firms that specialize in it. Within the past decade, newer components such as Teflon-glass needle valves, O-ring seals, and oil diffusion pumps have been introduced into vacuum systems. 

Figure 18.1 Diagram of the glass vacuum line showing the major components mechanical pumps, diffusion pump, vacuum gauge, main vacuum manifold, and purge valve.

The catch to the vacuum method is that you must have a controlled boil without which the material and/or solvent are liable to be sprayed all over your vacuum system. Although a solvent can easily be pumped out of a vacuum system, it can cause serious problems if it remains in contact with stopcock grease, O-rings, or mechanical and/or diffusion pump oils. Any particulate material deposited within a vacuum line can only be removed from the vacuum line by disassembly and cleaning. With a glass vacuum system, such a cleaning may be difficult or impossible. 

What typically happens with a glass vacuum system is that first a mechanical pump removes a great deal of the loose, or free, gas particles. Then, greater vacuum is achieved with the combination of a diffusion pump (or similarly fastpumping unit) and traps that remove or bind up the various vapors within the system (for example, oil, mercury, and water). The only way a system can achieve a vacuum lower than 10 6 to 10 7 torr is if the pump can remove water vapor faster than the water vapor can leave the walls. Most diffusion pumping systems cannot achieve this goal, but even if they could, there is such a substantial amount of water vapor within the glass that, unless the walls are baked, a better vacuum cannot be obtained. 

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