High vacuum benches

To estimate the outgassing contribution, specific tests can be carried out using high vacuum benches equipped with a quadrupole mass spectrometer. 

Figure 4.6 High vacuum bench for helium permeation tests (Reproduced by courtesy of SAES Getters SpA, Italy).

The reaction described here of methylzinc iodide with sodium hydride, to produce Na[Zn2(CH3)2H3], may be pictured as a combination of the two reaction types named above. Unlike many hydride preparations, the synthesis of sodium trihydridodimethyldizincate(l —) is easily performed using ordinary Schlenk-type bench-top techniques and commercially available starting materials.4 Sodium trihydridozincate( 1 —) is easily prepared by exposing Na[Zn2(CH3)2H3]-xTHF to high vacuum.4... 

This apparatus is invaluable when preparing relatively small quantities of high-boiling liquids. It is relatively mobile and can be moved from bench to bench and attached to various vacuum sources. However, it is useful to locate it near to a high vacuum pump, where it is normally used. 

If sufficient resources are available each bench should have a medium or high vacuum pump, connected to a double manifold, but in many labs vacuum pumps are considered to be communal equipment (see Chapter 8 for more details about vacuum pumps). 

These are so universally useful that we suggest you have at least three of the B14 size and two of the B24 size as part of your personal bench set. They are particularly useful when used in conjunction with a double manifold. With the inert gas from the manifold connected to the horizontal inlet and the tap in position A (Fig. 4.10), a reaction flask can be kept under a slight positive inert gas pressure. If the gas flow is increased and the tap is turned to position B, liquids can be introduced via the vertical inlet, whilst maintaining an inert atmosphere (see Chapters 6 and 9 for more details). Another simple use is for connecting flasks to a high vacuum system for removal of last traces of solvent. 

Figure 4.2 Scheme of the stainless steel high vacuum bakeable bench equipped with mass spectrometer for outgassing tests on vacuum components. IG ionisation gauge. 

Another way to measure the pressure is by puncturing the panel and analysing the gas atmosphere with a mass spectrometer. The sample is connected to the analytical bench of Figure 4.3 by a glass tube which is sealed with a high vacuum epoxy resin, as in Figure 4.23. 

If at all possible, a laboratory bench or a table ca. 1 m high should be available at right angles to the vacuum frame, for ancillary equipment such as oscilloscopes, electrical instruments, microcomputers etc. and there should be within easy reach of the operator a bench or table for Dewar vessels, reagents, etc. 

Two very different kinds of pump fluids have been employed in diffusion pumps. For many years, mercuiy diffusion pumps, were used in small laboratory-bench glass vacuum systems. Mercury pumps are now seldom used owing to the health hazards associated with mercury and the high probability of contamination of the vacuum system with mercury unless a cold trap is used (the vapor pressure of mercury at room temperature is —1.5 mTorr). The oil diffusion pump eliminates the safety hazard and can serve for both small glass and larger metal vacuum systems. 

Models bench uses exhaust ventilation, returns airway connected inverter type axial fans, and tails roadway directly vented to the air. in order to ensure the negative of tail Lane meet the requirements, the tail is also connected to an inverter Lane axial fan speed adjustment and controlled range from 0 m/s to 4 m/s, and high drainage roadway is connected to vacuum pumping to meet high negative pressure requirements. 

A typical thermogravimetric system is illustrated in Figs. 7.2 and 73, based on the classical, high-precision instrument, the Mettler Thermoanalyzer, first described in detail by Wiedemann in 1964. The left top sketch in Fig. 72 shows a view of a basic thermoanalyzer installation. The center table provides space for the high temperature furnace, the balance, and the basic vacuum equipment. The cabinet on the right houses the control electronics and the recorder. On the left is the work bench and gas cleaning setup. 

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