Manometer, mercury McLeod pressure gauge

Fig. 19.4. A vacuum system for manufacture of EDL 1, rotary vacuum pump 2, mercury manometer 3, tilting-type McLeod pressure gauge 4, EDL blank 5, modified microwave oven ...

If the pump is a filter pump off a high-pressure water supply, its performance will be limited by the temperature of the water because the vapour pressure of water at 10°, 15°, 20° and 25° is 9.2, 12.8, 17.5 and 23.8 mm Hg respectively. The pressure can be measured with an ordinary manometer. For vacuums in the range lO" mm Hg to 10 mm Hg, rotary mechanical pumps (oil pumps) are used and the pressure can be measured with a Vacustat McLeod type gauge. If still higher vacuums are required, for example for high vacuum sublimations, a mercury diffusion pump is suitable. Such a pump can provide a vacuum up to 10" mm Hg. For better efficiencies, the pump can be backed up by a mechanical pump. In all cases, the mercury pump is connected to the distillation apparatus through several traps to remove mercury vapours. These traps may operate by chemical action, for example the use of sodium hydroxide pellets to react with acids, or by condensation, in which case empty tubes cooled in solid carbon dioxide-ethanol or liquid nitrogen (contained in wide-mouthed Dewar flasks) are used. 

Cold traps must be used if mercury is used in your system (such as manometers, diffusion pumps, bubblers, or McLeod gauges) and if your mechanical pump has cast aluminum parts. Mercury will amalgamate with aluminum and destroy a pump. Even if your mechanical pump does not have aluminum parts, the mercury may form a reservoir in the bottom of the mechanical pump, which may cause a noticeable decrease in pumping speed and effectiveness. Aside from a cold trap between the McLeod gauge and the system, place a film of low vapor pressure oil in the McLeod gauge storage bulb. This oil will limit the amount of mercury vapor entering the system that makes its way to the mechanical pump. In addition, an oil layer should be placed on the mercury surface in bubblers and other mercury-filled components. 

In the laboratory, gas pressures have been measured with a manometer, or a McLeod gauge, using mercury as the manometric liquid, and the unit of such pressures has become "the pressure due to a column of mercury 1 mm high", usually written mmHg and now known as Torr. 

These manometers are not satisfactory below 5 to 10 mm, because of the relatively large error in reading the pressure. It is, therefore, desirable to have available a tilting McLeod gauge (Fig- 1 22), which will read up to 5 or 10 mm. The mercury is normally contained in the large bulb A, and when the pressure in the system is to be... 

The pressure on the high-vacuum side may be given by mercury manometers, McLeod gauges, or Pirani gauges according to the rate at which the pressure rises on this side. At Kg. 48. Apparatus for mea-high pressures robust methods of through... 

Gas was bled into the H2 gas container to the desired pressure. Low pressures were measured with an ion gauge, and a Stokes-McLeod gauge or mercury manometer was used for those runs where much larger amounts of H2 gas were admitted. The mass of H2 gas added was calculated from PVT relationships. This known amount of gas was then allowed to enter the connecting piping and calorimeter vacuum space. 

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