Vacustat gauge

The vacustat f (Fig. 11,23,2) is another useful gauge it covers the range 10 to 0 01 mm. of mercury and is usually employed in conjunction with an oil pump. It is direct reading, compact, and is charged with only about 8 ml. of mercury. The gauge must be swivelled to the vertical position when a reading of the pressure is required it does not automatically record a variable pressure. 

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. 

Fig. 2.13. The Vacustat or swivel gauge. This instrument is plugged into an outlet from the vacuum line by means of the horizontal B.14 cone A (see (c)). The normal position of the gauge is, as shown in (a), horizontal, so that the whole of it is evacuated the mercury is in reservoir B. When a measurement of the pressure is to be made, the instrument is turned into the vertical position, (b), so that the mercury traps the content of bulb D and compresses it into capillary which has the same diameter as capillary E. The pressure of the gas compressed into is given by the difference in height of the mercury in E and E. ...

S.6. Choice of gauges For the general operation of a vacuum system, a vacuum gauge is usually not required, but it may be useful, especially to the less experienced operator. For general monitoring purposes the small U-tube manometers and the Vacustat -type mini-McLeod gauge are adequate. 

A severe mercury vapour hazard may occur through misuse of mercury-containing vacuum gauges (e.g. the vacustat , Section 2.30) attached to oil vacuum pumps. If the gauge is turned about its axis too quickly, mercury may be sucked into the pump and circulated with the hot oil to release large quantities of mercury vapour into the atmosphere. If there is any possibility of this having happened, the pump must not be used and should be stripped down and cleaned as soon as possible. 

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