McLeod type gauge

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. 

Nowadays a shortened McLeod type compression vacuum gauge according to Kammerer is used to measure the partial final pressure of mechanically compressing pumps. Through the high degree of... 

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 ...

A simple bench type McLeod gauge is shown in Figure 64, III. Before starting to make such a gauge an estimate should be made of... 

Figure 65. A small rotating type of McLeod gauge...

According to the type of scale division, a distinction is made between two forms of compression vacuum gauges those with a linear scale (see Fig. 3.7) and those with a square-law scale (see Fig. 3.8). In the case of the compression vacuum gauges of the McLeod linear-scale type, the ratio of the enclosed residual volume Vc to the total volume V must be knovm for each height of the mercury level in the measurement capillary this ratio is shown on the scale provided with the instrument. In the case of compression vacuum gauges with a square-law scale, the total volume and the capillary diameter d must be known. 

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. 

Gauges which are sensitive in this range are primarily used to determine ultimate vacuum on a system and to hunt leaks. This pressure range is measurable by a variety of gauge types ranging from the manually operated mercury-filled McLeod gauge to various electronic gauges. 

Now p is usually very small compared with h (in a common type of McLeod gauge p is 1 x 10 torr when /i is 1 mm), so we may write... 

The McLeod gauge will measure the pressure of dry gas to an accuracy of about 1 % and is used to calibrate other types of vacuum gauges. It is slow in operation and does not give continuous readings. It does not measure the pressure accurately if condensable vapours are present, and is limited to 10 torr if no refrigerated trap is used. 

Pressure-measuring devices may be divided into two groups those which measure the pressure directly and can be calibrated without reference to another intrument and those which involve some other physical measurement which will be related to the pressure. Of the former type, the barometer, manometer, and McLeod gauge are the most common. 

When pressures get below about 10" torr (0.013 Pa), McLeod gauges are not practical. There are several types of gauges that can be used. Two of the most common gauges that cover the range from 10 to 1 O torr are (1) the Pirani gauge, and... 

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