Vacuum McLeod

This is sometimes made of mirror glass in order to eliminate the error due to parallax, t Manufactured by Edwards High Vacuum Ltd. This is essentially a form of McLeod 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. 

With the rotary and diffusion pumps in tandem, aided by a liquid-nitrogen trap, a vacuum of 10 Torr became readily attainable between the wars by degrees, as oils and vacuum greases improved, this was inched up towards 10 Torr (a hundred-billionth of atmospheric pressure), but there it stuck. These low pressures were beyond the range of the McLeod gauge and even beyond the Pirani gauge based on heat conduction from a hot filament (limit Torr), and it was necessary to... 

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. 

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. 3.7 McLeod compression vacuum gauge with iinear scale (equatbn 3.1b)...

Fig. 3.8 McLeod compression vacuum gauge with square-law scale (equation 3.11)...

In the past the McLeod vacuum gauge was also used for calibration purposes. With a precision-made McLeod and carefully executed measurements, taking into account all possible sources of error, pressures down to 10" mbar can be measured with considerable accuracy by means of such an instrument. 

Compression vacuum gauge (McLeod vacuum gauge)... 

The vacuum line used in the following preparations is similar to that described by Shriver.14 It consists of a pump station, a main reaction manifold with six reaction stations, a fractionation manifold with four U-traps and a reaction station at each end, a McLeod Guage, and a Topler pump. The pump station employs a two-stage mechanical forepump and a two-stage mercury diffusion pump. Operating vacuum is 1.0 X 10"S torr. Teflon valves are employed throughout. 

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. 

Before each experiment the catalyst was treated with pure 02 at 1 atm at 500°C and evacuated at the same temperature until a flat vacuum in a McLeod gage was obtained. 

MCLEOD GAGE. A liquid-level vacuum gage in which a known volume of gas. at the pressure to he measured, is compressed by the movement of a liquid column to a much smaller known volume. a> which the resulting higher pressure is measured,... 

This chapter is primarily devoted to pumps for high vacuum-operation (10 3-10 5 torr), which is the vacuum range of greatest interest in chemical vacuum lines. In addition, rough-vacuum systems (760-0.1 torr) are discussed in connection with their use in manipulating mercury-filled apparatus, such as Toepler pumps and McLeod gauges. 

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. 

The McLeod gauge is not suitable for the determination of pressures of easily condensed gases, such as water vapor, and it has the additional disadvantage of being slow and sometimes clumsy to operate. Because of this naturally slow response, the electronic vacuum gauges are superior for tracing leaks. 

The tilting McLeod gauge (Fig. 7.8) is a simple, inexpensive, and portable gauge which may be used to measure pressures down to about 10 3 torr. These gauges are very useful for checking rough vacuum systems, Schlenk systems, and for the calibration of thermal conductivity vacuum gauges. 

In 1851, Newman developed a mechanical pump that achieved a vacuum of 30.06 in. of mercury on a day that the barometer was reading 30.08 in. This pump was very impressive for the time. Vacuum technology was further enhanced by the invention of the Toepler pump in 1862, the Sprengel pump in 1865, and the McLeod gauge in 1874. 

Be aware that the old oil from a pump (and any solvent used to clean out the old oil) more than likely contains any toxic materials that may have come from the vacuum system. For example, if the system had a McLeod gauge, it is likely that the old oil is contaminated with mercury. The amount of contamination concentration determines how the oil or solvent can be disposed of. Unfortunately, because of the possibility that specific EPA-established concentration levels will change before you read this book, no disposal procedures are provided. Therefore, contact the EPA, or local regulatory agencies, to verify the various toxicity levels and the proper disposal procedures for materials of those levels. 

The beauty of liquid traps is that once in place they require no further oversight, care, or maintenance. Once you have seen the damages caused by a manometer or a McLeod gauge that has burped, you understand the value of liquid traps. However, the value of liquid traps can be overemphasized, and they should not be used as panaceas for clumsy vacuum work. They will not stop all the mercury (or other fluids) that are being battered around within a system, so do not depend on liquid traps to make up for carelessness. 

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