Pressure McLeod 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. 

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. 

The pressures were measured on a thermocouple gauge calibrated with a McLeod gauge, using dry air. (Pressure gauges and diffusion pumps will be discussed later in this chapter.)... 

One optional, but highly recommended accessory to a double manifold is an active pressure gauge that monitors the pressure in the vacuum line. Since leaks from the line can ruin experiments with air-sensitive compounds, it is critical to be aware of the status of the vacuum at all times. A traditional mercury McLeod gauge can provide accurate readings of pressure, but thermocouple and Pirani gauges can more quickly and... 

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... 

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... 

S.4 The McLeod gauge The principle of operation of the McLeod gauge is that a large volume V of gas at low pressure P is compressed into a small volume v contained in a glass capillary. If V and v are known from a calibration and the pressure p in the capillary is known from the difference in height h between the levels of the mercury in it and an evacuated capillary of the same diameter (to eliminate surface tension effects), then... 

McLeod gauges have the advantage of being absolute , and they are therefore used for calibrating electrical gauges. Their disadvantages include the use of a large volume of mercury and that they cannot measure the pressure of a condensable gas. 

If vapour pressure measurements are to be an essential part of the work to be undertaken, a cold cathode manometer is probably the best choice, despite the fact that it needs to be calibrated for each molecular species, and its use with mixtures of gases containing two or more species is correspondingly more difficult. If such mixtures are to be investigated, or if the chemicals concerned are corrosive, it is probably most efficient to use a mechanical gauge as a null-point instrument and to measure the pressure by means of a McLeod gauge. 

To measure the low pressures associated with krypton adsorption a thermocouple or thermistor can be used after calibration against a McLeod gauge. 

Langmuir made measurements on the amounts of gas adsorbed at very low pressures by means of direct readings with a McLeod gauge, and came to the conclusion that saturation was reached with the completion of a unimole-cular layer. 

Fig. 7.7. The McLeod gauge. The principles of operation follow. Let the unknown pressure in a system be P when the Hg level is below point 1. Let the volume of the bulb and closed capillary above I be V, which is known. When the mercury is allowed to rise past point I, the gas is trapped and finally compressed into the capillary. Suppose that when the mercury in the reference capillary is at 0, the mercury in the dead-ended capillary is B mm below 0 (i.e., the pressure of the compressed gas is B mm). Since the initial pressure-volume product equals the final pressure-volume product, PV = pv, the volume in thecapillary v will be the height B times the area of the capillary bore A. Thus P = pv/V = B (A/V). Since A and V are known and B is measured, the original pressure (P) may be calculated. Most commercial gauges are provided with a calibrated scale which presents pressures directly. Alternatively, it is possible to devise a linear scale for the McLeod gauge, in one such method the mercury height in the closed capillary is always adjusted to the same point (B0), and then the difference in meniscus heights between the two capillaries is measured (AB). For this case the pressure being measured is P = pv0/V = (B0A/V)AB. As in the previous example, the quantity in parentheses represents the gauge calibration constant.

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. 

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