McLeod gauges

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. 

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... 

ITie McLeod gauge is one of the oldest instriunents for the measurement of vacua and it has remained virtually imchanged since its introduction in 1874 (H. McLeod, 1874). 

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... 

When parts of an apparatus contain mercury, as in cut-ofiFs and McLeod gauges, it very often happens that air rushing in or out of the system will displace merciu in an undesirable manner. With cut-offs across a differential pressiue, bubbling of gas through mercury is unavoidable, and in tubing of internal diameter less than 3-4 mm bubbling will not occur but the gas will raise a column of mercury in front of it. It is therefore desirable to insert anti-splash traps wher-... 

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

McLeod gauge ebulliometry summary of literature data selected values ... 

C, ebulliometry-McLeod gauge, measured range 78.4-288.6°C, Myers Fenske 1955)... 

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. 

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. 

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. 

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