The McLeod Gauge

Rather than using mercury as a piston that is pushed about by the forces within a vacuum system, the McLeod gauge traps a known volume of gas of unknown pressure and compares it to a known volume of gas at a known pressure using Boyle s law  

Its great accuracy (McLeod gauges are used to calibrate electronic gauges). 

Readings are unaffected by the type of gas species within the system (although condensable vapors can affect readings). 

It can only read the pressure at single points in time, not continuously as 

The McLeod gauge uses mercury, which can be a nuisance because the gauge is difficult to clean and may be illegal in some areas. In addition, backstreamed mercury can sometimes affect your work. 

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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... 

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... 

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.

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. 

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. 

Cold traps must be used if mercury is used in your system (such as manometers, diffusion pumps, bubblers, or McLeod gauges) and if your mechanical pump has cast aluminum parts. Mercury will amalgamate with aluminum and destroy a pump. Even if your mechanical pump does not have aluminum parts, the mercury may form a reservoir in the bottom of the mechanical pump, which may cause a noticeable decrease in pumping speed and effectiveness. Aside from a cold trap between the McLeod gauge and the system, place a film of low vapor pressure oil in the McLeod gauge storage bulb. This oil will limit the amount of mercury vapor entering the system that makes its way to the mechanical pump. In addition, an oil layer should be placed on the mercury surface in bubblers and other mercury-filled components. 

The McLeod gauge has no ability to compensate for condensable vapors. 

A liquid trap can be placed between the McLeod gauge and the rest of the system to prevent mercury from accidentally spraying throughout your system. If you do not want condensable vapors affecting the McLeod gauge readings or do not want mercury vapors to enter your system, a cold trap can be placed between the liquid trap (shown in Fig. 7.41) and the main vacuum line. 

Fig. 7.41 Some traps should be placed between the McLeod gauge and the rest of the system to prevent mercury from spilling into your system.

After reading the pressure, rotate the plug of the three-way stopcock 180° to the 2 position to draw the mercury back into the storage bulb. Once the mercury is back in the storage bulb, turn the three-way stopcock 90° to a closed position. Once a vacuum reading has been made, turn the two-way stopcock connecting the McLeod gauge and the vacuum system 90° to a closed position. 

Slowly open the three-way stopcock to evacuate the McLeod gauge, drawing the mercury back into the storage bulb. 

Slowly open the two-way stopcock (to system) allowing mercury into the McLeod gauge. 

There are two mechanisms for contamination from McLeod gauges (1) contamination which is backstreamed into the system from the McLeod gauge and (2) contamination which comes from the McLeod gauge storage bulb during the evacuation process. 

Fig. 7.45 By making linear measurements as the mercury rises within a McLeod gauge, it is possible to determine whether there are condensable vapors within the McLeod gauge.

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. 

Because it operates with a Hg column, the McLeod gauge does not read the partial pressure of mercury (ca. 10-3 torr) which continually fills those portions exposed to liquid Hg. 

Connect the McLeod gauge (CC), the manostat (M), and the bleeding valve (P) into the vacuum line as shown in Figure 6-20, p. 68, if not already done. 

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