Vacuum measurement with mercury

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. 

The dilatometer is generally made of glass and is the vessel where the mercury is intruded into the sample pores. The design is dependent on the pressure source and monitoring system of the instrument. The dilatometer consists of a sample holder and a calibrated stem, which is used to measure the amount of mercury intruded into the sample. The sample in the dilatometer must be cleaned from adsorbed species by degassing the material in a vacuum [42], Most commercial instruments degas the sample in the instrument before mercury intrusion. Once the sample is degassed, the dilatometer (sample holder and stem) are filled with mercury. 

Relation 9.77 is usually called the Washburn equation [55,237], One should consider it as a special case of the fundamental Young-Laplace equation [3,9-11], Washburn was the first to propose the use of mercury for measurements of porosity. Now, it is a common method [3,8,53-55] of psd measurements for a range of sizes from several hundreds of microns to 3 to 6 nm. The lower limit is determined by the maximum pressure, which is applied in a mercury porosimeter the limiting size of rWl = 3 nm is achieved under PHg = 4000 bar. The measurements are carried out after vacuum treatment of a sample and filling the gaps between pieces of solid with mercury. Further, the hydraulic system of a device performs the gradual increase of PHg, and the appropriate intmsion of mercury in pores of the decreasing size occurs. 

U-tube vacuum gauges fiiied with mercury are the simpiest and most exact instruments for measuring pressure in the rough vacuum range (1013 to a... 

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. 

An example of such an instrument is the U-tube vacuum gauge, with which the measurement of the pressure in the measurement capillary is based on a measurement of the weight over the length of the mercury column. 

U-Tube Manometers. These are generally made from a glass tube bent in the shape of the letter U and partly filled with a working liquid, most often with mercury. The operation of the manometer is based on the displacement of the levels of the working liquid in both arms of the tube depending on the difference in the pressures over these levels. One arm of a manometer is connected to the vacuum setup in which the pressure is to be measured, while the other arm is either closed (soldered) or remains open, i.e. is constantly at atmospheric pressure. 

The apparatus in Fig. 9.7.b is the simplest to operate because the entire manometer system, the sample, and its vapor are immersed in the constant-temperature bath. The mercury reservoir permits the removal of mercury from the U-manometer portion of the apparatus, and the material to be measured is then condensed into the terminal bulb. While this material is still condensed and there is a high vacuum in the system, the mercury is reintroduced to the U, thus isolating the sample. The apparatus is immersed in a constant-temperature bath to the level of the wavy lines. With a vacuum on the upper portion of the apparatus the vapor pressures can be measured directly. If vapor pressures beyond the range of the immersible manometer must be measured, the mercury in the... 

Pressure can be measured with many different types of devices. A common instrument used to measure atmospheric pressure is called the barometer. A simple barometer consists of a tube closed at one end and open at the other. The tube is filled with a high density liquid such as mercury and inverted in a larger reservoir of mercury which is open to the atmosphere. This geometry will result in a vacuum at the top of the glass tube as the mercury inside the tube runs downward, out of the tube and into the reservoir of mercury (Figure 3.2). 

For example, suppose we measure the pressure with a simple U-tube manometer filled with mercury. Suppose the manometer is set up with a 1 cm diameter tube exposed to 1 atm nitrogen at room temperature (298K) on one end, and exposed to vacuum on the other end (which of course will be approximately 760 mm higher). The observed pressure can only change when the column of mercury has time to flow the device (and any other measuring device) will have a nonzero response time. A reasonable estimate for the response time of a manometer might be 0.1 seconds, so the amount the pressure will appear to fluctuate will depend on the number of collisions with the top of the column in that time. 

The liquid gauge family is identified simply as vacuum gauges that have some liquid (usually mercury or a low-vapor-pressure diffusion pump oil) directly in contact with the vacuum. The amount of liquid movement is directly proportional to the force exerted on it, and the (measured) amount of movement is read as the vacuum. Because mercury has traditionally been used for vacuum measurement, the term millimeters of mercury is commonly used even with nonliquid gauges. 

Fig. 7.38 In this example, take the measurement from the closed side (439 mm) of the manometer, and subtract from it the measurement from the side of the manometer connected to the vacuum system (426 mm) to obtain the vacuum reading (13 mm Hg). Remember, avoid parallax problems, and with mercury, read the top of the meniscus.

It is very important to be able to measure the pressure in a vacuum system, particularly when carrying out a distillation. For low vacuum measurement a simple manometer, such as that shown in Fig. 8.3a, is commonly used and the pressure is taken by subtracting the heights of the mercury levels. Dial gauges are also useful for in-line measurements and they are particularly valuable when used with rotary evaporators. For high vacuum... 

The pressure of gas being pulled though a line by a vacuum pump is measured with an open-end mercury manometer. A reading of -2 in. is obtained. TOat is the gas gauge pressure in inches of mercury What is the absolute pressure if Paim = 30 in. Hg ... 

There are two practical difficulties. One is the measurement of the very small photocurrent, which may be as low as 10" amp, requiring the use of a vibrating reed electrometer or similar instrument. The second is that, for work functions above 5 eV, Vq lies in the far ultraviolet. This makes the study of some adsorptions very difficult and 6 eV is about the practical limit of such measurements. A suitable light source is the quartz mercury arc. The energy of the incident beam can be measured with a calibrated photocell, a vacuum thermopile or a radiometer. A suitable cell for adsorption studies is shown in Fig. 11. The sample being studied forms the cathode B. It can be a metal foil or a film formed by evaporation from the filament E. A wire C is fused through the glass to make contact with the... 

Pressure is the force per unit area acting perpendicular to a surface. The unit of pressure in the SI system of units is Newtons per square meter. This unit is also called the Pascal and abbreviated as Pa. Another unit frequently encountered in practice is the torr. This unit corresponds to a millimeter of mercury in a standard barometer. The standard barometer is a glass tube filled with mercury connected to vacuum on one side and to the measured pressure on the other. The mercury is at 0 °C in a location having gravity corresponding to the standard gravitational acceleration, g = 9.807 m s-2. One atmosphere (1 atm) is 760 torr exactly, which corresponds to 101325 Pa. 

The synthesis is carried out in a 500-ml., single-necked, round-bottomed flask which is connected to a standard vacuum line by means of a stopcock adapter. The stopcock adapter is important, since it allows for the easy attachment and removal of the reaction vessel from the vacuum line without exposing the contents of the reactor to air. The vacuum line must be equipped with a mercury manometer in order to measure the pressure of PH3 during the preparation of the Li[Al(PH2)4] solution. A conventional vacuum line with greased stopcocks is satisfactory. However, germyl-phosphine does attack conventional stopcock lubricants so a greaseless system is preferred if available. 

The vapor-pressure measurements were carried out with hexa sublimed in high vacuum. Pressures of 10 to 10 mm. Hg, corresponding to 20 to 85°, were measured with a quartz filament manometer 2, S). At temperatures of 120 to 210° a simple mercury manometer was used, and the vapor pressures were obtained by extrapolating the pressure-time curves plotted in Fig. 1 to time zero. 

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