Measuring Vacuum

The pH scale is a convenient way of measuring acidity or alkalinity. The definition is pH = -logi0[H+], 

In bakery products the pH of the product is important for several reasons. Acid foods are normally relatively safe microbiologically as a low pH inhibits the growth of bacteria. Fruit flavoured products such as fruit pie fillings normally have some acid component added to complement the fruit flavour. Where a hydrocolloid is present the pH of the product can be critical in terms of product stability or gelling. A hydrocolloid held at its isoelectric point, i.e. the pH at which there is no net charge, will likely come out of solution. 

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It is helpful to understand the relationship of vacuum to the other pressure measurements. Vacuums can range from atmospheric pressure down to zero absolute pressure , representing a perfect vacuum (a theoretical condition involving the total removal of all gas molecules from a given volume). The amount of vacuum is measured with a device called a vacuum gage. 

A differential gage that shows the difference in the system and the atmospheric pressure surrounding the system usually measures vacuum. This measurement is expressed as ... 

Pressure. Pressure, defined as force per unit area, can be expressed as an absolute or relative value. Although atmospheric pressure constantly fluctuates, a standard value of 101.3 kPa (14.7 psia) has been assigned as the accepted value at sea level. The MaM in the psia stands for absolute, ie, the pressure is 14.7 psi (101.3 kPa) above zero pressure or a vacuum. Most ordinary pressure-measuring instruments do not measure true pressure, but rather a pressure relative to the barometric or atmospheric pressure. This relative pressure is called gauge pressure. The atmospheric pressure is defined to be 1 psig, in which the "g" indicates that it is relative to atmospheric pressure. Vacuum is the pressure below atmospheric pressure and is, therefore, a relative pressure measurement as well. The relationship between absolute and relative pressure is shown in Figure 3 (see Pressure measurement, Vacuum technology). 

Ion source Atomization Ionization Plasma temperature Electron density Ions measured Vacuum condition Mass analyzer utilized Application... 

We will need to use this table to calculate a jet s compression ratio, when we measure vacuum pressures with an American-type (in Hg) gauge. 

These results are again confirmed by CP MAS NMR measurements. Vacuum pretreated vacuum cured samples were prepared, with minimal water contact. The influence of air humidity in the curing stage was tested by precuring one sample in air for 2 h (sample 7). I3C NMR spectra are presented in figure 9.8. 

A.M. Sachs, A. Blaer, J. French, M. May and E. Zavattini An experiment to measure vacuum polarization in 3D-3P transitions in muonic helium atoms. Proposal Exp. 745, Brookhaven National Laboratory. 

Second only to the mechanical gauge as the easiest device to measure and read a vacuum (and decidedly easiest in construction) is the liquid manometer (see Fig. 7.37). A well-made mercury manometer, kept very clean, can measure vacuums of up to 10 3 torr. This sensitivity can be increased by up to 15 times if a liquid with less density, such as diffusion pump oil, is used. However, diffusion pump oil is far more difficult to keep clean and can require either (a) a very tall (and thereby impractical) column or (b) a manometer of very limited range. In addition, because of the strong surface tension between diffusion pump oil and glass, long waiting periods between readings are required as the oil settles into place. 

Table I. Coincidences between transitions in the B liy-X Tg Systems of Na2 and Li2 and argon laser lines. Dimer laser emission has been observed from all the underlined transitions. The accuracy is 5 x lO" A for all resolved lines. A is the difference in A between measured vacuum wavelength and the calculated wavelength from the constants of References 6 and 7.

Onsager model—reaction field effects. In the simplest form of this model a chosen molecule is represented by a spherical cavity of suitable volume filled with fluid of relative permittivity c , containing a rigid dipole of value fi. This p, is chosen so that if py is the measured vacuum dipole moment of the molecule, = (c , + 2)py/3. Correct calculation of the orienting couple on the dipole due to a given external field leads to the Onsager relation... 

Negative pressure gauge - measures vacuum (pressure below atmospheric pressure)... 

Higher pressures are commonly measured with Bourdon tube gauges as seen in Figure 33.15. Bourdon tube gauges can be designed to measure vacuum pressures or positive pressures. Bourdon tube gauges display the pressure by the displacement of a... 

There have been many theoretical determinations of the rates of distillation under vacuum, but none that appeared to be applicable to vacuum dezincing when its development was commenced at Port Pirie in 1946. Thus it was felt to be desirable to develop the theoretical side (37) simultaneously with the practical development, as a guide to understanding and possible later application to optimising of the process. Figure 7 illustrates the concept of the distillation process which was developed. It was necessary to discard some faulty ideas or misconceptions, which derived fix>m implicit notions associated with equilibrium, but not kinetic conditions. Carman (38), for example, assumed that the partial pressure of the vapour of the condensing species is equal to its partial pressure in the condenser. It is not, unless the condensation rate is zero. Richardson (6) assumed that the measured vacuum is equal to the distilling species, which it is not, but is instead the partial pressure of the inert atmosphere. Warner (40) assumed the partial pressure of zinc to be constant across the distillation space, which is not correct unless the distillation rate is zero. 

Figure 1 is a front view of the completed apparatus and shows a gas cylinder attached. The apparatusisessentially self-contained, and requires only a 110-volt ac power source, a source of cooling water, a supply of liquid helium and an external wet-test meter. The apparatus incorporates vacuum equipment, two Pirani gauges for measuring vacuum, two coil-and-trap assemblies to contain the impurities and a fixed orifice for controlling flow in the apparatus. Stainless steel tubing is used wherever possible and all stainless steel connections are silver-brazed. The apparatus is assembled in a relay-rack cabinet with overall dimensions of 17 X 22 X 67 in. 

Directly and Indirectly Measuring Vacuum Gauges and Their Measurement Ranges... 

In directly measuring vacuum gauges, the elastic restoring force of a spring element (diaphragm) or of gravitation (liquid column) is used for pressure measurement. 

Because of the large measurement range of the indirectly measuring vacuum gauges (up to nine decades) the scale or electrical output of this type of instruments often has a logarithmic graduation, digital instruments show... 

Bias—Since there is no accepted reference material suitable for determining the bias for the procedure in this test method for measuring vacuum distillation characteristics. ... 

There are a couple of ways of measuring vacuum. The method we will be using is inches of mercury (in. hg). This refers to the amount of vacuum that it takes to lift a column of mercury 1". A mercury gauge with a vacuum at the closed top of the tube will support a mercury column of 29.92 in. Hg in height at sea level under standard conditions. This represents the maximum vacuum we can expect to achieve under idea conditions at sea level. 

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