Pressure units, conversion

Pressure is defined as force per unit of area. The International System of Units (SI) pressure unit is the pascal (Pa), defined as 1.0 N /m. Conversion factors from non-SI units to pascal are given in Table 1 (see also Units and conversion factors front matter). An asterisk after the sixth decimal place indicates that the conversion factor is exact and all subsequent digits are 2ero. Relationships that are not followed by an asterisk are either the results of physical measurements or are only approximate. The factors are written as numbers greater than 1 and less than 10, with 6 or fewer decimal places (1). 

Pressure units are summarized in Table 4.1. It is important to be familiar with them and to be able to make conversions between them. In Example 4.2, for instance, the pressure in pascals could have been obtained by using a conversion factor derived from Table 4.1 ... 

The measuring unit of pressure in SI system is called pascal (Pa) = newton/m2. One Pa corresponds to a very small pressure. Also for the latter reason, several other units are commonly used in vacuum practice and instrumentation. In Table 1.1, the conversion among the most frequently used pressure units is reported. 

Table I Permissible pressure units including the torr 1) and its conversion...

TABLE 2.5 Conversion between Units of Concentration in ppm, pphm, ppb, ppt, and Molecules cm 3, Assuming 1 atm Pressure and 25°C ... 

There s no one specific pressure unit you have to use when doing problems with Dalton s pcirtial-pressures equation. As long as the pressure units for all the gases are the same, you re good to go. However, if all the pressures given aren t in the Scime units, then some conversion must take place ... 

The third calculation is the conversion of pressure units from pounds per square inch read from a guage (psig) to pounds per square inch absolute (psia). Strictly speaking, the value of barometric pressure in psi read from a barometer at the time the pressure gauge was calibrated should be added to psig to get psia. At sea level, a value of 14.7 psi is usually sufficiently accurate. 

Conversion factors for various pressure units and flow units are given in Table VI.1. Table VI.2 lists values of the ratio d,/do (the density of mercury at a temperature t divided by the density of mercury at 0°C) over the temperature range 0-99°C. Capillary depression corrections for mercury in glass tubes are given in Table VI.3. The complete set of corrections for a pressure measurement is made as follows ... 

New catalysts have helped increase the conversion and yields. The older, high-pressure processes used zinc-chromium catalysts, but the low-pressure units use highly active copper catalysts. Liquid-entrained micrometer-sized catalysts have been developed that can convert as much as 25 percent per pass. Contact of the synthesis gases with hot iron catalyzes competing reactions and also forms volatile iron carbonyl that fouls the copper catalyst. Some reactors are lined with copper. 

Conversion among Units of Pressure, Temperature, and Energy... 

Table lisa conversion table for pressure units. In a mass spectrometer, the mean free path should be at least 1 m and hence the maximum pressure should be 66 nbar. In instruments using a high-voltage source, the pressure must be further reduced to prevent the occurrence of discharges. In contrast, some trap-based instruments operate at higher pressure. 

Section 7.1 gives examples illustrating the use of quantity calculus for converting the values of physical quantities between different units. The table in section 7.2 lists a variety of non-SI units used in chemistry, with the conversion factors to the corresponding SI units. Conversion factors for energy and energy-related units (wavenumber, frequency, temperature and molar energy), and for pressure units, are also presented in tables inside the back cover. 

Various units are used for expressing pressures (see Chapter 1, Footnote 8). A pressure of one standard atmosphere, or 0.1013 MPa, can support a column of mercury 760 mm high or a column of water 10.35 m high. As indicated in Chapter 1, the SI unit for pressure is the pascal (Pa), which is 1 N m-2 an SI quantity of convenient size for hydrostatic pressures in plants is often the MPa (1 MPa = 10 bar = 9.87 atm). (An extensive list of conversion factors for pressure units is given in Appendix II, which also includes values for related quantities such as RT.) Pressure is force per unit area and so is dimensionally the same as energy per unit volume (e.g., 1 Pa = 1 N m-2 = 1J m-3). Vw has the units of m3 mol-1, so VWP and hence /aw can be expressed in J mol-1. 

A"ab is the equilibrium constant for an equilibrium in which the number of moles change, and so will have the units of (concentration)". It is thus dependent on our choice of units. If for example AJb is expressed in units of concentration, for example, then A"Jb( ), where P is pressure units, is obtained (for ideal gases) by using the conversion factor from the ideal gas law, P = cJKT, A ab(I ) = -Kab(c) Similarly, to obtain AJb(A) where the units are mole fractions, we have Kab N) = A Jb(c) C total Eab P) Ptotal. These relations will be of importance when we consider standard states and various thermodynamic relations (see footnote pg. 271). 

The conversion table on the inside front cover of this book lists values of a pressure expressed in several common force/area units and as heads of mercury and water. The use of this table for pressure unit conversion is illustrated by the conversion of 20.0 psi to cm Hg ... 

Steps 8 and 9 (Continued) Enthalpy data have been taken from Table 4.3. The heat of reaction at 25°C (77°F) and 1 atm from Example 4.31 after conversion of units is — 121,672 Btu/lb mol of CO. We can assume that the slightly higher pressure than SC of 2 atm has no effect on the heat of reaction or the enthalpy values. Figure E4.40b shows the sensible heat (enthalpy) values for the entering and exiting materials. 

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