Total Pressure Measurement

Introductory comments on pressure were made in Section 2.1. Particularly important is the fact that the pressure of gas in a system is a convenient expression of the particle number density (n) in that system (see Equation (1.5)). 

There are two general procedures for pressure measurement direct and indirect. 

Direct methods are based on the fact that the force that constitutes pressure can physically shift a surface to which it is applied. Indirect methods monitor either the particle number density or a property of the gas that depends on n. Direct methods yield pressure information that is independent of the nature of the gas whilst pressures indicated by indirect methods are dependent on gas type. The need to have the two methods arises from the fact that, at low pressure, the forces involved are 

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A. Berman, Total Pressure Measurements in Vacuum Technology, Academic Press, Inc., Orlando, Fla., 1985, pp. 140—146. 

Test run the curves 2 in Fig. 1.71 are taken from the test run, as shown in Fig. 1.63, but with pressure control 0.36 mbar (total pressure measured with Capacitron). The ice temperature has been -22 °C (constant) for 3 h and DR reached 0.05 %/h after 10 h. Secondary drying could have been started much more early, thus shortening the drying process. 

Barton, D.P., Bhetanabotla, V.R., and Campbell, S.W., Binary total pressure measurements for methanol with 1-pentanol, 2-pentanol, 3-pentanol, 2-methyl-l-butanol, 2-methyl-2-butanol, 3-methyl-l-butanol, and 3-methyl-2-butanol at 313.15 K,/ Chem. Eng. Data, 41(5) 1138-1140, 1996. 

Test run curves 2 in Figure 1.71 are taken from the test run, as shown in Figure 1.63, but with pressure control 0.36 mbar (total pressure measured with Capac-... 

After integrating this equation, the difference of total pressures measured at the outer and inner cylinders is given by... 

Due to lack of data on vapor compositions over PbF2 at various temperatures, the total pressure measurements reported by Wartenberg and Bosse (2.), using boiling point method, and those of Nesmeyanov and lofa (2) are not used for evaluation. 

Barker, J. A. Determination of activity coefficients from total pressure measurements. Aust. J. Chem. 1953, 6, 207-210. 

Thomas, S. Bhethanabotla, V.R. Campbell, S.W. Total pressure measurements for n-pentane-methanol-2-butanol at 303.15 K. J. Chem. Eng. Data 1991, 36, 374-378. 

To compute the rate constant from total-pressure measurements we first write the rate equation in the usual way. 

Figure V-2 Comparison between the saturated pressure of selenium calculated from the selected thermodynamic quantities and the total pressure measured in various investigations. A logarithmic unit of 0.1 corresponds approximately to a relative error of 25%.

Se2(g) can be found to within + 10% at high temperatures. This allows the evaluation of fairly reliable values of the enthalpy of formation of Se2(g) from high temperature total vapour pressure data. The mass spectrometric investigations are treated first and are followed by a discussion of the high temperature total pressure measurements. 

Table V-3 Enthalpy of formation of Se2(g) at 298.15 K evaluated from total pressure measurements by the third law. The values derived in [84GRO/DRO] are included for comparison. The mole fraction and temperature columns do not apply to these values.

In the present review, the enthalpies of formation at 625 K in Table III of the paper were recalculated to 298.15 K using the selected heat capacities of Se(cr, I), Se2(g), and Se5(g)-Ses(g) to provide for a comparison with other investigations. Similarly, third law enthalpies of formation and second law entropies were evaluated by combining the data of Table 111 in the paper and the total pressure measurements by the Knudsen torsion-effusion method in the same paper with the selected values for the heat capacities and entropies. The results are summarised in the Table A-70. 

New density and total pressure measurements of selenium vapour are presented. A previous investigation by the same author, [67RAU], is re-evaluated and combined with the new data in an evaluation of the thermodynamic properties of the molecules Se2(g)-Seg(g). The enthalpies of formation and entropies were varied in a procedure to obtain a good fit to the observed densities. The procedure contains many parameters that can be varied and different sets may fit the experimental data almost equally well. The thermodynamic properties derived for the species Se2(g)-Seg(g) are too dependent on the choices made by the author in order to be considered by the review. However, the new measurements of the total vapour pressure in the temperature range 1073 to 1373 K were evaluated by the review for the thermodynamic properties of Sc2(g) using the third law and mole fractions of Se2(g) estimated from the selected selenium data. The enthalpy of formation derived was Af//°(Se2, g, 298.15 K) = (138.9 1.9) kJmoP. ... 

Oppermann et al. studied some of the equilibria in the system Se-O-Br by total pressure measurements. Thermodynamic quantities were derived from the temperature dependence of the equilibrium constant. The extrapolations of the results to standard conditions do not use experimental thermal functions but employ various estimates. 

In more recent studies, mass spectrometry has commonly been used to supplement or replace total pressure measurement as the means of directly... 

ABSOLUTE PRESSURE - Total pressure measured from an absolute vacuum. It equals the sum of the gauge pressure and the atmospheric pressure corresponding to the barometer. 

The progress of this gas phase reaction was monitored using total pressure measurements to obtain the following data. 

For reaction at 80.4°C, the data consist of total pressure measurements as a function of time. This irreversible gas-phase reaction takes place in a constant volume reactor. 

After the preparation, the panel was put in a oven and kept at the indicated temperature for 15 to 20 minutes. The panel was then removed and the total pressure measured as a function of time. To allow the continuous monitoring of the pressure, a SpiroTorr spinning rotor gauge was mounted in the panel as shown in Figure 4.21. 

Aizawa, K. Kato, M. Vapor-liquid equilibrium determination by total pressure measurements for three binary systems made of 1,2-dimethoxyethane with toluene, methylcyclohexane, or (trifluoromethyl)benzene 7. Chem. Eng. Data 1991,36, 159-161... 

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