Vapour pressure absolute calculation

The measurement of the dielectric constants of the various vapours was carried out under the conditions described in the previous section, i.e. at the vapour pressures found to correspond to the various temperatures. In this way we obtain the effect of temperature on the dielectric constants when the density has a constant known value. In the following tables the absolute temperature is given in the first column and its reciprocal in the second. The third column gives the pressure calculated from the equation of state and used in the measurement of the dielectric constants. Finally, the last column gives the measured values of the dielectric constants. The results obtained arc illustrated graphically in figs. 6, 7 and 8. 

In this equation, known as the Clausius-Clapeyron equation, represents the heat absorbed, per gram, in the transformation of one phase into the other, and Vi are the specific volumes of the two phases, and T is the absolute temperature at which the change occurs. The above equation enables one to calculate only the slope of the curve at a given point, not the actual values of the pressure. It is possible, however, to derive an expression by means of which the individual points on the vapour-pressure curve can be calculated approximately. 

Kelvin Equation. An equation useful in sorption studies for the calculation of pore size and pore size distribution. It is rRTln(p/po) = -2yV cos 0 where p is the equilibrium vapour pressure of a curved surface (as in a capillary or pore) of radius r p is the equilibrium pressure of the same liquid on a plane surface R is the gas constant T is the absolute temperature y is the surface tension V the molar volume 0 the contact angle of the adsorbate. When the Kelvin equation is satisfied, vapour will condense into pores of radius r. (W.T. Thompson, Phil. Mag. 42,1871, p.448). 

Please note that function y has included two calculations estimation of vapor pressure aud then estiniation of absolute humidity y. As a result, we have to define a new variable within the function called vapour pressure with the corresponding equation. In order to be able to use the variable vapour pressure, we have to include the senteuce Dim vapour pressure Double so the program is able to understaud that as a numerical variable. 

During this type of measurement, the volume of gas adsorbed by a unit mass of solid denoted by VyW depends on the equilibrium pressure, the absolute temperature, the solid, and the gas. The adsorbable gas will be below it s critical temperature and the pressure inside the equipment as measured by the manometer shown is expressed as a fraction of the saturation vapour pressure P. The resulting ratio P/P is called relative pressure. The amount of gas adsorbed can be calculated using the gas law relationships, the measured pressures, and known volumes of the equipment. 

Fig. 14. Change in ealcile saturation state during single-step adiabatic boiling (vapour saturation pressure) of aquifer water from four wet-steam wells in three areas Krafla. Iceland Momotnmho. Nicaragua and Zunil. Guatemala. A positive Sl-value corresponds to oversaluralion and a negative value to undersaturation. An SI of zero corresponds with equilibrium. SI is on a log Scale so an Sl-value of +1 indicates tenfold oversaturation. The numbers indicate well numbers. The calculated Sl-values for the aquifer waters (dots) depart a little from equilibrium. In view of all errors involved in the calculation of the Sl-values. the departure front equilibrium is. however, not significant. Nine that variations in Sl-values are more accurately calculated than absolute values.

Let us test the approximate equation (Ha) by calculating with its aid the dissociation of water vapour. This is, perhaps, the best gas reaction for the purpose, as it has been carefully investigated in several different ways by Nernst and his pupils. (The degree of dissociation has been determined between 1300° and 2300° absolute.) Let x per cent, be the degree of dissociation of water vapour at a total pressure of 1 atmosphere. The equation for the dissociation is 2H20 = 2H2H-02, and hence the... 

Prom the definition of relative humidity it is seen that the partial pressure of the water vapour p in fact assumes the value ps when RH = 100 %. To calculate the absolute humidity of a mass of air it is necessary to know both the relative humidity RH and the temperature 6. 

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