Saturation vapor density

Extension of tbe pressure range to P,. = 14 is available in tbe Technical Data Book. For saturated vapor densities, tbe values of Z and Z are tabulated as a... 

Saturation vapor density The density of a saturated vapor. 

The Kelvin equation can be combined with the relative humidity, RH, if water is involved as the fluid relative humidity indicates how moist the air is. The amount of water vapor in the air at any given time is usually less than that required to saturate the air. The relative humidity is the percentage of saturation humidity, generally calculated in relation to the saturated vapor density. Relative humidity may be defined as the ratio of the water vapor density (mass per unit volume) to the saturation water vapor density, usually expressed in percent. Relative humidity is also approximately equal (exactly equal when water is assumed as an ideal gas) to the ratio of the actual water vapor pressure to the saturation water vapor pressure, RH = PJP°. The P° values corresponding to each temperature are given in tables which can be found in handbooks. If RH is measured in an experiment, then Pv can be calculated by using the saturation water vapor pressure tables and can be inserted into the Kelvin equation. 

A similar calculation for lindane, using the figure in Table 8.3 (9.4 x 10 mmHg), gives a saturation vapor density of 150 ng/1, which indicates that lindane disappears by evaporation much more easily than DDT. It is important to note that the vapor density, and thus evaporation velocity, will be reduced by adsorption in the soil, but will be enhanced by higher moisture content in the soil due to co-distillation. A parameter called Henry s constant (H) is important to determine the volatilization of pesticides when dissolved in water. According to Henry s law there will be equilibrium of concentrations in water and air at a specified temperature ... 

Volatilization. The volatilization flux of pesticide is usually determined by first considering its aqueous solubility and sorption. Excess pesticide beyond that which will dissolve in soil water and be sorbed by the soil is considered available for diffusion across the soil surface and into the atmosphere. Most models that consider volatilization therefore require as input the pesticide aqueous solubility and the saturated vapor density. One method of partitioning between the liquid and vapor phases is (9). 

An initial problem with Equation 15 is loss of significant figures as Za 1. The more serious problem is that if independent equations are used for the vapor pressures, Pa[Ta(p)], and for the saturated vapor densities, papproach values from zero to infinity, depending on the formulations for Pa and pa. 

A solution for the problems mentioned above is to replace conventional formulations of the saturated vapor densities by a formulation of the compressibility factor, Za(T), for saturated vapor (see Saturated Vapor Densities). By using this new formulation for substitutions, Equation 15 can be transformed to... 

Saturated Vapor Densities. We formulate the compressibility factor for saturated vapor as a function of temperature by using the vapor-pressure equation. Subscripts are omitted because we refer always to saturated vapor and to the vapor pressure. We define the constant, A0 Zc — 1, where Zc is the value of the compressibility factor at the critical point, and the variables... 

Data at low pressures were estimated from the present vapor-pressure equation and the virial equation from Ref. 5. The rms relative deviation for 30 equally weighted, saturated-vapor densities is 0.17%. For most substances, the vapor pressures of the solid are extremely small hence we assume that differences at T < Tt will be negligible for present purposes. 

In previous reports we have shown graphically the behavior of functions (p,T), (p,T), B(p), and C(p) for Equation 4. In Ref. 3 we illustrated nonanalytic behavior in relation to the maximum in specific heats at the critical point. In this chapter we have given a solution for the long-standing problem of behavior in the limit of low densities, namely, a completely new type of formulation for the saturated-vapor densities, which extrapolates to Za = 1 at p = 0, T = 0. 

This approximation is often useful for estimation enthalpies of vaporization in the vacuum region. For pressures greater than approx. 1 bar, a qualitatively wrong curvature is obtained thus, Eq. (3.65) cannot be recommended for general use. For v/, a cubic equation of state is more appropriate, for example, the Peng-Robinson equation of state, which is appropriate for the calculation of saturated vapor densities from the triple point to temperatures close to the critical point. 

FIGURE 13.3 The saturated vapor density of helium at buffer-gas cooling temperatures. The He curve is extrapolated from ITS-90 [8] and the He curve is from Ref. [7] with the old TTS-90 curve (thin line) shown for reference. The shaded region represents the range of saturated vapor densities used for buffer-gas loading, which sets a minimum temperature of 180 and 500 mK for He and He buffer gas, respectively. 

One way to achieve this thermal disconnect is to remove the buffer gas. This can be done either by cooling the cell walls [5] to lower the saturated vapor density of helium (see Figure 13.3) or by pumping the gas out through an orifice in the side of the cell [19]. In order for the molecule number after buffer gas removal to be large, the timescale for pump-out must be small compared to the valley of death lifetime in Figure 13.5. 

The vapor pressures were calculated from the equations of Armstrong et al. [24]. The saturated compressibilities were computed from the saturated vapor densities of Matthews and Hurd [25]. 

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