Helium density

Helium is a colorless, odorless, and tasteless inert gas that is noncombustible and is the least soluble of any gas in water and alcohol. As a gas, it diffuses well in sohds. Hehum s freezing point is -272.2°C, and its boiling point is -268.93°C. Both temperatures are near absolute zero (-273.13°C, or-459.4°F), where all molecular and thermal motion ceases. Liquid helium has the lowest temperature of any known substance. Heliums density is 0.0001785g/cm. ... 

DENSITY MEASUREMENTS. Helium densities of the extracts were determined so that the gravimetric sorption data could be converted to volume fractions needed to calculate x parameters. The results of these measurements are shown in Table III. The density of the extract was observed to decrease with increasing size of the added alkyl group. These results are qualitatively similar to those of Liotta t al.(14), who studied the effects of O-alkylation on the physical structure of the native Illinois No. 6 coal. 

TABLE III. Helium Densities, CO2 Surface Areas, and Solubilities of Illinois No. 6 Extracts... 

This work was supported by a grant from the U.S. Department of Energy, Grant No. DE-FG22-88PC88924, and the Research Corporation. We thank D. Smith for determining helium densities and R. Porter for obtaining NMR spectra. 

The resulting polymer foam composition is substantially of the closed cell type. This is evidenced by the fact that for equivalent densities, foams of EVA and acid copolymer are found to exhibit lower helium densities than foams of EVA alone. This is an indication that more of the cells in the EVA/acid copolymer foam are closed. Particularly for an acid copolymer content in the range of about 3-15%, the acid copolymer has been observed to be uniformly dispersed within the EVA in micron-sized particles when analyzed by transmission electron microscopy. 

Comparison of ternary moments with measurements. The density dependence of the helium-argon collision-induced absorption spectra has been studied at the temperature of 165 K, helium densities from 66 to 130 am-agats, and argon densities from 156 to 280 amagats. Ternary moments of... 

S. P. Nandi. Many parameters (e.g., helium density, heat of wetting, butane adsorption, etc.) for correlating coal rank pass through a minimum at ca. 85-90% carbon content. In the present paper another parameter—the average size of the micropores—has been shown to follow the same pattern. 

Helium and mercury densities were determined on the 6-8 mesh fraction. The larger mesh size was used to avoid the possibility that mercury would not penetrate the space between particles in the mercury density measurements. The coal was placed in a calibrated density tube, evacuated at room temperature for one hour, and then heated at 100°C. in vacuo for 2 hours. The weight of the coal after this treatment was used to compute the densities. Helium densities were determined at 30°C. by the method of Rossman and Smith (11). Mercury densities were determined by admitting mercury at an absolute pressure of 1140 torr to the coal sample after evacuation, following the helium density measurement. 

An alternative explanation of the data presented in Figure 3 is that the majority of the macropore structure is available for reaction, and by breaking up the coal one is simply opening up hitherto sealed pores. However, this does not seem likely since—to explain the results in Figure 3— one would have to assume that from 2% to 90 mesh the exposed internal area of the coal increases by a factor of 12. Mercury penetration data on Coal C indicates that the macropore area increased only 7% going from 2% to 90 mesh (see Discussions). Also, Malherbe (16) found for three bituminous coals that in going from 5 to 270 mesh the BET area only increased by a factor of 2. Furthermore, the helium density of coal is independent of particle size indicating no sealed pores exist at least in respect to the accessibility of helium (14). 

P represents the fluid density. p0 corresponds to the normal liquid helium density. 

The precise determination of true density requires complete filling of the pore structure with a fluid that has no interaction with the solid. No fluid meets these requirements completely. Helium has traditionally been considered as the best choice since it is not significantly adsorbed by coal at room temperature and that the use of helium gives a more accurate determination of coal density, but there is evidence (Berkowitz, 1979, and references cited therein) that part of the pore system may be inaccessible to the helium. Thus, when helium is used as the agent for determining coal density, the density (helium density) may differ from the true density and may actually be lower than the true density. 

The true density of coal is usually determined by helium displacement and therefore is often referred to as the helium density. Helium is used because it has the ability to penetrate all the pores of a given sample of coal without (presumably) any chemical interaction. In the direct-pressure method, a known quantity of helium and a weighed sample of coal are introduced into an apparatus of known volume, whereupon the pressure of the helium at a given temperature allows calculation of the volume of the coal. In the indirect method, mercury is used to compensate for the helium displaced by the introduction of the coal. 

The true density (in fact, the helium density) has been correlated with the elemental composition of coal. Thus,... 

The density of coal shows a notable variation with rank for carbon content (Figure 6.1) and, in addition, the methanol density is generally higher than the helium density because of the contraction of adsorbed helium in the coal pores as well as by virtue of interactions between the coal and the methanol, which results in a combined volume that is notably less than the sum of the separate volumes. Similar behavior has been observed for the water density of coals having 80 to 84% w/w carbon. 

Coal with more than 85% w/w carbon have usually been shown to exhibit a greater degree of hydrophobic character than the lower-rank coals, with the additional note that the water density may be substantially lower than the helium density for the 80 to 84% carbon coals, there is generally little, if any, difference between the helium and water densities. However, the hydrophobicity of coal correlates better with the moisture content than with the carbon content and better with the moisture/carbon molar ratio than with the hydrogen/carbon or oxygen/carbon atomic ratios. Thus, it appears that there is a relationship... 

During the course of laser resonance experiments it was noticed that the central wavelengths shift depending on the helium density. Thus, the resonance line shapes at various target gas conditions were measured precisely with a reduced laser bandwidth and an improved wavelength calibration [18]. Figure 5 shows resonance profiles taken for the 597.26 nm line at different pressures ranging from 530 mb to 8.0 bar at temperatures of 5.8-6.3 K. The results are summarized in Table 2. 

Fig. 5. Resonance profiles of the 597.26-nm line showing red-shifts of the center with helium density. The linear scale for the y axes is not the same for different target conditions. From Torii et al. [18]...

Thus, from (9), (11), and the measured value of the T, we could evaluate the quantity I. For this calculation we assupied that the coal consisted entirely of carbon, that—, y= 4.60 cnr/gm, and that the mass density of the coal 1.30 gm/cm. This density is a compromise between the helium densities determined for coals of different rank and origin—., 5... 

Several catalyst densities are used in the literature. True density may be defined as the mass of a powder or particle divided by its volume excluding all pores and voids. In a strict physical sense, this density can be calculated only through X-ray or neutron diffraction analysis of single crystal samples. The term apparent density has been used to refer to the mass divided by the volume including some portion of the pores and voids, and so values are always smaller than the true density. This term should not be used unless a clear description is given of what portion of the pores is included in the volume. So-called helium densities determined by helium expansion are apparent densities and not true densities since the measurement may exclude closed pores. 

The helium density is calculated by measuring the volume accessible to helium. This is the true density and is also spoken of as the crystallographic density. 

Equations (54) and (55) are applicable both for an ordinary cluster and for a cluster with a bubble. To characterize the density profile for the cluster with a bubble, we choose the helium atom density function in the form of a void at r < Rb — t /I, a rising profile toward a constant density with increasing r beyond the void boundary at r > Rb — t jl, and an onset of the cluster exterior decreasing density profile for r > / — tijl. Here Rb is the bubble radius, R is the cluster radius, t is an effective thickness parameter for the density profile of the bubble wall, and 2 is the thickness of the cluster surface density profile. The explicit form of the helium density profile was taken as... 

In this density, volume is defined as the sum of the volume of the solid material and any closed pores within the solid. These pores cannot be penetrated by any fluid and become part of the powder volume. A mass of catalyst is placed in a flask of known volume, and the amount of helium needed to fill the flask measured, giving the powder volume by difference. Care should be taken to dehydrate all pores thoroughly. Because helium is used as the displacing fluid, this density is sometimes called the helium density. See Table 7.1 for an example of typical values. 

When only a single phase is present, the system pressure depends on the temperature, the volume, and the quantity of material. Rather large pressures can be produced by completely vaporizing a cryogenic fluid in a closed chamber. For example, using the helium density data of Figure 6, we see that an unyielding container filled initially with liquid at 1 atm (p = 0.125 g/cm ) would experience a pressure of 100 atm at 27°K and about 1000 atm at 270°K if the container were initially half full of liquid, it would experience a pressure of about 50 atm at 27°K (p = 0.0625 g/cm ) and about 400 atm at 270°K. In practice, if the container remained intact, its volume would increase somewhat so that the final density and the pressure would be reduced. 

In this equation, p g is the mercury density and pHe is the helium density, decreases with carbon content (Figure 9.4). In addition, the surface area of coal varies over the range of 10-200 va g and also tends to decrease with the carbon content of the coal. 

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