Gas pycnometers

True Density or Specific Gravity. The average mass per unit volume of the individual particles is called the tme density or specific gravity. This property is most important when volume or mass of the filled composition is a key performance variable. The tme density of fillers composed of relatively large, nonporous, spherical particles is usually determined by a simple Hquid displacement method. Finely divided, porous, or irregular fillers should be measured using a gas pycnometer to assure that all pores, cracks, and crevices are penetrated. 

The gas pycnometer is based on the ideal gas law. A known quantity of gas (characterized by a determined temperature and pressure) is allowed to flow from a calibrated reference volume into a calibrated sample cell containing the solid. A second pressure reading is obtained, and the sample volume is calculated. 

When measuring powder volume in the manner described above it is necessary to avoid using any gas which can be even slightly adsorbed. If so much as a thousandth of a monolayer were adsorbed the equivalent volume of gas would be in the order of 0.001 cm for each 2.84 m of area, if nitrogen were used. Since the sample cell used in the apparatus described in Fig. 21.1 can hold 130 cm, the total surface area of the sample can be hundreds or even thousands of square meters. Thus errors of O.l-l.O cm can be incurred due to very small amounts of adsorption. This is another reason helium is recommended in any gas pycnometer. 

Methods of measurement of coal density include use of a gas pycnometer and particle density by mercury porosimetry. However, the difference in density values using different gases must be recognized since, for example, density values measured by nitrogen may be greater than those obtained when helium is used. Density measurement depends on adsorption of gas molecules, and differences (between nitrogen and helium) may be due to nitrogen adsorption on the coal surface. 

The skeletal density, also called the true density, is defined as the density of a single particle excluding the pores. That is, it is the density of the skeleton of the particle if the particle is porous. For nonporous materials, skeletal and particle densities are equivalent. For porous particles, skeletal densities are higher than the particle density. Measurements of the skeletal density can be made by liquid or gas pycnometers. 

ASTM. (2003), Standard Test Method for Specific Gravity of Soil Solids by Gas Pycnometer. Designation D5550-00. Volume 04.08. American Society for Testing Materials, accessed at Website www.astm.org. 

Table 1. Manufacturer of volumetric / carrier gas sorption instruments, gas pycnometers...

In Table 1 manufacturers of carrier gas and volumetric adsorption measuring instruments are compiled. Most of these companies offer also mercury porosimeter and gas pycnometer. Mercury porosimetry extends the measuring range of the sorption method towards larger pores. Table 2 gives a survey on the commercial offer of gravimetric apparatus. 

The bulk density of a powder is obtained by dividing its mass by the bulk volume it occupies. The volume includes the spaces between particles as well as the envelope volumes of the particles themselves. The true density of a material (i.e., the density of the actual solid material) can be obtained with a gas pycnometer. The bulk density of a powder is not a definite number like true density or specific gravity but an indirect measurement of a number of factors, including particle size and size distribution, particle shape, true density, and especially the method of measurement. Although there is no direct linear relationship between the flowability of a powder and its bulk density, the latter is extremely important in determining the capacity of mixers and hoppers and providing an easily obtained valuable characterization of powders. 

Gas pycnometry is probably the most commonly used method in the pharmaceutical industry for measuring true density. Gas pycnometers rely on the measurement of pressure changes, as a reference volume of gas, typically helium, added to, or deleted from, the test cell. 

Fig. 11.11 Photographs showing non-mercury instruments for the determination of porosity and related data, (a) Capillary flow po-rometer for gas as well as liquid permeability and the testing of filter integrity (b) gas pycnometer (c) bulk/absolute density analyzer (d) BET sorptometer (courtesy PMI, Ithaca, NY, USA).

The density of packing of the particles is of primary importance in considering the behavior of sediments. There are four ways to determine sample volume in practice—(A) wet total volume from a constant volume sampling ring to extract sample cylinders from soft sediments (V,Ring), (B) wet total volume from the gas pycnometer (V,pyJ, (C) dry volume from the gas pycnometer (V pyJ, and (D) volume of sampling tube (V,). The three volume... 

Some laboratories use a gas pycnometer to determine the density with Ar or He to intrude into the sample and, not surprisingly, results do differ from methods that use a liquid. The gas pycnometer uses the ideal gas law to determine the volume of a sample and given a known volume of the sample, test chamber and the gas reservoir, together with the change in pressure, the absolute density can be calculated from the volume of the sample and its weight. The method does require a larger sample (0.5-10 g), but the test is non-destructive. Normally 10 iterations are taken to ensure accuracy. 

This procedure is simple and effective and several types of He-gas pycnometers are commercially available, cp. Tab. 1.5. 

Determination of the volume of activated carbon Norit R1 Extra by helium expansion measurements at 298 K in a commercial gas pycnometer (Micromerites, Accu Pyc 1330). 

Figure 1.7. Measurements of the (specific) He-volume of activated carbon fibres (ACF) at 298 K for a set of pressure step up experiments (upper data ) and respective step down pressures (lower data o). Measurements were performed in a commercial gas pycnometer using He (5.0) [1.47, 1.48].

Table 2.1 Manufacturer of volumetric and carrier gas sorption measuring instruments and of gas pycnometers.

For non-porous solids, this is easily measured by a gas pycnometer or specific gravity bottle, but these devices should not be used for porous solids since they give the true or absolute density p bs the material of which the particle is made and this is not appropriate where interaction with fluid flow is concerned ... 

Illustrations of severai methods to determine specimen voiume or density (a) pycnometer for liquid samples, (b) pycnometer for solid samples, (c) gas pycnometer, and (d) density gradient column showing sample suspended in liquid... 

Gas pycnometers are popular because they avoid wetting and bubble issues. Because of this, they work well on materials with porosity and on granular and powdered materials, as well... 

ASTM D 5550-00 Standard Test method for specific gravity of soil solids by helium gas pycnometer. Annual Book of ASTM Standard, 04.08, ASTM, Philadelphia, USA... 

Method B of ASTM D 792 uses a gas pycnometer to measure the volume of a powder or a granulated or irregular solid. The displacement of gas, usually helium, from a vessel of known volume is measured by sensitive differential pressure indicators. Suitable laboratory equipment is manufactured by Quantochrome (UltraFoam Pycnometer), Micromeiitics (Accu Pyc 1330), and others. This is also a convenient route to foam density. In troubleshooting production problems, SpG should always be checked, because of its sensitivity to misformulation. 

The porosity of the samples was determined using QuantaChrome Stereopycnometer with Nitrogen gas medium using the gas pycnometer test method ASTM D6226-98. This method measures the accessible cellular volume of a material. The remaining volume is that occupied by closed cells, cell walls, and unfoamed sohd polymer. A gas pycnometer is designed to measure the volume of sohd objects. This is accomplished by employing the Archimedes principle of fluid displacement to determine the volume. The displaced fluid is a gas,... 

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