Gas permeability method

Carman s Gas Permeability Method. A gas or a wetting liquid is made to flow through the porous... 

Carman s Gas Permeability Method. A gas or a wetting liquid is made to flow through the porous material in a tube by applying vacuum or pressure. The pressure drop or flow rate is measured. For pigments, a modified procedure is used in which mainly nonlaminar flow takes place [1.16]. For standards, see Table 1.1 ( Specific surface Permeability techniques ). 

Gas permeability method. Like the gas flow method, this technique is also based on the measurement of the flow rate of a gas through a porous medium such as a membrane. The flow rate is monitored as a function of the pressure drop across the thickness of the membrane. But unlike the gas flow method, the gas used in this method is a pure, nonadsorbable and noncondensable gas. 

In addition to the afore mentioned methods there are still special instrumental techniques applied such as the laser diffraction method for the determination of the particle size and the gas permeability method for the determination of the specific surface of a powder [17]. A further discussion of these methods is beyond the scope of this book. More information is available in textbooks [18]. 

Various techniques and equipment are available for the measurement of particle size, shape, and volume. These include for microscopy, sieve analysis, sedimentation methods, photon correlation spectroscopy, and the Coulter counter or other electrical sensing devices. The specific surface area of original drug powders can also be assessed using gas adsorption or gas permeability techniques. It should be noted that most particle size measurements are not truly direct. Because the type of equipment used yields different equivalent spherical diameter, which are based on totally different principles, the particle size obtained from one method may or may not be compared with those obtained from other methods. 

Stretching a polymer in two perpendicular directions, either successively or by blowing a bubble of molten material, leads to its biaxial orientation, which strongly improves mechanical properties in the stretching directions and/or gas permeability (e.g., biaxial orientation of polypropylene leads to BOPP (for biaxially oriented polypropylene) or biaxial orientation of poly(ethylene terephthalate) gives CC>2-impermeable bottles for carbonated beverages.) (Characterisation methods for determining molecular orientation are considered in Chapter 8.)... 

In this method the sample is acidified and the inorganic carbon is removed with nitrogen. An aliquot is resampled for analyses. Buffered persulfate is added and the sample is irradiated in the ultraviolet destructor for about 9 min. The hydroxylamine is added and the sample stream passes into the dialysis system. The carbon dioxide generated diffuses through the gas-permeable silicon membrane. A weakly buffered phenolphthalein indicator solution is used as the recipent stream, and the colour intensity of this solution decreases proportionately to the change in pH caused by the absorbed carbon dioxide... 

Dual lifetime referencing (DLR) is another powerful technique that enables referenced measurements in case of fluorescent indicators [23]. In this method, the analyte-dependent signal from an indicator is referenced against the signal from an inert luminophore. This can be realized in both the time domain [24] and in the frequency domain [25]. Often, a luminescent reference dye is embedded into gas blocking nanobeads to avoid oxygen quenching. Polymers with very low gas permeability such as poly(acrylonitrile) [24] or poly(vinylidene chloride-co-acry-lonitrile) [26] are the best choice here. 

In order to determine the viscous and inert through-plane gas permeabilities of diffusion layers at varied compression pressures, Gostick et al. [212] designed a simple method in which a circular specimen was sandwiched between two plates that have orifices in the middle, aligned with the location of the material. Pressurized air entered the upper plate, flowed through the DL, and exited the lower plate. The pressure drop between the inlet and the outlet was recorded for at least ten different flow rates for each sample. The inert and viscous permeabilities were then determined by fitting the Forchheimer equation to the pressure drop versus flow rate data as explained earlier. 

P. Koschany. Electrodes with adjustable gas permeability, and method of producing such electrodes. US Patent 2003138689 (2003). 

The detection of a test gas using mass spectrometers is far and away the most sensitive leak detection method and the one most widely used in industry. The MS leak detectors developed for this purpose make possible quantitative measurement of leak rates in a range extending aaoss many powers of ten (see Section 5.2) whereby the lower limit = 10 mbar l/s, thus making it possible to demonstrate the inherent gas permeability of solids where helium is used as the test gas. It is actually possible in principle to detect all gases using mass spectrometry. Of all the available options, the use of helium as a tracer gas has proved to be especially practical. The detection of helium using the mass spectrometer is absolutely ( ) unequivocal. Helium is chemically inert, non-explosive, non-toxic, is present in normal air in a concentration of only 5 ppm and is quite economical. Two types of mass spectrometer are used in commercially available MSLD s ... 

ASTM D1434, 1982 (2003). Standard test method for determining gas permeability characteristics of plastic film and sheeting. 

Most of the physical properties of the polymer (heat capacity, expansion coefficient, storage modulus, gas permeability, refractive index, etc.) undergo a discontinuous variation at the glass transition. The most frequently used methods to determine Tg are differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and dynamic mechanical thermal analysis (DMTA). But several other techniques may be also employed, such as the measurement of the complex dielectric permittivity as a function of temperature. The shape of variation of corresponding properties is shown in Fig. 4.1. 

J.Y. Park and D.R. Paul, Correlation and Prediction of Gas Permeability in Glassy Polymer Membrane Materials via a Modified Free Volume Based Group Contribution Method, J. Membr. Sci. 125, 29 (1997). 

In the mid-1950s, Leland Clark developed an electrochemical method for oxygen measurements in biological fluids and made the discovery that if the Pt electrode used for detection could be separated from the biological medium by a gas permeable... 

The PDMS channels can also be primed with C02 gas which is readily dissolved in aqueous solution, and so no bubbles will be formed after solution filling [160], It was reported that gas bubbles in PDMS channels, if formed, can be removed by blocking the output port, and pressurizing the channel via the input port. This method is successful because of high gas permeability of PDMS [249],... 

Park, J. Y., and Paul, D. R. (1997). Correlation and prediction of gas permeability in glassy polymer membrane materials via a modified free volume based group contribution method, J. Membrane Sci. 125, 23. 

The area is an important surface parameter for catalytic studies. It is needed to evaluate the rate constant of the surface reaction from the kinetics as well as to allow a fair comparison to be made of the effectiveness of different catalysts. Areas are commonly determined by nitrogen or krypton gas adsorption interpreted by the Brunauer-Emmett Teller (BET) isotherm [30, 32], A number of other methods has been proposed and utilised including microscopy, isotopic exchange, chromatography, gas permeability, adsorption from solution, and negative adsorption (desorption) of co-ions [30, 33]. 

The particle size and surface area distributions of pharmaceutical powders can be obtained by microcomputerized mercury porosimetry. Mercury porosimetry gives the volume of the pores of a powder, which is penetrated by mercury at each successive pressure the pore volume is converted into a pore size distribution. Two other methods, adsorption and air permeability, are also available that permit direct calculation of surface area. In the adsorption method, the amount of a gas or liquid solute that is adsorbed onto the sample of powder to form a monolayer is a direct function of the surface area of the sample. The air permeability method depends on the fact that the rate at which a gas or liquid permeates a bed of powder is related, among other factors, to the surface area exposed to the permeant. The determination of surface area is well described by the BET (Brunauer, Emmett, and Teller) equation. 

Bubble of size of the order of tens of micrometers floating on the surfactant surface only little deviates from the spherical shape. This fact has been used in the method of diminishing bubble [128,129] which allows to measure the contact angle of the black film, the linear tension of the contact line film/meniscus and the coefficient of the gas permeability through the film. Fig. 2.24 presents the scheme of this device. 

In all methods for determination of the coefficient of gas permeability through foam films the driving force of gas transfer is the gas pressures difference Ap, when the gas phase is the same on either side of the film. If the gases are different, then it is possible to have a gas transfer through a flat film when the pressure on either film side is the same. Such a method has been realised in [131] by forming a foam film in a cylindrical tube between two vessels... 

The dependence of the gas permeability coefficient on surfactant concentration in the bulk solution is studied by the method of diminishing bubble (see Chapter 2). Fig. 3.108 presents the K(C) dependence for NaDoS bubbles at four different temperatures. 

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