Oscillometric-manometric measurement

As the pressure measurements in oscillometric-manometric experiments only are needed to calculate the density of the sorptive gas = p (p, T)) in a sorption equilibrium state, it seems to be worthwhile to consider direct measurements of (p ), for example by using the buoyancy of a sinker at a magnetic suspension balance. Experiments of this type can be called oscillometric-densimetric measurements . They can be recommended to measure solubilities of gas mixtures in swelling (polymeric) sorbing materials. A scheme for such measurements is sketched in Figure 5.15. Equations to calculate the total mass of gas (m ) sorbed in the sorbent mass (m ) and the volume of the mass (m + m ) are the same as those given for oscillometric-manometric measurements in the next section (4.3). 

For swelling sorbents like polymers and resins, combined osciUometric-gravimetric or oscillometric-manometric measurements allow one to determine simultaneously the total mass (m + m ) and the volume (V ) of the sorbent / sorbate system. In this way, for example, a shift in the glass transition temperature of a polymer due to gas absorption can be observed. 

For rigid sorbent materials, combined oscillometric-gravimetric or oscillometric-manometric measurements in a saturation state of an adsorption system also allow one to determine both the total mass (m m ) and the Volume (V ) of the system without using the so-called helium volume hypothesis, cp. Fig. 5.8 and Chap. 1. 

V =Vsv + Vac volume of gas storage vessel (Vsv) and adsorption vessel(VAc) in oscillometric-manometric measurements. Fig. 5.13. 

These numerical values of dispersions (Ox) provided one can expect relative uncertainties of Gibbs excess masses measured with the rational pendulum to be limited by (a GE/mQg)<2%. This is about the accuracy of standardized volumetric / manometric measurements, cp. Chap. 2. To realize accurate oscillometric measurements it is most important to determine the (A) and ( ) parameters related to the various oscillations of the pendulum as accurate as possible, cp. Eq. (5.39). For this it is necessary to choose an initial amplitude (uo) of about 2-3 times the size of (ai, but less than 60 ° as for higher values of (Uo) measurable deviations of the oscillation from ideal harmonic behavior occur. Also it is necessary to observe at least 10 periods within an oscillation if not more, to get reliable (A, co)-data. To ensure this, the use of diodes with characteristic detection times less than 0.1 ms is recommended [5.7]. 

We also would like to draw reader s attention to the possibility to combine oscillometric, gravimetric, and manometric measurements in a single experiment. The schematic diagram of such an instrument is given in Figure 5.16. It basically consists of a rotational pendulum, a microbalance (two beams as indicated in Fig. 5.16 or single beam as sketched in Fig. 5.15) and a storage vessel for the sorptive gas. This instrument would allow cosorption... 

The mass of gas (m ) adsorbed or absorbed in a swelbng sorbent material of mass (m ) and volume (V ) in the sorbate state at given pressure (p) and temperature (T) of the sorpbve gas can be calculated from combined oscillometric and volumetric/manometric measurements. To derive the respective equations we consider again the result of oscillometric measurements, i. e. equation (5.25)... 

The solubility of carbon dioxide (Messer-Griesheim, 5.0) in polymethylmethacrylate (PMMA) Makrolon 2400 (Bayer AG), cp. Tab. 5.3 and [5.29], and the swelling of the polymer have been measured at T=308 K using the oscillometric-manometric method [5.27]. Results are presented in Figure 5.19. The reduced masses (Q =m -p V ) resulting from manometric measurements, cp. Eq. (5.74), are indicated by ( ). As clearly can be seen, these data increase at low pressures (p < 3.5 MPa) to have a maximum at p = 3.5 MPa, then to decrease and even assume negative values... 

Figure 5.16. Scheme of an instrument for manometric-gravimetric-oscillometric measurements of binary coadsorption equilibria in swelling materials (polymers, resins etc.) without sorptive gas analysis by GC or MS. 

Combining the dielectric measurements with either manometric, gravimetric or oscillometric measurements of gas adsorption equilibria states, one gets calibration curves allowing one the determination of Gibbs excess adsorbed masses by purely electric measurements which normally can be performed fairly quickly and on site in industrial situations. 

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