Page 273 - Gas Adsorption Equilibria

5. Oscillometry 259

Combined oscillometric and gravimetric measurements provide a basis to
determine simultaneously the mass and the volume of a swelling sorbent
material like polymers or resins in a sorptive gas atmosphere. However
measurements seem in practice to be restricted to determinations of sorption
equilibria of these materials as the kinetics of mass uptake often is very slow.
To give an example we mention that in case of sorption of on pellets of
Makrolon 2400 at 35 °C, p = 2 MPa, it took more than 4 days till equilibrium
was reached.

Figure 5.11.
Rotational Pendulum for com-
bined oscillometric-gravimetric
sorption measurements of gases in
swelling (polymeric) materials.
© IFT University of Siegen, 1999.

According to our experience, data of oscillometric measurements were
highly reproducible, but were more sensitive to changes in temperature than
gravimetric measurements. Relative uncertainties in oscillometry are
comparable to those in gravimetry. The pendulum can be used in a fairly large
range of pressure and temperature comparable to that of commercial
thermogravimetric instruments available today. For proper design the
condition that the disk of the pendulum must be “thick” to make sure
inequalities (5.29) hold, should be observed. Otherwise, the pendulum cannot
be used for low gas pressures, i. e. p < 1 MPa for our instrument [5.7]. Choice