Page 290 - Gas Adsorption Equilibria

276 Chapter 5

2. Calibration
As the mechanical properties of the torsional wire (retarding moment,
internal damping, length) depend on temperature, calibration of the
pendulum is necessary for all temperatures at which measurements should
be performed.

3. Pressure limitations
The pendulum should not be used for adsorption measurements at low
pressures as then not only the sorptive gas density but also
the kinematic viscosity and hence the Reynolds-number may
increase considerably This indicates that for turbulent
secondary flows may occur leading to non reproducible oscillations of the
pendulum. The low pressure limit of the pendulum strongly depends on its
geometry (thick disk criterion (5.29)). It can be shifted to lower values by
increasing the height of the ring slit on the disk, i. e. using slim hollow
cylinders filled with pellets as sorbent material.

4. Duration of measurements
Measurements of gas solubilities in continuous materials like soft matter or
highly viscous liquid polymers filled in the ring slit of the pendulum are
normally very slow and may last for days and even weeks. Hence, it is
often not easy to decide whether thermodynamic equilibrium really has
been attained. Accompanying gravimetric and / or dielectric impedance
measurements can be very helpful to decide this question,
cp. Chapts. 3, 6.

5. Kinetics
The kinetics of a sorption process of gas in a sorbent material is hardly
observable by monitoring rotational oscillations. At least our experiments
aiming at this problem have been unsuccessful [5.7].