Page 289 - Gas Adsorption Equilibria

5. Oscillometry 275

5. PROS AND CONS OF OSCILLOMETRY

In this section some of the advantages and disadvantages of sorption
measurements by using slow rotational oscillations of a pendulum coming
from our experience are described in brief.

5.1 Advantages

Oscillometry is based on the inertia of masses observed and neither on
their extension as in manometry nor on its weight as in gravimetry. Hence,
from a physical point of view oscillometry delivers basically new information
on the masses observed and can be operated even under zero gravity
conditions.

1. Versatility
For swelling sorbents like polymers and resins, combined oscillometric-
gravimetric or oscillometric-manometric measurements allow one to
determine simultaneously the total mass and the volume 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.

2. Measuring techniques
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
and the Volume of the system without using the so-called
helium volume hypothesis, cp. Fig. 5.8 and Chap. 1.

By choosing proper materials for the pendulum, for example titanium for
the disk, gold alloy for the torsional wire etc., oscillometric measurements
can be performed in a fairly broad range of temperature and pressure and
even in corrosive gas atmospheres where other techniques may fail or
simply become inapplicable.

5.2 Disadvantages

1. Availability
Torsional pendulums designed for adsorption measurements of gases on
solids are not available commercially today. Also principles for optimal
design aiming at high sensitivity and reproducibility of data have not yet
been formulated though hints for this are given in [5.2, 5.7].