Page 249 - Gas Adsorption Equilibria

Chapter 5

OSCILLOMETRY

Abstract The physical and experimental technique of gas adsorption measurements by slow
oscillations of a rotational pendulum or, likewise, the relaxational motion of a
freely floating rotator are described. Combinations of the pendulum with either
gravimetric or volumetric measurements are outlined. These especially are suited to
measure the absorption or solubility of gases in non-rigid or swelling sorbent
materials like polymers. Pros and cons of these methods are discussed in brief.
List of symbols. References.

1. INTRODUCTION

The inertia of mass against acceleration provides another possibility to
measure it, i. e. to compare it with a standardized sample mass. For practical
measurements periodic motions, i. e. oscillations at high or low frequencies in
*)
linear or circular modes are used, cp. Ref. [3.1] . For gas adsorption
measurements it always should be taken into account that

changes of mass due to adsorption are normally small, and
the sorbent / sorbate is surrounded by a sorptive gas atmosphere
causing additional damping to the sample’s motion depending on
the viscosity of the gas.

For these reasons we will be here restricted to consider only the slow
rotational oscillations of sorbent masses in a sorptive gas atmosphere such
that the masses moved geometrically are transformed into themselves and
only initiate flows in the surrounding gas which are rotationally symmetric.

* )
Today this method is used in practice to determine the mass of astronauts in zero gravity
(g = 0) conditions like in “Spacelab”. The astronaut is put on a sledge connected to a linear
spring of known constant (k) and the frequency or the time period of
oscillations (T) is measured. Then the astronaut’s mass (m) can be calculated as
this quantity being an important indicator of his health status [5.9].