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5. Oscillometry 249
practice we experienced that for as sorptive gas at T = 300 K
for pressures MPa oscillations become irreproducible which
probably is mainly due to turbulent secondary flows initiated by the
motion of the pendulum. To avoid theses, it is recommended to
place thin plates above and below the pendulum, cp. Fig. 5.4.
2. The analytic method leading to Eq. (5.38) in principle also allows
one to determine the kinematic viscosity of the sorptive
fluid [5.1, 5.2]. This can be of interest if gas mixtures are used for
which viscosity data often are scarce.
3. To avoid certain difficulties with the rational pendulum, cp. Sect.
4.2, it should be mentioned that on principle the pendulum can be
substituted by a floating rotator. By this we understand a cylinder
rotating freely, i. e. floating in either vacuum or a gaseous
atmosphere within another hollow cylinder and bearing on top a
permanent magnet coupled to a magnetic suspension, cp. Chaps. 3,
4, and at its bottom a bowl filled with sorbent material [5.4],
Fig. 5.5.
An instrument of this type has been designed a couple of years ago for
gas viscosity measurements [5.5, 5.6]. After initializing rotator’s motion
electromagnetically, a rotational relaxation motion of the rotator represented
by a sequence of time intervals n = 1, 2, 3... needed for n = 1, 2, 3...
rotations can be observed.
This motion can be represented by its angular velocity which is
for vacuum
for a gaseous atmosphere
with being characteristic relaxation times to be determined from the
respective sets via a Gaussian minimization procedure. The parameters
depend – among other quantities – on the moment of inertia of the rotator
and hence of its (cylindrically symmetric distributed) mass. By analogous
reasoning as for the pendulum, one can present the mass ratio