Page 263 - Gas Adsorption Equilibria
P. 263

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
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