Biomedical Engineering Reference
In-Depth Information
involving more than one isotherm point is therefore inherent to
the method. Any uncertainties introduced to the determination
of the adsorbed quantity for each aliquot will contribute to the
accumulative error, including those due to pressure or temperature
measurement, real gas behavior, leaks, calibration of the internal
volume of the measurement system and the location of the Gibbs
dividing surface. This is in contrast to the gravimetric technique in
whicheach isotherm point, or the gas uptake at a particular pressure,
is referenced directly to the sample mass reading at vacuum [24].
It is therefore important to consider this in the development of
volumetric measurement procedures because an unnecessarily large
number of isotherm points or gas doses will lead to an unnecessarily
large accumulative error.
1..1
Leaks
Leakage can be a practical problem in volumetric instrumentation,
in particular. Although it is best avoided in gravimetric apparatus, in
volumetric measurement, leakage will produce a false measurement
of adsorption or desorption because the accompanying pressure
change could be misinterpreted as either process, depending on
the experimental configuration and the direction of leakage. Leaks
should, however, be distinguishable from a sorption process because
the kinetic profile for either adsorption or desorption will typically
show curvature, leading to an equilibrium pressure, whereas
leakage will be approximately linear and continuous. Leaks have
been identified as a practical problem in volumetric measurement
by a number of authors [11, 20, 26, 43, 52]. Kiyobayashi
[43]
measured the hydrogen leak rate of their system over a 10-20
h period to use as a reference for comparison with the pressure
reduction during adsorption. Van Hemert
et al.
[26], meanwhile,
account for leakage in the calculation of the adsorbed quantity; in
this case, by using the helium leak rate of the system measured
prior to every carbon dioxide adsorption measurement. If
significant leakage is suspected, helium pressure hold tests and
the performance of pressure versus uptake measurements with an
empty cell, which should yield an approximately zero uptake as a
function of pressure in a leak-free system, should help determine
the likely occurrence of leakage.
et al.
