Biomedical Engineering Reference
In-Depth Information
1..11
Dead Volume Corrections
As for the buoyancy effect corrections in gravimetric measurement,
it is also necessary in volumetric measurement to account for the
presence of the sample in the sample cell. For the calculation of
the adsorbed quantity, the volume used for the sample cell must
be the volume that is unoccupied by the sample (including the
accessible pore volume). This so-called
is typically
determined in one of two ways, which were termed the
dead volume
direct route
and the
[3]. In the first method, the
dead volume of the sample cell is determined directly using an inert
gas, which is typically helium as discussed previously. In the indirect
route, the volume of the sample is subtracted from the known volume
of the empty sample cell. In the latter case, the sample volume can
be calculated from the mass of the sample used for the measurement
and an independent determination of the sample density, either
experimental or theoretical. In both cases, the source of the error
is the uncertainty in our knowledge of the sample volume. The
direct determination of the dead volume is essentially a (helium)
pycnometry measurement, which is the common method used for
the experimental determination of sample density. The use of the
theoretical density will almost certainly introduce a significant error
because the theoretical density is unlikely to equal the real sample
density (see Section 1.5.6). Furthermore, for carbon nanomaterials,
we cannot calculate a theoretical density from the crystallography of
their structure, at least not in the same way as we can for periodic
crystalline microporous materials. In addition, in many materials
developed today, there is more than one phase present. This can
include, for example, catalytic dopants, which are likely to alter the
density compared to a pure carbonaceous sample, or remnants from
the synthesis or sample purification process. It is therefore important
to ensure that such density variations do not contribute significantly
to perceived differences in the measured gas sorption behavior due
to their effect on the dead volume corrections.
indirectroute
by Rouquerol
et al.
1..1
Accumulative Errors
It can be seen from Eq. (1.3) that the calculation of the adsorbed
quantity using the manometric method is an iterative procedure.
Error accumulation throughout the course of a measurement
