Biomedical Engineering Reference
In-Depth Information
for hydrogen. The version of the mBWR EOS used was only valid at
273 K and above, but in this region it showed higher precision than
the SRK EOS. According to the authors, however, it did not yield more
“correct” isotherms. Kiyobayashi
[43] discussed the choice of
EOS in their work on the measurement of hydrogen sorption by a
number of carbonaceous materials. They compared results from
their volumetric apparatus calculated with the ideal gas law and a
32-term mBWR EOS, as implemented by the NIST database at the
time, showing again the significance of the accuracy of the hydrogen
compressibility description. Zhou
et al.
[44], meanwhile, compared
the use of the ideal gas law, the van der Waals equation and another
mBWR EOS, concluding that the latter describes the real gas
behavior of hydrogen with sufficient accuracy, but the use of the
van der Waals equation can lead to significant errors, particularly at
lower
et al.
temperatures.
These
studies,
however,
pre-date
the
development of the Leachman
[45] EOS for hydrogen, as
currently implemented by the NIST database, and this EOS appears
to represent the current state of the art.
With regard to other adsorptives, Zhou
et al.
[44] also chose
the mBWR EOS for methane, while the current NIST database uses
the multi-parameter EOS by Setzmann and Wagner [46]. For carbon
dioxide, the Span and Wagner [47] EOS has been used widely in
recent high-pressure carbon dioxide adsorption studies [26, 48-51]
and is also implemented by the NIST database. The equations
of state used by the NIST database for many other adsorptives,
including argon, oxygen and nitrogen, were tabulated by Span
et al.
et al.
[40] in 2001, although some of the equations for other species have
been developed in the intervening period. The problems associated
with the accuracy of the chosen EOS will generally increase with
the strength of the dependence of the compressibility on
temperature and pressure. As a consequence, carbon dioxide
sorption measurements in the near-ambient, near-critical regime
are particularly susceptible to such errors.
1.5.10 Buoyancy Effect Corrections
Corrections are necessary in gravimetric measurement to account
for the presence of the sample in a gas of differing density at different
pressures and temperatures. The so-called
manifests
itself as an upthrust on the balance and is dependent on the volume
buoyancy effect
