Biomedical Engineering Reference
In-Depth Information
Isotherm measurements at the elevated pressures required
for gas storage and separation applications can be subject to a
number of errors that are relatively insignificant for typical gas
adsorption measurements up to ambient pressure. These include
those due to the accurate representation of the gas compressibility
and our knowledge of the sample volume or density. Gas sorption
measurements can also be susceptible to impurities in the gas
supply and reaction with contaminants contained on or within the
sample; both of these aspects can affect TPD measurements as well
as volumetric and gravimetric isotherm determination.
Prior to discussing the experimental techniques and methods,
and some of the possible error sources that can affect gas sorption
measurements, we will look briefly at the definition of some terms
relevant to this chapter. Firstly,
is defined in
the most recent international vocabulary of metrology [1] as being
the “closeness of agreement between a measured quantity value
and a true quantity value of a measurand” and is qualitative, not
quantitative. A measurement is considered more accurate when
it offers a smaller
measurement accuracy
. The measurement error is
the “measured quantity value minus a reference quantity value”,
where the latter is a value used for the basis of a comparison. This
can be a “true” value or an agreed value (
measurement error
conventional quantity
value
). For our purpose, the measurement accuracy defines the
qualitative agreement between the measured uptake of a material
at a particular temperature and gas pressure and the real uptake of
a material under the same conditions, and the measurement error
expresses the quantitative difference between a particular measured
uptake and the real uptake of the material. The real uptake itself will
have an intrinsic uncertainty, termed the
,
which will be dependent partially on the properties of a particular
material and will limit the
definitional uncertainty
, or the closeness
of agreement between repeated measurements on the same
material under specified conditions. The latter part of this chapter
is concerned with factors that can contribute to measurement
error in the determination of the gas sorption properties of carbon
nanomaterials and can, therefore, lead to a lower accuracy assessment
of their potential storage capacity. In addition, for gas adsorption,
the nonadsorbed gas is commonly known as the
measurement precision
adsorptive
, the
adsorbed gas is known as the
adsorbate
, and the solid onto which
adsorption occurs is known as the
adsorbent
[2, 3].
