Biomedical Engineering Reference
In-Depth Information
of any phenomena that can lead to inaccuracy and also to reduce the
repetition of avoidable methodological mistakes, such as insufficient
consideration of the importance of gas purity, temperature stability
and gas leakage during sorption property characterization.
The objective comparison of the relative importance of different
error sources in each measurement technique and their potential
interrelation is complicated, because so many factors play a role
in any particular measurement. Although a brief summary of the
issues discussed in this chapter is susceptible to over-simplification,
a few broad statements based on the discussion presented here
can be made. Firstly, a high-purity gas supply is a prerequisite to
accurate characterization using all techniques. The dead volume
and buoyancy effect corrections are equivalent and the significance
of both increase with decreasing sample density and increasing gas
pressure. Both corrections are affected by our knowledge of the
sample volume or the definition of the location of the Gibbs dividing
surface. The accuracy of pressure and temperature measurement
in volumetric instrumentation inherently limits the accuracy
of a volumetric gas adsorption measurement. Any volume in a
volumetric instrument, which is at an uncontrolled temperature,
also adds to the measurement error. In volumetric measurement,
errors accumulate, which is not the case for gravimetry. Finally,
any additional uncertainty in the chemical and microstructural
composition of a sample will create an inherent uncertainty in
the characterization of the gas sorption properties of the material
that the sample is supposed to represent. Any further research
that improves our understanding of these different factors will be
valuable.
Before concluding the chapter it is worth noting the role
that Round Robin (
) exercises can play in the
investigation of measurement accuracy. Such studies can help
validate experimental methodology and aid the investigation of
the effects of different error sources on measurement accuracy.
Due to the importance of both adsorption measurement and gas
adsorption-based technology, interlaboratory test exercises have
been performed recently for high-pressure hydrogen [58] and high-
pressure carbon dioxide adsorption [48, 49, 51] by carbonaceous
materials, as well as low-pressure nitrogen and carbon dioxide
adsorption for the purpose of surface area and pore volume
determination [59]. Generally speaking, the discrepancies observed
have been large enough to warrant further investigation, particularly
interlaboratory test
