Biomedical Engineering Reference
In-Depth Information
calibration or conformance measurements can be performed with
a standard sample to check the accuracy of the sample temperature
measurement. An example of such a measurement is the use of
thermomagnetometry
[25], in which a ferromagnetic sample and
a permanent magnet are used for calibration purposes; if the
sample is mounted on the balance in the vicinity of the magnet and
the temperature is changed, a response will be seen in the
microbalance reading when the temperature exceeds the Curie point
of the sample.
In volumetric measurement, the calibrated dosing volume
temperature should be maintained constant, typically just above
ambient. The isothermal conditions of the main system are
assumed for the calculation of the adsorbed quantity. Therefore,
the accuracy with which the main system temperature is both
known and maintained contributes significantly to the accuracy
of the measurement. The system should be resistant to ambient
temperature fluctuations; if this is not the case, then accuracy may
be compromised, unless the ambient laboratory temperature is
particularly stable. For high-pressure carbon dioxide adsorption
measurement, it is common to perform measurements in a system
that is held entirely at the temperature of the measurement [26].
Temperature measurement inaccuracy in this case can have an
important effect on the calculated compressibility due to the
strong temperature (and pressure) dependence of the pressure-
density relationship for CO
(see Section 1.5.9). Van Hemert
et al.
2
[26] emphasize the significant effect that even a 0.1 K error in the
temperature can have on the calculated gas density, because a 0.1 K
and 10 kPa change at 10 MPa and 318 K can alter the gas density by
2%. According to the authors, this temperature measurement error
could lead to an error of up to 70% in the measurements performed
on the activated carbon material used in the study (Filtrasorb 400).
High-accuracy temperature measurement is therefore clearly crucial
for the performance of a high-accuracy sorption measurement.
1..
Pressure Measurement
Pressure measurement is important in both the gravimetric and
volumetric techniques. In the gravimetric case, it affects the accuracy
of the equilibrium pressure at each isotherm point, but for volumetric
measurement it directly affects the calculation of the adsorbed
