Biomedical Engineering Reference
In-Depth Information
in the pressure measurement will lead to an error in the calculated
number of moles adsorbed at any given isotherm point, which will
contribute to the total accumulated error (see Section 1.5.12). It
will also affect the measured equilibrium pressure for a particular
isotherm point, and this will also be the case for the gravimetric
technique. In addition to the calibration of the temperature and
pressure measurement systems, gravimetric instruments also
require calibration of the balance, which can be performed using
traceable mass standards. Volumetric instruments, meanwhile,
require the calibration of the internal volumes of the system,
which includes the measurement or determination of the dead
volume of the sample cell (see Section 1.5.11). Volume calibration
can be performed using helium and traceable volume standards, if
required.
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1..
Temperature Measurement and Control
Accurate temperature measurement is clearly important in all
techniques. Firstly, in isotherm determination, the assumption of
isothermal conditions requires accurate monitoring of the sample
temperature. Secondly, in volumetric measurement, the assumption
of an isothermal sample and dosing volume environment is
implicit in the calculation of the adsorbed quantity; in gravimetric
measurement, the equivalent concern is the thermal stability of the
microbalance chamber because thermal fluctuations will result in a
potentially unstable weight reading. This could lead to an erroneous
measurement, even if the microbalance is well-calibrated and
accurate. Thirdly, for thermal desorption experiments, a constant
temperature ramp and confirmation that the sample temperature
closely follows the applied ramp is important to ensure high-quality
data.
With regard to sample temperature measurement, in volumetric
apparatus the temperature sensor can be in direct contact with
the sample. This is favorable to the gravimetric case in which the
sensor can only be in the vicinity of the sample because it will
otherwise interfere with the weight measurement. In gravimetry,
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A series of measurements can be performed both with and without the calibrated
volume displacers included in the system and the resultant set of simultaneous
equations solved to give both the dosing and empty sample cell volumes, assuming
no adsorption occurs on the volume displacer.
