Biomedical Engineering Reference
In-Depth Information
quantity, as mentioned in Section 1.5.1. Although regular calibration
is important, the accuracy of the chosen pressure transducer is
a significant factor in the overall accuracy of a volumetric system.
Perhaps rather obviously,higher accuracy pressure transducers
generally require higher expenditure, and so this is often a question
of the financial constraints on the development of the instrument.
The accuracy of measurements in different pressure regimes can,
however, be improved by using different pressure measuring
devices in the high and low ranges; so, for example, a strain gauge
is used for relatively high pressures and a capacitance manometer
for low pressures. When selecting a pressure measuring device for a
volumetric instrument, one must ensure that the uncertainty in the
pressure measurement is not significant compared to the anticipated
reduction in pressure upon adsorption, or vice versa, for the chosen
sample size. Blach and Gray [7] suggest a figure of merit for hydrogen
sorption measurement,
, for a manometric instrument, which
incorporates the useable resolution of the transducer,
η
δP
,
s
P
k
(1.9)
where
is a measure of the sensitivity of the system to changes in
the hydrogen content of the sample. According to Blach and Gray [7],
η
s
k
≥ 100 allows high quality data to be obtained from a manometric
instrument. Van Hemert
[26] also emphasize the importance
of high-accuracy pressure measurement, as shown in the example
given in the previous section, for high-pressure carbon dioxide
adsorption.
et al.
1..
Sample Size Considerations
Sample size can potentially affect the accuracy of a measurement
in a number of ways. A sample that exhibits a particular amount
of adsorption may, for example, simply be too small for the lower
detection limit of the instrument or technique. In gravimetric
measurement, this will be dependent, to a certain extent, on the
accuracy and sensitivity of the balance. This is particularly relevant
to hydrogen due to its low molar mass. In volumetric measurement,
it will depend primarily on the system volume and the accuracy of
the pressure measuring devices (see Section 1.5.3). However, the
accuracy with which the degassed sample mass can be determined is
