Biomedical Engineering Reference
In-Depth Information
PV
=
nZRT
(1.1)
where
P
is the pressure,
V
is the volume,
n
is the number of moles,
Z
is the gas compressibility factor,
R
is the universal gas constant
and
is the temperature. There are several volumetric-based
methods, but the most common implementation is the
T
manometric
method
, which is performed in a system of a fixed, known volume.
The principle, in this case, is the same as that commonly used in
commercial instruments for the determination of the Brunauer-
Emmett-Teller (BET) surface area of powders and porous solids
[3, 6], which is typically performed with nitrogen. However,
the required compatibility of the adsorption apparatus for use
with hydrogen, in particular, and the need for elevated pressure
measurements for storage and separation applications, in general,
means that there are significant practical differences between
systems used for the two types of measurement. Typical apparatus
of this kind consists of a stainless steel (ideally 316L) pressure
system with a sample cell contained in a thermostat and a calibrated
volume mounted in a temperature-controlled cabinet along with one
or more pressure measuring devices. All-metal seals are required
for hydrogen and for high-pressure operation, with the primary
requirement being a very low leak rate. Gas is delivered to, and
removed from, the calibrated volume in a controlled manner and
a valve separates the calibrated dosing volume and the sample cell
to allow the dosing of the gas to the sample. In hydrogen storage
studies, the manometric method or manometric apparatus is often
referred to as
, which is the
terminology commonly used in metal hydride research [7, 8].
The stepwise measurement method proceeds in the following
manner. As for the gravimetric method described above, the sample
is first degassed and the measurement then begins at vacuum. The
sample cell is sealed by closing the valve separating the cell and the
calibrated dosing volume. A chosen pressure of the adsorptive is
then applied to the dosing volume and the pressure recorded. The
gas is dosed to the sample by opening the separation valve, and the
pressure monitored. Once the pressure has reached equilibrium, the
amount of gas adsorbed by the sample in moles, Δ
Sieverts' Method
or
Sieverts' apparatus
, is determined
from the drop in pressure beyond that which is expected from the
ratio of the dosing volume to the total system volume, according to
the following expression,
n
