The other set-up consists of the introduction of the transducer
inside the reactor. This solution, proposed by Riera et al. (2002),
is based on a piezoelectric sandwich transducer designed and built
for this application and inserted in the upper part of the vessel
with a 100W power capacity. The transducer is driven by an electronic
generator, which incorporates a system to follow the resonance
frequency (Fig. 6). This is an essential device due to the
changes that different process conditions may provoke in the characteristic
impedance of the supercritical fluid (Riera et al., 2004). In
fact, these authors achieved stable operation conditions when the
values of density, pressure and temperature were kept at the operational
values. Moreover, any change in the flow rate or density
was immediately detected and followed by the control system of
the transducer. Therefore, it is possible to use this control system
to monitor the extraction process.