Biomedical Engineering Reference
In-Depth Information
analytes by the coating materials. Thereafter the mass change on the sensor surface
is converted to the frequency change.
Advantages:
• For large dynamic range its linear.
• High sensitivity and fast response time.
• Can be implemented in a variety of measurement locations and various situa-
tions as its stabile long term.
• It can be used as a single sensor or as a group of sensors (sensor array).
• Detection of trace gases is special application.
• Low power utilization.
• Miniature size.
• Robustness.
Disadvantages:
• Temperature and humidity dependence.
• The poor reproducibility in the deposition of the coating material, i.e., complex
fabrication process.
• Need of a thin crystal to increase the sensitivity that prevents their inclusion in
multisensory arrays.
• Poor signal-to-noise performance because of the size of crystal thickness.
Application:
• Used to detect gases and volatiles such as NO
2
,NH
3,
xylene, toluene, tetra-
chloroethylene, H
2
, and toxic gases.
• Organophosphorous compounds, SO
2.
• To analyze the headspace of foodstuffs as olive oil samples.
6.11 SAW
The sensors having their design based on surface acoustic wave (SAW) platform
are perhaps the most sensitive, miniature of E-nose gravimetric type of sensors.
Oscillator circuits whose resonance frequencies are controlled by resonator devi-
ces in the feedback path are defined as SAW sensors. Chemical vapor sorption by
depositing broadly selective polymer thin films in the acoustic wave propagation
region is the functional principle of the SAW devices. An oscillator frequency is
shifting due to exposure to vapor and it is taken to the chemical signal [
21
]. The
schematic of SAW sensor is shown in Fig.
6.22
.
SAW sensors require waves to travel over the surface of the device. SAW
sensors operate at higher frequencies (100-1,000 MHz) and thus generate a larger
change in frequency. SAW sensors also suffer from reduced long-term stability
and are highly sensitive to humidity. Main characteristics of SAW sensors are its
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