Page 314 - gas transport in porous media

Chapter 18: Measurement of Vapor Concentrations
are not very durable and have short shelf lives. Subject to interfering gases such as
hydrogen. 313
18.4 MASS SENSORS

18.4.1 Surface Acoustic Wave Sensors/
Portable Acoustic Wave Sensors

Surface Acoustic Wave Sensors (SAWS) are small miniature sensors used to detect
VOCs. A SAW device consists of an input transducer, a chemical adsorbant film, and
an output transducer on a peizoelectric substrate (see Figure 18.2). The piezoelectric
substrate is typically quartz. The input transducer launches an acoustic wave which
travels through the chemical film and is detected by the output transducer. The device
runs at a very high frequency 100MHz. The velocity and attenuation of the signal are
sensitive to the viscoelasticity as well as the mass of the thin film which can allow
for the identification of the contaminant. Heating elements under the chemical film
can also be used to desorb chemicals from the device. A signal pattern recognition
system that uses a clustering technique is needed to identify various chemicals.
SAWS have been able to distinguish organophosphates, chlorinated hydrocarbons,
ketones, alcohols, aromatic hydrocarbons, saturated hydrocarbons, and water.
Pros: Small, low power, no moving parts other than the high-frequency
excitation, good sensitivity to various chemicals. Can detect chemicals in very low
concentrations.
Cons: May not be able to discriminate among unknown mixtures of chemicals.
Some polymers react strongly to water vapor; uncertain durability in subsurface
environments. Requires high-frequency excitation.

Chemically
adsorbent
film
Input
transducer Output
transducer
Acoustic wave

Heater and
temperature sensor

Piezoelectric quartz substrate

Figure 18.2. Schematic of SAW device