handbook of modern sensors

17.3 Classification of Chemical-Sensing Mechanisms 501
platinum metal results in a change in the resistance of the wire that introduces offset
error into the sensor reading, and it leads to early burnout of the heating platinum
coil.
Chemical poisoning can affect many sensors like the catalytic bead devices where
silicone and ethyl lead bind to the sensing element, inhibiting the oxidation of the
hydrocarbon species and producing an inaccurate, false low reading. Filters are commonly
used with any chemical sensor if it is to be subjected to an environment containing
a characteristic poison. Judicious selection of the filter material is required
to eliminate only the poisoning agent without an associated reduction in the target
analyte (the chemical species being exposed to the sensor).
Surface-acoustical-wave (SAW) devices that use species-selective adsorptive
films can be poisoned mechanically by species that adsorb but which do not desorb
returning the mass of the device back to its original (calibrated) state. Similarly,
gas-selective coatings on fiber-optic devices also may be poisoned by nonremovable
species, permanently reducing the optical reflectance and indicating a false positive.
Another problem unique to chemical sensors is the significant chemical reaction
changes that occur throughout the concentration levels. Reactive hydrocarbon devices
(metal-oxide devices, voltammetric devices, etc.) require mixtures near stoichiometric
(balanced chemical reactions) so that required minimal levels of both target analyte
hydrocarbons and needed oxygen are available to feed the measurement reaction. If
the hydrocarbon levels are too high (or better stated as the accompanying oxygen
levels are too low), then only a fraction of the hydrocarbons will react producing a
false-negative reading again.
17.3 Classification of Chemical-Sensing Mechanisms
All chemical sensing can be classified by the actual indicator phenomena employed
for sensing, and they also can be classified by the measurement strategy employed.
We will separate chemical sensors into two major groups, direct (simple) and indirect
(complex), and will also distinguish between chemically reactive devices and
physically reactive devices in each group (Fig. 17.1).
Direct chemical sensors utilize any of a variety of chemical reaction phenomena
that directly affect a measurable electrical characteristic such as resistance, potential,
Fig. 17.1. Direct versus complex and chemically versus
physically reactive groupings.