handbook of modern sensors

502 17 Chemical Sensors
Fig. 17.2. Direct devices.
Fig. 17.3. Complex devices.
current, or capacitance (Fig. 17.2). These devices require some sort of electrical signal
conditioning, but no transducing (converting the sensor phenomena from one form
of energy to another). Complex devices (Fig. 17.3) employ chemistry-influenced phenomena
that do not directly affect an electrical characteristic and will require some
form of transducing to obtain electrical signal in order to interface with common
measurement electronics. Nondirect phenomena include physical shape change, frequency
shifts, modulation of light, temperature or produced heat change, and even
mass change.
Some of the simplest chemical-sensor designs require that the sensing element
chemically react with the analyte to effect a measurable change in the indicator (phenomena)
or signal. This often adversely influences the device and introduces stability
problems. The chemically reactive devices suffer when there is incomplete reversibility,
when there is depletion or consumption of the sensor/analyte chemicals (electrochemical
cells use up electrolyte and some electrodes get consumed), or when there
is no species-specific reaction (including interference from other species).
Physical chemical sensors do not require a chemical reaction to take place, but
isolate and employ a physical reaction to indicate the presence of a chemical species.
These devices regularly demonstrate less drift and better stability than true chemically
reactive devices, but often at the cost of significant additional instrumentation and
slower reaction times.