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the sensing medium (like a well) or a reservoir in the sensor package, which can
dry out. Ho et al.
Cons: May not be able to discriminate among unknown mixtures of chemicals. Some
polymers react strongly to water vapor. Uncertain durability of polymers in subsurface
environments; need to develop robust packaging. May need pre-concentrator to detect
very low limits (for regulatory standards).Although reversible, signal may experience
hysteresis and a shift in the baseline when exposed to chemicals.
18.3.1.2 Catalytic bead sensors
Catalytic bead sensors are low-power devices (50–300 mW) that have been used
for many years in the detection of combustible gases, particularly methane in air
(U.S. EPA 1995). They are used widely in portable gas detection instruments. The
catalytic bead sensor is comprised of a passive and active element, both made from an
embedded coiled platinum wire in a porous ceramic (Figure 18.1). The active element
is coated with a catalyst such as platinum, and the passive element is coated with an
inert glass to act as a reference element to compensate for environmental conditions.
Both elements are heated to a prescribed operating temperature ranging from 300 to
◦
800 C. When a combustible gas such as methane contacts the elements, the vapor
combusts on the active element and the active element increases in temperature. As
a result, the resistance of the platinum coil changes. The two elements are connected
to a wheatstone bridge circuit, so the changes in resistance are measured as a change
in voltage.
Portable soil vapor monitors have been commercialized that use the catalytic bead
sensor for detecting combustible hydrocarbon gases. These devices may have the
ability to eliminate methane from the readings. Methane occurs naturally in the
subsurface, so it can provide false readings. Hand-held devices based on catalytic
bead sensors can provide discrete readings of real-time gas concentrations. Sampling
occurs through a probe, and an internal pump draws the sample.
Pros: Commercial units are portable and can allow the operator to distinguish between
methane and other volatile hydrocarbon vapors.

Ceramic
coating
Platinum Deactivator
catalyst

Platinum alloy
wire

Active Reference
Common

Figure 18.1. Diagram of catalytic bead sensor