handbook of modern sensors

514 17 Chemical Sensors
17.5.2 Pellister Catalytic Sensors
These sensors operate on the principle similar to thermal enzymatic sensors. Heat is
liberated as a result of a catalytic reaction taking place at the surface of the sensor
and the related temperature change inside the device is measured. On the other hand,
the chemistry is similar to that of high-temperature conductometric oxide sensors.
Catalytic gas sensors have been designed specifically to detect a low concentration
of flammable gases in ambient air inside mines. These sensors often are called pellistors
[8]. The platinum coil is imbedded in a pellet of ThO 2 /Al 2 O 3 coated with a
porous catalytic metal: palladium or platinum. The coil acts as both the heater and the
resistive temperature detector (RTD). Naturally, any other type of heating element
and temperature sensor can be successfully employed. When the combustible gas
reacts at the catalytic surface, the heat evolved from the reaction increases the temperature
of the pellet and of the platinum coil, thus increasing its resistance. There are
two possible operating modes of the sensor. One is isothermal, where an electronic
circuit controls the current through the coil to maintain its temperature constant. In
the nonisothermal mode, the sensor is connected as a part of a Wheatstone bridge
whose output voltage is a measure of the gas concentration.
17.5.3 Optical Chemical Sensors
Optical sensors are based on the interaction of electromagnetic radiation with matter,
which results in altering (modulating) some properties of the radiation. Examples
of such modulations are variations in intensity, polarization, and velocity of light in
the medium. The presence of different chemicals in the analyte affects which wavelengths
of light are modulated. Optical modulation is studied by spectroscopy, which
provides information on various microscopic structures from atoms to the dynamics
in polymers. In a general arrangement, the monochromatic radiation passes through
a sample (which may be gas, liquid, or solid), and its properties are examined at the
output. Alternatively, the sample may respond with a secondary radiation (induced
luminescence), which is also measured.
Chemiluminescence devices (reaction produces measurable light) phosphoresce
when light hits them and that emission of light is an indication of chemical species
presence. Nondispersive infrared (NDIR) absorbance involves the absorption of specific
wavelengths of light and, when tuned through experimental methods, can be
used for single-analyte target gases such as CO 2 . Spectroscopic absorption optical
sensors are useful for UV and IR wavelengths and can be used to target O 3 detection
by producing a more complex absorbance signature versus a simple attenuation. In all
strategies, the wavelength of the light source is routinely matched to the reactive energy
of the optrode indicator to achieve a best possible electronic signal. The detection
of the original and resultant light is obtained with a photodiode or photomultiplier tube.
Optical chemical sensors can be and are designed and built in a great variety of
ways, which are limited only by the designer’s imagination. Here, we will describe
only one device just to illustrate how an optical sensor works. Figure 17.12 shows a
simplified configuration of a CO 2 sensor [18]. It consists of two chambers which are
illuminated by a common LED. Each chamber has metallized surfaces for better internal
reflectivity. The left chamber has slots covered with a gas-permeable membrane.