Catalytic bead sensors are used primarily to detect combustible gases.
They have been in use for more than 50 years. Initially, Combustible Gas Sensors were
used for monitoring gas in coal mines, where they replaced canaries that had
been used for a long period of time.
The sensor itself is quite simple in design and is easy to manufacture. In
its simplest form, as used in the original design, it was comprised of a single
platinum wire. Catalytic bead sensors were produced all over the world by a
large number of different manufacturers, but the performance and reliability of
these sensors varied widely among these various manufacturers.
Principle of Operation
Combustible gas mixtures will not burn until they reach an ignition temperature.
However, in the presence of certain chemical media, the gas will start to burn
or ignite at lower temperatures. This phenomenon is known as a catalytic
combustion.Most metaloxides and their compounds have these catalytic
properties. For instance, volcanic rock, which is comprised of various metal
oxides, is often placed in gas burning fireplaces.
This is not only decorative, but it also helps the combustion process and
results in cleaner and more efficient burning in the fireplace. Platinum,palladium,
and thoria compounds are also excellent catalysts for combustion. This explains
why the auto-mobile exhaust system is treated with platinum compounds and is
called a catalytic converter. This kind of gas sensor is made on the basis of
the catalytic principle, and therefore is called the
catalytic combustible gas sensor.
A gas molecule oxidizes on the catalyzed surface of the sensor at a much
lower temperature than its normal ignition temperature. All electrically
conductive materials change their conductivity as temperature changes. This is
called the coefficient of temperature resistance(Ct). It is expressed as the
percentage of change per degree change in temperature.
Platinum has a large Ct in comparison to other metals. In addition, its Ct
is linear between 500°C to 1000°C, which is the temperature range at which the
sensor needs to operate. Because the signal from the sensor is linear, this
means that the concentration of gas readings are in direct proportion to the
electrical signal. This improves the accuracy and simplifies the electronic
circuitry. Also, platinum possesses excellent mechanical properties. It is
physically strong and can be transformed into a fine wire which can be
processed into small sensor beads.
Furthermore, platinum has excellent chemical properties. It is corrosion
resistant and can be operated at elevated temperatures for a long period of
time without changing its physical properties. It is capable of producing a
constant reliable signal over an extended period of time.
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