What Is a Pellistor Gas Sensor?
In
simple terms, a pellistor gas sensor is a solid state device designed to
detect combustible gases. It’s also used to detect gases that
significantly differ in thermal conductivity to the surrounding air. Thanks to
their predisposition to change resistance when exposed to certain vapours,
compact catalyst-loaded ceramic pellets are used in the detection process. The
fusion of ‘pellets’ and ‘resistor’ is where the term ‘pellistor’ comes from –
ingenious!
There
are two types of pellistor contraptions used to detect gases - Catalytic and
Thermal Conductivity (TC).
- Catalytic sensors
This
is the most common form of pellistor sensor which actively burns target gases
to produce additional heat. In turn, this initiates a change in the resistance
of the ceramic pellets which can be used to determine concentration of a target
gas.
- Thermal Conductivity sensors
Like
its Catalytic counterparts, Thermal Conductivity sensors use changes in heat to
detect target gases. Although rather than actively generating heat, TC sensors
measure changes in heat loss and use these to identify the presence of certain
gasses.
How do they work?
It may
sound complicated, but the gas detection process is actually quite simple.
Pellistors are stored in flameproof housing which is bordered by a gas
permeable membrane. Pellistors raise the alarm when gas diffuses through the
permeable membrane and creates an exothermic reaction. This reaction is due to
the fact that the maximum concentration of flammable gas has been exceeded.
This is more commonly referred to as the Lower Explosion Limit (LEL) and varies
between different substances.
Where are they used?
Pellistor
gas sensors first made an appearance in the 1960s thanks to British scientist
and inventor, Alan Baker. The concept was put forward as a solution to the
long-running flame safety lamp and canary techniques. The sensors are
highly reliable and are capable of detecting almost 100% of combustible gases
that fall within their explosive range. Today, pellistor gas sensors are still
used as an effective way to detect combustible gases in a range of potentially
explosive environments. These include industrial applications such as mines,
oil refineries and oil rigs.
Are there any risks?
While
pellistor gas sensors are incredibly useful, they are still liable to fall
victim to a number of risks. Poisoning is one of the most common dangers
wherein compounds begin to decompose on the catalyst to form a dense barrier.
Common poisons include silicon and organic lead. Decomposition of these
compounds reduces the sensitivity of the catalyst and hinders its ability to
detect target gases. Inhibition is another risk which sees catalysts absorb
compounds such as halogenated hydrocarbons and hydrogen sulphide. This can block
a catalyst’s reaction sites and impede its capacity to display resistance.
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