Chemical Engineering October 2021 - 29

sensing element.
MOS sensors are designed to respond to the widest
possible range of toxic and flammable gases and vapors.
The idea is to provide a " broad range " response to
the presence of contaminants. MOS sensors can detect
chlorofluorocarbon refrigerants, as well as other gases
that are difficult to detect by other means, as well as ammonia,
carbon monoxide, hydrogen, alcohols and many
other gases and vapors.
Their non-specificity is advantageous where unknown
toxic gases are present, and a simple go/no-go determination
is sufficient. As the sensors are not specific to
ammonia, however, this can lead to false alarms if the
sensors are installed where interfering contaminants are
present. The benefits of MOS sensors are their long operational
life and low cost. MOS sensors can be used to
detect ammonia concentrations as low as 30 ppm, up to
concentrations in the flammable range.
Electrochemical sensors
Target-gas-specific
electrochemical (EC) sensors are
available for many of the most common toxic gases.
Electrochemical sensors are compact, require very little
power, exhibit excellent linearity and repeatability, and
are comparatively inexpensive. The detection technique
is very straightforward in concept.
Electrochemical sensors function on principles similar
to those found in batteries. The target gas enters the
sensor through an inlet, and then generates a chemical
reaction with a catalyst that is in contact with an electrode.
When the target gas encounters an electrolyte solution
on the working electrode, a reaction occurs (Figure
5).
This reaction causes a release of electrons, whereby
the flow of the electrons is measured as current within
the sensor. Consequently, the current is proportional to
the gas concentration and the reaction is measured in
ppm of the gas. In the case of ammonia sensors, the
electrolyte includes an active ingredient that is consumed
in the electrochemical reaction used to detect
the ammonia. Thus, the lifespan of the sensor is directly
related to its amount of exposure to ammonia.
The sensor is filled with an organic gel electrolyte mixture
in which the reaction occurs (Figure 6). Active ingredients
in the electrolyte are incrementally consumed as the sensor
is exposed to ammonia. Once the exposure life of the
sensor is exceeded, it is no longer capable of detecting
gas, and will need to be replaced. Next-generation technology
utilizing ionic-liquid electrolytes have an extremely
low vapor pressure, and they do not evaporate in extreme
environments, even with long periods of low humidity.
The positive benefits for this type of sensor include good
resolution in the low-ppm range and acceptable coldtemperature
performance.
Catalytic bead sensors
These sensors use a catalytic bead to oxidize mainly combustible
gases, but also higher-volume-percentage concentrations
of ammonia (Figure 7). A bead is represented
by wire coil coated with a catalyst-coated glass or ceramic
material and is electrically heated to a temperature that allows
it to burn the gas being monitored, releasing heat
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