1. 66 Climate Control Middle East August 2014
perspective CARBON MONOXIDE SENSORS
Regularly
scheduled
recommended
maintenance
procedures
should extend
the lifespan
of the sensor
if properly
followed
GETTINGSENSITISED
TOSENSORS
Warning about the dangers posed by sensors that are not calibrated
and designed by an authorised professional company, Syed Taqi Hussain
offers advice on the maintenance of electrochemical carbon monoxide
sensors.
Introduction
The purpose of this article is
to provide an instructional
and informative document
regarding gas sensing
technology, primarily the
electrochemical sensor, its
principles of operation,
maintenance and typical
lifespan.
What is a gas
sensor?
A gas sensor measures the
concentration of gas in its
vicinity. The sensor interacts
with a gas to measure its
concentration. Each gas
has a unique breakdown
voltage, ie, the electric field
at which it is ionised. The
concentration of the gas can
be determined by measuring
Carbon monoxide is an
odourless, tasteless, invisible
gas primarily produced
through a combustion
process. It is highly lethal
in humans, and even small
concentrations of CO can
cause headaches, nausea,
vomiting, dizziness and
fatigue. It is often referred
to as “the silent killer”.
Therefore, it is extremely
the current discharge in the
sensing device.
Electro-
chemical
sensing
technology
Gas sensing devices
are available in several
technologies or principles
of operation. Most
common are the electronic,
electrochemical and metal
oxide semiconductors.
The selection of a specific
sensing technology is usually
determined by their purpose
– to sense a toxic gas or to
sense a combustible gas.
Here, I will deal specifically
with electrochemical sensing
technology used for the
purpose of measuring the
carbon monoxide (CO) gas.

2. August 2014 www.climatecontrolme.com 67
The writer is the
Director – Sales &
Services (HVAC)
at Eco Evergreen.
He can be
contacted at taqi@
ecoevergreenuae.
com
Figure 1: Nemoto NAP-505 3-Electrode CO Sensing Element
important that CO sensing
devices are installed in close
proximity to all sources
of combustion, like boiler
rooms, industrial kitchens
and enclosed car parks.
The typical
electrochemical CO sensors
consist of chemical reactants
(electrolytes or gels) and
two terminals – an anode
and a cathode. The anode
is responsible for an
oxidisation process and the
cathode is responsible for
a reduction process. As a
result, current is created
by way of positive ions
flowing to the cathode and
the negative ions flowing
to the anode. The output is
directly proportional to the
concentration of the carbon
monoxide.
A more superior three
terminal configuration
consists of the anode or
“working” electrode, the
cathode or “counter”
electrode, and a third,
“reference” electrode.
The “reference” electrode
maintains the healthy
operation of the cell. It is
surrounded by electrolyte,
sees that no gas and no
current is allowed to be
drawn from it. Therefore,
its electrochemical potential
remains constant at a
level referred to as “rest
air potential”. It is used to
regulate the potential of the
working electrode, regardless
of the current generated
during operation. Some
of the benefits achieved
with a three-electrode
sensor are extended range,
improved linearity and other
performance advancements.
(See Figure 1)
As with all gas detectors,
their operation and accuracy
will deteriorate over time to
an extent dependent on their
installation and environment.
The life expectancy of an
electrochemical CO sensor
will depend on several
factors, such as the specific
gas detected, the total
amount of target (CO) gas
the sensor is exposed to,
temperature, humidity,
and pressure. Typically, an
electrochemical gas sensor
should operate as designed
for one-three years as
the electrolyte within the
sensor cell will eventually
get used up. Regularly
scheduled manufacturer’s
recommended maintenance
(calibration) procedures
should extend the lifespan
of the sensor if properly
followed.
It is recommended that
CO detectors be recalibrated
at a minimum annual
schedule and preferably
every six months in order
to ensure accurate and
proper operation, given the
potential detrimental or fatal
consequences which could
occur without the required
monitoring or indication of
harmful CO concentrations
present.
The duration of the
recalibration process will
depend on the amount of
target gas the sensor is
exposed to, or the extent of
deterioration of the sensor,
whereby one sensor can have
its calibration re-established
quicker than another,
depending on the installation
location. In doing so, I
advise that a typical CO
detector should have an
operational lifespan of five-
seven years.