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IIn the fi rst of a two-part series, gas
detection expert Andy Avenell of
Crowcon outlines the factors which affect
the operating lifespan of electrochemical
toxic and oxygen gas sensors.
An explanation is given on how An explanation is given on how
these sensors work, their normal life
expectancy followed by a review of
factors that can reduce operational life.
Suggestions are also given to help predict
sensor life and plan for replacement.
IntroductionGas detectors are used extensively in industry
to protect personnel and equipment from
dangerous gases and their effects. Users of
portable and fi xed-point gas detectors will
be very familiar with the potentially signifi cant
costs of keeping their instruments operating
safely over their operational life.
Part 1: Electrochemical sensorsHow they workThese small cells contain electrodes wetted with
an aqueous or gel electrolyte (often sulphuric
acid: H2SO4). The working electrode is treated
with a catalyst to generate a tiny current as
the target gas (e.g. carbon monoxide: CO,
hydrogen sulphide: H2S etc.) is either oxidised
or reduced. The sensor is connected to an
amplifi er which is set to indicate the gas
concentration in the required scale.
Electrochemical sensors are most often
used in diffusion mode whereby gas in the
ambient environment enters through a hole
in the face of the cell (driven by the natural
mobility of the gas). Some instruments supply
the air/gas sample to the sensor via a pump.
A PTFE membrane is fi tted over the hole to
prevent water or oils from entering the cell.
Sensor ranges and sensitivities can be varied
in design by using different size holes. Larger
holes provide higher sensitivity and resolution,
whereas smaller holes reduce sensitivity and
resolution but increase the range.
Understanding gas sensor lifespan
Fig 2. Typical Construction of an Electrochemical Sensor
Fig 1. Electrochemical sensors
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Gas detection38
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Gas detection 39Gas detection
Galvanic oxygen sensors operate using
a similar principle to that described for
electrochemical oxygen sensors, however
their life is predictable and replacement their life is predictable and replacement
periods are pre-defi ned: usually two
or three years. Unlike most toxic gas
sensors, oxygen sensors are exposed
to the target gas continuously. In normal
oxygen depletion monitoring applications
the sensor is exposed to 20.9% volume
oxygen, which causes a galvanic reaction
on the lead anode which is gradually
consumed by the reaction. Hence the
sensors’ ability to continue producing a
current in reaction to oxygen is limited
by the lead content available to the
electrolyte.
The gas detection instrument
manufacturer adds a vital component
to the performance of the sensor:
temperature compensation. Sensitivity to
gas (and also the zero base-line signal)
often varies with temperature, resulting in
non-linear gas response as the ambient
temperature changes.
A great deal of time is taken during the
development of a gas detector, applying a
number of gas concentrations to multiple
sensors of the same type at temperature
increments between the sensor minimum
and maximum (typically -30˚C to +50˚C).
The data collected is then averaged to
produce a temperature compensation
algorithm used in the gas detector to
ensure the sensor reading is consistent
across the full operating range.
Factors affecting sensor lifeTemperature extremes can affect sensor
life. The manufacturer will state an operating
temperature range for the instrument:
typically -30˚C to +50˚C. High quality
sensors will, however, be able to withstand
temporary excursions beyond these limits.
Short (1-2 hours) exposure to 60-65˚C
for H2S or CO sensors (for example) is
acceptable, but repeated incidents will result
in evaporation of the electrolyte and possible
shifts in the base-line (zero) reading and
slower response.
Sensitivity is lost at low temperatures;
sensors may operate down to -40˚C but will
become signifi cantly less sensitive to gas
(sensitivity may be reduced by as much as
80%) and take much longer to respond.
There is also a risk the electrolyte will freeze
below -35˚C.
Exposure to very high gas concentrations
can also compromise sensor performance.
Electrochemical sensors are typically tested
by exposure to as much as ten-times
their design limit. Sensors constructed
using high-quality catalyst material should
be able to withstand such exposures
without changes to chemistry or long-term
performance loss. Sensors with lower
catalyst loading may suffer damage.
The most signifi cant infl uence on sensor
life is humidity. The ideal environmental
condition for electrochemical sensors is
20˚Celsius and 60%RH (relative humidity).
When the ambient humidity increases
beyond 60%RH, water will be absorbed into
the electrolyte causing dilution. In extreme
cases the liquid content can increase by 2-3
times, potentially resulting in leakage from
the sensor body, and then through the pins.
Below 60%RH, water in the electrolyte will
begin to de-hydrate. The response time may
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Gas detection42
be signifi cantly extended as the electrolyte is
dehydrated.
A quick and simple method of testing for
absorption or dilution of the electrolyte is to
weigh the sensor. Changes of +/-250mg
of the original weight indicate a leakage
or change in performance is possible.
Electrolyte dilution and evaporation are
reversible if the sensor is exposed to
humidity levels of the opposite extreme.
The sensor can return to the original weight
and electrolyte concentration within 5-25
days, with performance restored.
It should be noted that sensor sensitivity
may adjust to the ambient environment:
a sensor that has a lower response or
extended response time may improve as
the ambient humidity changes, and this
can even be dependent on the time of year
in countries with large seasonal changes.
Hydrogen sulphide sensor performance in
particular depends on ambient conditions.
The sensitivity and response time of a sensor
in a fi xed-point detector is likely to change
during the fi rst two to three weeks after
commissioning as it stabilises according
to the local temperature and humidity. This
effect will be especially prevalent where
sensors have been stored in very dry
environments (e.g. an air conditioned offi ce)
prior to installation.
Sensor electrodes can in unusual conditions
be poisoned by interfering gases that adsorb
onto the catalyst or react with it, creating by-
products which inhibit the catalyst.
Extreme vibration and mechanical shocks
can also harm sensors by fracturing the
welds that bond the platinum electrodes,
connecting strips (or wires in some sensors)
and pins together. This is unusual for well-
constructed sensors, however.
‘Normal’ life expectancyElectrochemical sensors for common gases
such as carbon monoxide or hydrogen
sulphide have an operational life typically
stated at 2-3 years. More exotic gas sensors
such as those for hydrogen fl uoride may
have a life of only 12-18 months.
In ideal conditions, stable temperature
and humidity in the region of 20˚C and
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43Gas detection
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60%RH with no incidence of contaminants, 60%RH with no incidence of contaminants,
electrochemical sensors have been known electrochemical sensors have been known
to operate in excess of 11 years! Periodic to operate in excess of 11 years! Periodic
exposure to the target gas does not limit exposure to the target gas does not limit
the life of these tiny fuel cells: high quality the life of these tiny fuel cells: high quality
sensors have a large amount of catalyst sensors have a large amount of catalyst
material and robust conductors which do material and robust conductors which do
not become depleted by the reaction.not become depleted by the reaction.
Quoted sensor ‘shelf life’ or ‘storage life’ Quoted sensor ‘shelf life’ or ‘storage life’
may cause confusion and frustration for may cause confusion and frustration for
users, service companies and manufacturers users, service companies and manufacturers
alike. Electrochemical sensors typically have alike. Electrochemical sensors typically have
a stated storage life of six months from a stated storage life of six months from
manufacture (if stored in ideal conditions manufacture (if stored in ideal conditions
at 20˚C). After this period, their output at 20˚C). After this period, their output
signal may begin to drift. Inevitably, a small signal may begin to drift. Inevitably, a small
proportion of this period is consumed in the proportion of this period is consumed in the
manufacture of the instrument or sensor manufacture of the instrument or sensor
module and in shipping to the customer. module and in shipping to the customer.
It is vital, therefore, to plan the purchase of It is vital, therefore, to plan the purchase of
spare sensors carefully so that there is not spare sensors carefully so that there is not
an excessive delay in putting them into use.an excessive delay in putting them into use.
Filtered sensorsFiltered sensorsChemical fi lters are used on some sensors
to limit the effect of interfering gases,
particularly hydrogen sulphide. These fi lters
usually have a limited life defi ned as ppm/
hours to indicate their tolerance to the
interfering gas. The ppm/hour metric may be
imprecise given varying gas concentrations.
A fi lter with a stated capacity of 1000ppm/
hours will not necessarily last twice as long
at half the gas exposure.
Sensor cross-response to the interfering gas
(e.g. hydrogen sulphide, H2S, or sulphur
dioxide, SO2, sensors) will increase as the
fi lter becomes saturated. The user will of
course not be able to determine if their
sensor is responding to SO2 or H2S as
this occurs.
Organic (carbon based) fi lters are effective,
but they are non-regenerative and can
saturate as the pores become blocked as
ambient humidity exceeds 50%RH. The
effi cacy of chemical fi lters may reduce in
high humidity environments.
How do I know when my sensor has failed?There have been several patents and
techniques applied to gas detectors over the
past few decades which claim to be able to
determine when an electrochemical sensor
has failed. Most of these, however, only infer
that the sensor is operating through some
form of electrode stimulation and might
provide a false sense of security. The only
sure method of demonstrating that a sensor
is working is to apply test gas and measure
the response: a bump test or full calibration.
The reality is that electrochemical sensors
are not fail-safe. They produce zero signal
current in clean air, and at the end of their
operational lives will continue to produce
zero current even when exposed to gas. It is
not possible, therefore, for a gas detection
instrument to be guaranteed to automatically
determine when a sensor has failed.
The instrument can, however, report
incidents that are likely to have affected
sensor performance: intelligent gas
detectors and transmitters can monitor
the ambient environment and produce a
warning if the temperature exceeds the
upper or lower thresholds of the sensor.
The transmitter can also compare the
measured gas level to a maximum
permissible limit for a particular sensor and
warn if this is exceeded. In these examples,
the correct action is for the user to bump-
test the sensor using test gas to verify that
it responds correctly.
Planning gas sensor replacementSensor life predication is highly desirable
for instrument operators to enable sensor
replacement to be planned, and also to
ensure that service engineers attend site
already carrying replacement sensors and
thus avoid the risk of instrument down-time
or re-visits. Conversely, users could reduce
the cost of replacing sensors if they could
confi dently extend the period between
routine sensor replacements.
Electrochemical sensor life prediction is a
very inexact science - operational life-
spans are totally infl uenced by the factors
described in this article and thus are unique
to each application. In practice, sensors are
either replaced at a fi xed time period based
on manufacturers’ recommendations or
previous data (e.g. every 2 or 3 years), or
when they respond inadequately to test gas.
In the regular time period model, users have
assurance that sensors are always ‘fresh’,
however they may be paying a premium
for this re-assurance as it is very likely in
many cases that the replaced sensors
have signifi cant life remaining. Sensors that
are replaced only when they demonstrate
a signifi cant loss in sensitivity (or an
unacceptably long response time) are at
risk of failing between service intervals (often
only every 6 months).
AcknowledgementsSincere thanks to John Saffell of Alphasense
and Kevin Brown of SGX Sensortech for
their contributions to this article.
In the second part of this two-part series,
the author will examine the factors which
affect the lifespan of pellistor (‘catalytic
bead’) fl ammable gas sensors.
Andy Avenell has more than 20 years experience is the gas detection industry including many years spent commissioning large systems in industrial, petrochemical and oil and gas installations, both onshore and offshore. He was responsible for designing and engineering very large systems at Crowcon, and assumed the position of Fixed Systems Product Manager in 2004, followed by the position of Senior Product Manager in 2012. Avenell was appointed Senior Business Development Manager in 2015.
He is also a member of the UK’s industry council (CoGDEM), where applicable European standards and Directives are considered and reviewed.
About the author
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