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 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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