8/7/2019 25. Sensor_Life_Expectancy

1/1

Date 1/19/05

No AN20050119Application Note

Life Expectancy ofGas Detection Sensors

By Mike Burt

Page 1 of 1

Biosystems gas detectors are equippedwith one or more sensors all of which

have a certain life expectancy. Thisapplication note will address the natureof life expectancy for the most common

sensors when they are installed in a gasdetector.

Oxygen Sensors

Oxygen sensors are consumptive sen-

sors, meaning that something inside thesensor is used up over time when ex-posed to the target gas. Specifically, asoxygen enters the sensor and as it gen-

erates an electrical current in the gasdetector circuit, a chemical reactioncauses a metallic lead anode to be irre-

versibly oxidized.

2Pb + O2 2PbO

When all of the metallic lead is con-verted to the oxide, sensor current out-put will fall to zero and the sensor willneed replacement.

Oxygen sensors installed in a gas de-

tector circuit are always working, evenwhen the detector is turned off. There-fore, if the detector is stored in air, oxy-gen sensor life will be steadily decreas-

ing. The only ways to stop consump-tion of the lead anode (or slow it to anegligible rate) would be to either storethe detector in an inert gas, or to remove

the sensor from the gas detector and

leave it open circuit. The former step isnormally inconvenient, while the latterwould result in long warm-up time

(hours) once the sensor is re-installed inthe detector circuit. Oxygen sensors

installed in current Biosystems monitorsare designed to have a working life oftwo years.

CO and H2S Sensors

CO and H2S sensors are non-consumptive, or catalytic, meaning thatnothing is consumed by exposure to the

gases they are designed to detect.

In theory, CO and H2S sensors shouldlast forever, however, other factors such

as evaporation (drying out), leakage,catalyst contamination, etc. eventuallylimits the life of these sensors. Theexpected life of CO and H2S sensors is

between two to four years. However,depending on electrolyte reservoir size,general climate, as well as use and stor-age conditions, it is possible for these

sensors to last in some cases five toeight years.

Combustible (LEL) Sensors

Combustible sensors are different from

the other sensors in this application noteas they are not wet, electrochemical, butrather solid state, catalytic.

Catalytic combustible gas sensors aredesigned around two porous ceramicbeads, each encasing separate platinumwire coils. During operation they are

powered at constant voltage to keepthem at an elevated temperature. Oneof the beads contains a precious metalcatalyst system designed to promote the

oxidation of combustible gas/vapor thatenters the sensor. This bead is referredto as the active bead. The other bead

is chemically inert to serve as a thermaland electrical reference.

In ideal conditions the LEL sensor canlast for many years. In practice, LELsensors have an expected life of twoyears and rarely last longer than fouryears. There are many factors that can

cause the sensor to quickly fail or togradually lose sensitivity to combustiblegas.

Strong mechanical shock can break thefine platinum wire that suspends andpowers the beads in the sensor chamber.This results in an immediate failure via

a short circuit.

Combustible gas sensors require oxygento oxidize or burn combustible gases &

vapors. If a sensor encounters a fuelrich, but oxygen deficient environment,

carbon, tars, and unburned fuel residuecan build up on the active porous bead.Upon subsequent exposure to normaloxygen level air, the tars/carbon residue

may explosively burn off the activebead causing it to crack open. This oftenresults in a sensor baseline shift severeenough to render it unusable.

The precious metal catalyst supported

on ceramic has a chemistry and struc-ture very similar to catalytic converters

used in automobile exhaust systems. Assuch, it can be quickly rendered inef-fective when exposed to certain types ofmaterials referred to as catalyst poisons

or inhibitors. These materials includebut are not limited to: tetra-ethyl lead asfound in leaded fuel, volatile siliconeoils and silicone (RTV) rubber off-gas

products during cure, halogenated hy-drocarbons (Freons, and commonsolvents such as methylene chloride,

trichloroethylene and ethylene dichlo-ride), as well as chronic/high concen-

trations of hydrogen sulfide and othersulfur containing gases.

When LEL sensors become poisoned,often the first gas that they tend to losesensitivity to is methane. As such, thebest way to check sensor performance isto regularly test them using calibration

gas that is methane-based. In order totest sensors for poisoning while stillallowing the sensor to be calibrated to adifferent sensitivity scale than methane,

Biosystems offers methane-based LELgas blends that are equivalent to pro-pane (C3) or pentane (C5) sensitivities.

Please refer to Biosystems ApplicationNote Use of Equivalent Calibration

Gas Mixtures for details on usingmethane-based calibration gas.

Age will cause the sensors to slowlylose sensitivity over time. The highsurface area catalyst on the active bead

will gradually lose active sites as it isexposed to trace environmental con-taminants and some consolidation willoccur after many power-up cycles.

Conclusion

The only way to be fully confident thatsensors are functional and accurate is to

periodically verify accuracy with known

concentration calibration gas and cali-brate when necessary. Please refer toBiosystems application note Frequency

for Verifying Sensor Accuracy fordetails on how often to bump test and

calibrate Biosystems monitors.

All gas sensors need to be replaced fromtime to time. The most commonly re-

placed sensor is the oxygen sensor as ithas a finite life. LEL and toxic gassensors generally do not need replace-ment as frequently as oxygen sensors,

but anytime the sensor fails, it must bereplaced.

Biosystems sensors have either a one or

two year warranty. Sensors generallylast beyond their warranty period, but inthe case where it does not, Biosystemswill replace the sensor at no charge.

Arrangement for prompt replacementmay be made via Biosystems' TechnicalSupport at (800) 711-6776.