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Understanding gas sensor lifespan

29 November 2016

In this article, gas detection expert Andy Avenell of Crowcon outlines the factors which affect the operating lifespan of electrochemical and pellistor toxic and oxygen gas sensors. 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.

Introduction

Gas detectors are used extensively in industry to protect personnel and equipment from dangerous gases and their effects. Users of portable and fixed-point gas detectors will be very familiar with the potentially significant costs of keeping their instruments operating safely over their operational life.

Part 1: Electrochemical sensors

How they work

These 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 amplifier 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 fitted 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.

Galvanic oxygen sensors operate using a similar principle to that described for electrochemical oxygen sensors, however their life is predictable and replacement periods are pre-defined: 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.

Electrochemical Sensors
Electrochemical Sensors

Factors affecting sensor life

Temperature 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 significantly less sensitive to gas (sensitivity maybe 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 significant influence 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 be significantly 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 fixed-point detector is likely to change during the first 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 office) prior to installation.

Typical Construction of an Electrochemical Sensor
Typical Construction of an Electrochemical Sensor

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 expectancy

Electrochemical 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 fluoride may have a life of only 12-18 months.

In ideal conditions, stable temperature and humidity in the region of 20°C and 60%RH with no incidence of contaminants, electrochemical sensors have been known to operate in excess of 11 years! Periodic exposure to the target gas does not limit the life of these tiny fuel cells: high quality sensors have a large amount of catalyst material and robust conductors which donot become depleted by the reaction.

Quoted sensor ‘shelf life’ or ‘storage life’ may cause confusion and frustration for users, service companies and manufacturers alike. Electrochemical sensors typically have a stated storage life of six months from manufacture (if stored in ideal conditions at 20°C). After this period, their output signal may begin to drift. Inevitably, a small proportion of this period is consumed in the manufacture of the instrument or sensor module and in shipping to the customer. It is vital, therefore, to plan the purchase of spare sensors carefully so that there is not an excessive delay in putting them into use.

Filtered sensors

Chemical filters are used on some sensors to limit the effect of interfering gases, particularly hydrogen sulphide. These filters usually have a limited life defined as ppm/hours to indicate their tolerance tothe interfering gas. The ppm/hour metric may be imprecise given varying gas concentrations. A filter with a stated capacity of 1000 ppm/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 filter becomes saturated. The user will of course not be able to determine if their sensor is responding to SO2 or H2S as this occurs.

Sensor unit in place
Sensor unit in place

Organic (carbon based) filters are effective, but they are non-regenerative and can saturate as the pores become blocked as ambient humidity exceeds 50%RH. The efficacy of chemical filters may reduce in high humidity environments.

How do I know when my sensor has failed?

There have been several patents and techniques applied togas 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 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 replacement

Sensor 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 confidently extend the period between routine sensor replacements.

Electrochemical sensor life prediction is a very inexact science - operational life-spans are totally influenced by the factors described in this article and thus are unique to each application. In practice,sensors are either replaced at a fixed 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 significant life remaining. Sensors that are replaced only when they demonstrate a significant loss in sensitivity (or an unacceptably long response time) are at risk of failing between service intervals (often only every 6 months).

  

Part 2: Pellistors

Pellistor Sensor Construction
Pellistor Sensor Construction

How they work

The word ‘pellistor’ is an amalgam of ‘pellet’ and resistor’, and describes a sensor which comprises a tiny wire encased in a catalyst-loaded bead.

Pellistor sensors consist of two matched wire coils, each embedded in a ceramic bead. Current is passed through the coils, heating the beads to approximately 500°C. Flammable gas burns on the bead and the additional heat generated produces an increase in coil resistance which is measured by the instrument to indicate gas concentration. The detecting element is coated with a catalyst which promotes oxidation when in contact with flammable gases; the compensating element is treated so that catalytic oxidation does not occur. The compensating element is fitted to ensure that signals are not generated due to environmental effects (e.g. changes in ambient temperature or gas flow rate).

Because conventional pellistors are relatively high power devices, and as they operate at a temperature that will ignite flammable gases, they need to be sealed behind a flame arrestor (sinter).

Factors affecting sensor life

The capacity of the catalyst surface to oxidise the gas reduces when it has been poisoned or inhibited. Sensor life in excess of ten years is not uncommon in applications where inhibiting or poisoning compounds are not present. Higher power pellistors have greater catalytic activity and are less vulnerable to poisoning. More porous beads also have greater catalytic activity as their surface volume in increased. Skilled initial design and sophisticated manufacturing processes ensure maximum bead porosity.

The term ‘poison’ describes compounds that will have a non-reversible effect on the pellistor beads. Exposure to these compounds will permanently reduce sensitivity to the target gas. Organic silicone is the most common poison; it burns in contact with the bead surface and creates a silicone-oxygen bond which is difficult to break. This reduces the area of the bead available for reaction to gas. Response to methane is most affected as this gas is the most difficult to oxidise (and thus needs maximum bead surface area in order to be detected). It is possible that the pellistor can become insensitive to methane, but still react to butane, propane, hydrogen etc..

Examples of poisoning compounds are:

•  Organic silicon compounds

•  Organo-metallic compounds

•  Any organic metal component: lead, tin, copper compounds in fuels, paints etc.

Wheatstone Bridge Diagram
Wheatstone Bridge Diagram

Compounds that have a reversible effect on the pellistor are described as ‘inhibitors’; the pellistor sensitivity will recover when the compound is removed. Examples:

•  Compounds containing sulphur, hydrogen sulphide (10’s to 100’s of ppm)

•  Halogen-containing hydrocarbons fluorine (F), chlorine (Cl), bromine (Br), iodine (I).

Exposure to high gas concentrations (>100%LEL) may also compromise sensor performance and create an offset in the zero/base-line signal. Incomplete combustion results in carbon deposits on the bead: the carbon ‘grows’ in the pores and creates mechanical damage. The carbon may however be burned off over time to re-reveal catalytic sites. A common cause of pellistor damage from high gas exposure is ill-informed personnel testing sensors using gas cigarette lighters. These butane fuelled devices deposit gas at 100% volume concentration on the sensor causing extreme carbon deposits or even cracks in the bead surface due to sudden expansion.

Well designed gas detection instruments deploy a ‘pellistor save’ function to limit the damage caused by exposure to excessive gas concentrations. The instrument will remove the voltage from the pellistor when the measured gas concentration exceeds 95%LEL. The alarm on the instruments will remain active however indicating that there is a gas hazard. The pellistor bead will quickly cool minimising the risk of carbon formation or mechanical damage. The instrument can be reset manually once the sensor is returned to clean air conditions.

The pellistor beads are designed to operate in a low voltage ‘Wheatsone Bridge’ circuit (see diagram above) with a supply voltage typically of only 2Vdc. The supply voltage must be carefully applied, especially where it has to be set manually by a technician as excessive voltage will create too much heat and cause the coils to melt.

Extreme mechanical shock or vibration can in rare cases also cause a break in the pellistor coils. This issue is more prevalent on portable rather than fixed-point gas detectors as they are more likely to be dropped, and the pellistors used are lower power (to maximise battery life) and thus use more delicate thinner wire coils.

Filters

Pellistor sensors generally only use a sinter (a disk of compressed stainless steel fragments) or mesh without additional filters. Carbon filters may be used in mining and other applications to prevent poisons from reaching the sensor. These should be used with care, however, as the carbon will also filter-out long-chain hydrocarbons. Carbon filters are only recommended where methane or hydrogen are the primary gas risk.

The sinter and/or filter may also present a potential unrevealed failure mode: the pores can become blocked by dust/dirt, oil etc and prevent gas from reaching the sensor. The sinter and filter (if used) must be checked regularly by passing gas through, and replaced if there are signs of contamination.

How do I know when my sensor has failed?

As with electrochemical cells, the surest way to determine that a pellistor will correctly respond is to test it with gas. Manufacturers typically recommend that pellistors are re-calibrated every six months by applying %LEL concentration gas. The calibration frequency is entirely dependent on local conditions however. Pellistor sensitivity will degrade over time due to environmental influences: presence of poisons/inhibitors, contamination of the sinter or filter, effects of shock/vibration etc.

Pellistors may not fail safe: a pellistor that has been poisoned remains electrically operational but may fail to respond to gas. Hence the gas detector and control system may appear to be in a healthy state, but a flammable gas leak may not be detected.

Sensitivity degradation cannot practically be automatically monitored by the instrument, which is why regular checks and calibration with test gas are vital, especially in environments where poisons may be present. The European safety standard: EN60079-29-2:2015 (Explosive atmospheres. Gas detectors. Selection, installation, use and maintenance of detectors for flammable gases and oxygen) states: “Inspection and functional checks are intended to verify that the equipment is in a working state. It is recommended that they are done by personnel actually operating the equipment, and is performed before each day of use, particularly if the equipment has catalytic, electrochemical, or semiconductor sensors and is being used under arduous conditions.”

Planning gas sensor replacement

As described previously for electrochemical sensors, understanding factors that affect sensor life and deploying regular gas tests can help to determine the optimum time to replace pellistors. Sensor replacement costs can be minimised whilst retaining confidence that the sensor will respond to gas hazards effectively.

Acknowledgements

Sincere thanks to John Saffell of Alphasense and Kevin Brown of SGX Sensortech for their  contribution to this article.


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