What is a Pellistor (Catalytic Beads)? 

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 230˚C. The bead becomes hot from the combustion, resulting in a temperature difference between this active and the other ‘reference’ bead.  This causes a difference in resistance, which is measured; the amount of gas present is directly proportional to the resistance change, so gas concentration as a percentage of its lower explosive limit (% LEL*) can be accurately determined. 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. Pellistor sensors are widely used throughout industry including on oil rigs, at refineries, and for underground construction purposes such as mines, and tunnels. 

Benefits of Pellistor Sensors?

Pellistor sensors are relatively low in cost due to differences in the level of technology in comparison to the more complex technologies like IR sensors, however, they may be required to be replaced more frequently. With a linear output corresponding to the gas concentration, correction factors can be used to calculate the approximate response of pellistors to other flammable gases, which can make pellistors a good choice when there are multiple flammable gases and vapours present. 

Factors affecting Pellistor Sensor Life

The two main factors that shorten the sensor life include exposure to high gas concentration and poisoning or inhibition of the sensor. Extreme mechanical shock or vibration can also affect the sensor life.  

The capacity of the catalyst surface to oxidise the gas reduces when it has been poisoned or inhibited. Sensor lifetimes of up to ten years is known in some applications where inhibiting or poisoning compounds are not present. Higher power pellistors have larger beads, hence more catalyst, and that greater catalytic activity ensures less vulnerability to poisoning. More porous beads allow easier access of the gas to more catalyst allowing greater catalytic activity from a surface volume instead of just a surface area. Skilled initial design and sophisticated manufacturing processes ensure maximum bead porosity. 

Strength of the bead is also of great importance since exposure to high gas concentrations (>100% LEL) may compromise sensor integrity causing cracking. Performance is affected and often offsets in the zero/base-line signal result. Incomplete combustion results in carbon deposits on the bead: the carbon ‘grows’ in the pores and causes mechanical damage or just gets in the way of gas reaching the pellistor. The carbon may however be burned off over time to re-reveal catalytic sites.  

Extreme mechanical shock or vibration can in rare cases 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. 

What happens when a Pellistor is poisoned? 

A poisoned pellistor remains electrically operational but may fail to respond to gas as it will not produce an output when exposed to flammable gas. This means a detector would not go into alarm, giving the impression that the environment is safe.  

Compounds containing silicon, lead, sulphur, and phosphates at just a few parts per million (ppm) can impair pellistor performance.  Therefore, whether it’s something in your general working environment, or something as harmless as cleaning equipment or hand cream, bringing it near to a pellistor could mean you are compromising your sensor’s effectiveness without even realising it. 

Why are silicones bad? 

Silicones have their virtues, but they may be more common than you first thought. Some examples include sealants, adhesives, lubricants, and thermal and electrical insulation. Silicones, have the ability to poison a sensor on a pellistor at extremely low levels, because they act cumulatively a bit at a time.  

Products  

Our portable products all use low power portables pellistor beads. This prolongs battery life but can make them prone to poisoning. Which is why we offer alternatives that do not poison, such as the IR and MPS sensors. Our fixed products use a porous high energy fixed pellistor. 

To explore more, visit our technical page for more information.

How Long will my Gas Sensor Last?

Gas detectors are used extensively within many industries (such as water treatment, refinery, petrochemical, steel and construction to name a few) to protect personnel and equipment from dangerous gases and their effects. Users of portable and fixed devices will be familiar with the potentially significant costs of keeping their instruments operating safely over their operational life. Gas sensors are understood to provide a measurement of the concentration of some analyte of interest, such as CO (carbon monoxide), CO2 (carbon dioxide), or NOx (nitrogen oxide). There are two most used gas sensors within industrial applications: electrochemical for toxic gases and oxygen measurement, and pellistors (or catalytic beads) for flammable gases. In recent years, the introduction of both Oxygen and MPS (Molecular Property Spectrometer) sensors have allowed for improved safety.  

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. 

Electrochemical Sensor  

Electrochemical sensors are the most used in diffusion mode in which gas in the ambient environment enters through a hole in the face of the cell. Some instruments use a pump to supply air or gas samples to the sensor. 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. 

Factors affecting Electrochemical Sensor Life 

There are three main factors that affect the sensor life including temperature, exposure to extremely high gas concentrations and humidity. Other factors include sensor electrodes and extreme vibration and mechanical shocks.  

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 shifts in the baseline (zero) reading and slower response. 

Exposure to extremely 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 considerable 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 or dehydrated. 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.  

‘Normal’ Life Expectancy of Electrochemical Sensor 

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 sensor such as 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 more than 4000 days (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 do not become depleted by the reaction. 

Pellistor Sensor 

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. 

Factors affecting Pellistor Sensor Life 

The two main factors that affect the sensor life include exposure to high gas concentration and poising or inhibition of the sensor. Extreme mechanical shock or vibration can also affect the sensor life. The capacity of the catalyst surface to oxidise the gas reduces when it has been poisoned or inhibited. Sensor life more than ten years is common 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. 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. 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. 

How do I know when my sensor has failed? 

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. 

Oxygen Sensor 

Long Life 02 Icon

Our new lead-free, long-lasting oxygen sensor does not have compressed strands of lead the electrolyte has to penetrate, allowing a thick electrolyte to be used which means no leaks, no leak induced corrosion, and improved safety. The additional robustness of this sensor allows us to confidently offer a 5-year warranty for added piece of mind. 

Long life-oxygen sensors have an extensive lifespan of 5-years, with less downtime, lower cost of ownership, and reduced environmental impact. They accurately measure oxygen over a broad range of concentrations from 0 to 30% volume and are the next generation of O2 gas detection. 

MPS Sensor  

MPS sensor provides advanced technology that removes the need to calibrate and provides a ‘True LEL (lower explosive limit)’ for reading for fifteen flammable gases but can detect all flammable gases in a multi-species environment, resulting in lower ongoing maintenance costs and reduced interaction with the unit. This reduces risk to personnel and avoids costly downtime. The MPS sensor is also immune to sensor poisoning.  

Sensor failure due to poisoning can be a frustrating and costly experience. The technology in the MPS™ sensor is not affected by contaminates in the environment. Processes that have contaminates now have access to a solution that operates reliably with fail safe design to alert operator to offer a peace of mind for personnel and assets located in hazardous environment. It is now possible to detect multiple flammable gases, even in harsh environments, using just one sensor that does not require calibration and has an expected lifespan of at least 5 years. 

Pellistor sensors – how they work

Pellistor gas sensors (or catalytic bead gas sensors) have been the primary technology for detecting flammable gases since the ‘60s. Despite having discussed a number of issues relating to the detection of flammable gases and VOC, we have not yet looked at how pellistors work. To make up for this, we are including a video explanation, which we hope you will download and use as part of any training you are conducting

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Are Silicone Implants Degrading your Gas Detection?

In gas detection terms, pellistors have been the primary technology for detecting flammable gases since the 60s.  In most circumstances, with correct maintenance, pellistors are a reliable, cost-effective means of monitoring for combustible levels of flammable gases.  However, there circumstances under which this technology may not be the best choice, and infrared (IR) technology should be considered instead.

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Cross Calibration of Pellistor (Catalytic Flame) Sensors‡

After last week’s comparative levity, this week, I am discussing something rather more serious.

When it comes to detecting hydrocarbons, we often don’t have a cylinder of target gas available to perform a straight calibration, so we use a surrogate gas and cross calibrate. This is a problem because pellistor’s give relative responses to different  flammable gases at different levels. Hence, with a small molecule gas like methane a pellistor is more sensitive and gives a higher reading than a heavy hydrocarbon like kerosene.

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