A brief history of gas detection

The evolution of gas detection has changed considerably over the years. New, innovative ideas from canaries to portable monitoring equipment provides workers with continuous precise gas monitoring.

The Industrial Revolution was the catalyst in the development in gas detection due to the use of fuel that showed great promise, such as coal. As coal can be extracted from the earth through either mining or underground mining, tools like helmets and flame lights were their only protection from the dangers of methane exposure underground that were yet to be discovered. Methane gas is colourless and odourless, making it hard to know it’s presence until a noticeable pattern of health problems was discovered. The risks of gas exposure resulted in experimenting with detection methods to preserve the safety of the workers for years to come.

A Need for Gas Detection

Once gas exposure became apparent, miners understood that they needed to know whether the mine had any pocket of methane gas where they were working. In the early 19th century, the first gas detector was recorded with many miners wearing flame lights on their helmets to be able to see while they were working, so being able to detect the extremely flammable methane was paramount. The worker would wear a thick, wet blanket over their bodies while carrying a long wick with the end lit on fire. Entering the mines, the individual would move the flame around and along the walls looking for gas pockets. If found, a reaction would ignite and be noted to the crew while the person detecting was protected from the blanket. With time, more advanced methods of detecting gas were developed.

The Introduction of Canaries

Gas detection moved from humans to canaries due to their loud chirps and similar nervous systems for controlling breathing patterns. The canaries were placed in certain areas of the mine, from there, workers would check on the canaries to care for them as well as see if their health had been affected. During the work shifts, miners would listen to the canaries chirp. If a canaries began to shake its cage, that was a strong indicator of a gas pocket exposure in which it has started to affect its health. Miners would then evacuate the mine and noted that it was unsafe to enter. On some occasions if the canary stopped chirping all together, miners knew to make a swifter exit before the gas exposure had a chance to affect their health.

The Flame Light

The flame light was the next evolution for gas detection in the mine, as a result of worries about animal safety. Whilst providing light for the miners, the flame was housed in a flame-arrestor shell which absorbed any heat and captured the flame to prevent it from igniting any methane that may be present. The outside shell contained a glass piece with three incisions running horizontally. The middle line was set as the ideal gas environment while the bottom line indicated an oxygen-deficient environment, and the top line indicated methane exposure or an oxygen-enriched environment. Miners would light the flame in an environment with fresh air. If the flame lowered or started to die, it would indicate that the atmosphere had a low oxygen concentration. If the flame grew larger, the miners knew that methane was present with oxygen, both cases indicating that they needed to leave the mine.

The Catalytic Sensor

Although the flame light was a development in gas detection technology, it however, was not a ‘one size fits all’ approach for all industries. Therefore, the catalytic sensor was the first gas detector that has a resemblance to modern technology. The sensors work on the principle that when a gas oxidises, it produces heat. The catalytic sensor works through temperature change, which is proportional to the concentration of gas. Whilst this was a step forward in the development of the technology required for gas detection, it still initially required manual operation in order to receive a reading.

Modern Day Technology

Gas detection technology has been developed tremendously since the early 19th century in which the first gas detector was recorded. With now over five different types of sensors commonly used across all industries, including Electrochemical, Catalytic Beads (Pellistor), Photoionisation detector (PID) and Infrared Technology (IR), along with the most modern sensors Molecular Property Spectrometer™ (MPS™) and Long-Life Oxygen (LLO2), modern day gas detectors are highly sensitive, accurate but most importantly reliable, all of which allow for all personnel to stay safe reducing the number of workplace fatalities.

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

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.

The Dangers of Hydrogen

As a fuel, hydrogen is highly flammable and leaks generate a serious risk of fire. However, hydrogen fires are markedly different to fires involving other fuels. When heavier fuels and hydrocarbons, like petrol or diesel, leak they pool close to the ground. In contrast, hydrogen is one of the lightest elements on earth, so when a leak occurs the gas rapidly disperses upwards. This makes ignition less likely, but a further difference is that hydrogen ignites and burns more easily than petrol or diesel. In fact, even a spark of static electricity from a person’s finger is enough to set off an explosion when hydrogen is available. Hydrogen flame is also invisible, so it is hard to pin-point where is the actual ‘fire’ is, but it generates a low radiant heat due to the absence of carbon and tends to burn out quickly.

Hydrogen is odourless, colourless and tasteless, so leaks are hard to detect using human senses alone. Hydrogen is non-toxic, but in indoor environments like battery storage rooms, it may build up and cause asphyxiation by displacing oxygen. This danger can be offset to some extent by adding odorants to hydrogen fuel, giving it an artificial smell and alerting users in case of a leak. But as hydrogen disperses quickly, the odorant is unlikely to travel with it. Hydrogen leaking indoors quickly collects, initially at ceiling level and eventually fills up the room. Therefore, the placement of gas detectors is key in early detection of a leak.

Hydrogen is usually stored and transported in liquified hydrogen tanks. The last concern is that because it is compressed, liquid hydrogen is extremely cold. If hydrogen should escape from its tank and come in contact with skin it can cause severe frostbite, or even the loss of extremities.

Which sensor technology is best for detecting hydrogen?

Crowcon has a wide range of products for the detection of hydrogen. The traditional sensor technologies for flammable gas detection are pellistors and infrared (IR). Pellistor gas sensors (also called catalytic bead gas sensors) have been the primary technology for detecting flammable gases since the 1960s and you can read more about pellistor sensors on our solution page. However, their key disadvantage is that in low oxygen environments, pellistor sensors will not function properly and may even fail. In some installations, pellistors are at risk of being poisoned or inhibited, which leaves workers unprotected. Also, pellistor sensors are not fail-safe, and a sensor failure will not be detected unless test gas is applied.

Infrared-type sensors are a reliable way to detect flammable hydrocarbons in low oxygen environments. They are not susceptible to being poisoned, so IR can significantly enhance safety in these conditions. Read more about IR sensors on our solution page, and the differences between pellistors and IR sensors in the following blog.

Just as pellistors are susceptible to poisoning, IR sensors are susceptible to severe mechanical and thermal shock and are also strongly affected by gross pressure changes. Additionally, IR sensors cannot be used to detect hydrogen. So the best option for hydrogen flammable gas detection is molecular property spectrometer (MPS™) sensor technology. This does not require calibration throughout the life cycle of the sensor, and since MPS detects flammable gases without the risk of poisoning or false alarms, it can significantly save on total cost of ownership and reduce interaction with units, resulting in peace of mind and less risk for operators. Molecular property spectrometer gas detection was developed at the University of Nevada and is currently the only gas detection technology able to detect multiple flammable gases, including hydrogen, simultaneously, very accurately and with a single sensor.

Read our white paper to find out more about our MPS sensor technology, and for more info on hydrogen gas detection visit our industry page and have a look at some of our other hydrogen resources:

What do you need to know about Hydrogen?

Green Hydrogen – An Overview

Blue Hydrogen – An Overview

Xgard Bright MPS provides hydrogen detection in energy storage application

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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

A pellistor is based on a Wheatstone bridge circuit, and includes two “beads”, both of which encase platinum coils. One of the beads (the ‘active’ bead) is treated with a catalyst, which lowers the temperature at which the gas around it ignites. This 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 it, so gas concentration as a percentage of its lower explosive limit (%LEL*) can be accurately determined.

The hot bead and electrical circuitry are contained in flameproof sensor housing, behind the sintered metal flame arrestor (or sinter) through which the gas passes. Confined within this sensor housing, which maintains an internal temperature of 500°C, controlled combustion can occur, isolated from the outside environment. In high gas concentrations, the combustion process can be incomplete, resulting in a layer of soot on the active bead. This will partially or completely impair performance. Care needs to be taken in environments where gas levels over 70% LEL may be encountered.

For more information about gas sensor technology for flammable gases, read our comparison article on pellistors vs Infrared gas sensor technology: Are silicone implants degrading your gas detection?.

*Lower Explosive Limit – Learn more

Click in the top right hand corner of the video, to access a file that can be downloaded.

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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.

Continue reading “Are Silicone Implants Degrading your Gas Detection?”

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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.

Continue reading “Cross Calibration of Pellistor (Catalytic Flame) Sensors‡”

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