LEL — WHAT IS IT?
Flammable gases are commonly hydrocarbons, made up of carbon and hydrogen atoms. Such gases can burn in the presence of oxygen, in some cases such as furnaces, this planned combustion is very controlled, while unplanned gas leaks could cause a violent reaction or explosion. Industry offers a measure of the degree that an environment may be explosive due to combustible gases present. So-called lower explosive limit (LEL) indicates the minimum concentration of these gases required in the air to be explosive. This number is typically represented as a percentage of the lower explosive limit where 100% is considered to be explosive if a spark or other combustion source were introduced. An LEL level of 0% indicates that no detectable level of flammable gases is present.
The G7 not only provides its user a real-time reading of LEL but also communicates these gas readings to the Safety Network. Should G7 detect a sufficient LEL reading to trigger a high gas alarm — for example, 20% — a safety alert is communicated in real-time to monitoring personnel, who then contact the worker to ensure his or her safe egress from the environment.
PELLISTOR LEL SENSORS
Pellistor sensors use controlled combustion to detect and measure a wide variety of flammable gases. Pellistor sensors contain within them two platinum coils each embedded in separate ceramic beads. The first bead is coated with a catalyst to promote oxidation when exposed to flammable gases. The second bead is treated to discourage catalytic oxidation, and acts as a reference. The first bead allows for the combustion of a very small amount of flammable gas — generating heat and changing the resistance of the platinum coil. The resistance change is proportional to the amount of flammable gas present in an environment and is translated into an LEL% reading on the detector’s screen.
Infrared (IR) sensors (sometimes referred to as optical sensors, or non-dispersive infrared/NDIR) detect the presence of flammable gases by precisely measuring the absorption of infrared light at specific frequencies by various hydrocarbon molecules. Inside the sensor, an infrared emitter passes light through two paths. One path is used to measure the absorption of light by gases, the other is used as a reference. Light detectors on both paths allow the LEL sensor to measure the amount of flammable or combustible gases present by comparing how much light is absorbed in each path.
PROS AND CONS OF PELLISTOR LEL SENSORS
Pellistor sensors are very well established in many industries as a staple of safety equipment. These sensors detect a broader range of gases, including hydrocarbons, hydrogen and acetylene. Pellistor LEL sensors detect flammable gases using a catalyst that oxidizes gases behind an explosion-proof screen. As a result, this process requires sufficient oxygen to cause the combustion and detect flammable gases. Pellistor LEL sensors also should not be used in environments where there may be oxygen deficiencies.
Prolonged exposure to combustible gases may cause a pellistor LEL sensor’s zero reading to shift, resulting in inaccurate readings. Pellistor LEL sensors do not positively confirm a sensor fault and instead falsely indicate a 0% LEL reading. Exposing pellistor sensors to high gas concentrations, even for short periods of time, may stress the sensor leading it to produce poor readings or even causing sensor failure.
PROS AND CONS OF IR LEL SENSORS
IR LEL sensors are an ideal choice for many working scenarios, including those where pellistor function would be limited. Long-living, infrared sensors are not susceptible to poisoning by chemicals in the environment. Further, IR LEL sensors do not suffer a shift in zero reading due to long-term exposure to combustible gases. Due to their measurement method, LEL sensors confirm a failure if the system is not working correctly. As they do not utilize combustion, these low power sensors perform well in low/no oxygen environments.
IR LEL sensors should not be used in environments with risk of exposure to hydrogen or acetylene, as their single-atom structure does not absorb infrared the same way as hydrocarbons, and therefore do not produce accurate sensor readings.
SUITABILITY OF LEL SENSORS FOR DIFFERENT INDUSTRIES
Both pellistor and IR LEL sensors can be used in many of the same applications and industries, but each has their specializations.
Pellistor sensors are ideal for use in environments where combustible hydrocarbon gases could be present without low-oxygen levels. Industries for pellistor use include oil & gas, telecom, manufacturing and wastewater industries. Applications that incorporate hydrogen and acetylene into processing will require pellistor LEL sensors, including manufacturing of metals, semiconductors, petrochemicals and foods, plus flame-cutting, welding, brazing and heating applications. To maximize sensor performance over its operating life, contaminants that could poison the sensor and consistent exposure to high combustible gas levels should be avoided.
Infrared sensors can also be used in many similar scenarios but with the additional benefit of operating in low-oxygen environments or environments with consistently higher levels of combustible gases. Infrared sensors are ideal for use in many environments, including those with high flammable gas percentages and/or low oxygen environments where pellistor sensors may not provide a long-term, reliable LEL measurement. IR sensors are also ideal for scenarios where contaminants could poison a pellistor sensor’s catalytic chemical process. These sensors are suitable for oil & gas applications where the presence of hydrogen sulfide (H2S) is possible, and the use of silicone defoamers may be common. IR sensors are also ideal for waste water processing facilities, where explosive methane can accumulate.
WE’RE HERE TO HELP
They key to complete gas detection coverage is understanding your environment and the benefits and limitations of each sensor type. No one sensor accommodates every situation. The Loner G7 cartridges support both IR and pellistor LEL sensors, delivering flexibility with coverage for diverse environments. With our cartridge replacement program, it’s easy to quickly switch out cartridges at the end of their service life with another pre-calibrated cartridge of your choosing, increasing efficiency and minimizing device down-time.
Let us know how we can help you with your gas detection program.
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