How do LEL sensors work? Catalytic bead and infrared combustible-gas sensors, what LEL signifies, LEL to ppm, and which sensor detects LPG or methane
An LEL sensor reads how close the air is to burning, as a percentage of the lower explosive limit of the gas it was calibrated to. The common one is a catalytic bead: a tiny heated platinum coil coated with a catalyst burns a trace of the combustible gas that reaches it, the bead gets hotter, its electrical resistance rises against a matched reference bead, and the difference is displayed as percent LEL. It needs oxygen to burn the gas, reads most hydrocarbons and hydrogen, and can be poisoned by silicone, chlorinated and sulfur compounds, as RKI Instruments’ FAQ sets out. The other is infrared: the gas absorbs infrared light at its own wavelength and the loss across the sample path is measured; it needs no oxygen and cannot be poisoned, but cannot read hydrogen, acetylene or ammonia. LEL signifies the lower explosive limit itself, 5 percent by volume for methane, and a reading of 10 percent LEL means one tenth of that, 0.5 percent methane, which is OSHA’s hazardous-atmosphere line in a confined space. Methane and LPG are both read on a catalytic sensor; a methane-calibrated unit reads propane and butane through a correction factor, or is calibrated to isobutane instead.
RKI Instruments NC-6264A Combustible Gas (LEL) Sensor, 0-100% LEL, for GX-2009RKI catalytic LEL sensor, 0 to 100 percent LEL, as RKI states itWhy the reading is a percentage of a percentage. Every flammable gas burns only between its lower and upper explosive limits, and the limits differ by gas (RKI’s chart: methane 5.0 to 15.0 percent by volume, hydrogen 4.0 to 75.6, pentane 1.5 to 7.8). A monitor cannot know which gas it is seeing, so it is calibrated to one, methane as RKI’s standard, and reports every reading as a fraction of that gas’s LEL on a 0 to 100 scale. RKI’s FAQ puts it plainly: methane has an LEL of 5 percent, so an alarm at 10 percent LEL trips at 0.5 percent gas by volume while the display reads 0 to 100 percent LEL. The scale is what OSHA’s rule is written in: 1910.146(b) defines a hazardous atmosphere as a flammable gas, vapor or mist above 10 percent of its lower flammable limit, and RKI’s first alarm is set there, with the second and third at 25 and 50 percent.
The two combustible-gas sensors, as RKI Instruments describes them
What LEL signifies, and how to convert it
- LEL is the lower explosive limit: the smallest concentration of a flammable gas in air that can ignite. Below it the mixture is too lean; above the UEL too rich; between them it can explode, with the most energy near the midpoint (RKI’s FAQ). LFL and UFL are the same figures under other names.
- A reading of X percent LEL means X percent of the gas’s LEL is present: for methane 10 percent LEL is 0.5 percent gas (5,000 ppm), 20 percent LEL is 1.0 percent (10,000 ppm), 100 percent LEL is 5 percent (50,000 ppm).
- To convert to ppm: LEL in percent × 10,000 gives ppm at 100 percent LEL; multiply by the reading ÷ 100. Propane’s LEL of 2.1 percent (NIOSH) is 21,000 ppm, so 10 percent LEL of propane is 2,100 ppm, which is also NIOSH’s IDLH for propane; pentane’s 1.5 percent is 15,000 ppm.
- Which sensor detects methane: a catalytic bead calibrated to methane (RKI’s standard) or an infrared CH4 sensor; which detects LPG: a catalytic bead reading propane and butane through the correction factor in the manual, or calibrated to isobutane as the GX-Force allows, or an infrared hydrocarbon sensor.
- Why hydrogen sulfide is not an LEL question: its LEL is 4.0 percent by volume, 40,000 ppm, against a 20 ppm OSHA ceiling, so the toxic sensor, not the LEL sensor, is the warning.
- Response and accuracy, as RKI prints them: 0 to 100 percent LEL in 1 percent steps, ± 5 percent of reading or ± 2 percent LEL, T90 within 30 seconds for methane (40 seconds for hydrocarbons on the GX-Force); readings below 2 percent LEL are suppressed to zero on the GX-Force.
LEL and UEL of common combustible gases, as RKI prints them (percent by volume in air)
| Gas or vapor | LEL | UEL |
|---|---|---|
| Acetone | 2.15% | 13.0% |
| Acetylene | 2.5% | 100% |
| Benzene | 1.2% | 8.0% |
| Butadiene | 1.1% | 12.5% |
| Ethane | 3.0% | 15.5% |
| Ethyl alcohol | 3.3% | 19.0% |
| Ethyl ether | 1.7% | 36.0% |
| Ethylene | 2.7% | 36.0% |
| Hexane | 1.1% | 7.5% |
| Hydrogen | 4.0% | 75.6% |
| Isobutane | 1.8% | 8.5% |
| Isopropyl alcohol | 2.0% | 12.7% |
| Methane | 5.0% | 15.0% |
| Methanol | 6.0% | 36.0% |
| Pentane | 1.5% | 7.8% |
| Propylene | 2.0% | 11.1% |
| Toluene | 1.2% | 7.0% |
Source: RKI Instruments FAQ page (rkiinstruments.com/resources/faqs, rkiinstruments.com, fetched Nov 8 2026), "What is %LEL and %UEL". RKI notes the figures are empirically determined and that other authorities quote slightly different values; the NIOSH Pocket Guide figures elsewhere on this page are one such source.
Questions people ask
How do LEL sensors work?
A catalytic bead burns a trace of the gas on a heated catalyst and reports the rise in the bead’s resistance as percent of the lower explosive limit; an infrared sensor measures the light the gas absorbs. Catalytic needs oxygen and can be poisoned; infrared needs neither but cannot read hydrogen, as RKI Instruments’ FAQ sets out.
How does an LEL detector work?
The sensor is calibrated to one gas, usually methane, and shows every reading as a fraction of that gas’s LEL on a 0 to 100 scale, alarming at the set points loaded (RKI: 10, 25 and 50 percent). Other gases read through a correction factor from the manual.
What does LEL signify?
The lower explosive limit, the least concentration of a flammable gas in air that can ignite; a monitor reading in percent LEL signifies how far the air is toward that point. OSHA’s confined-space line is 10 percent of the LEL.
What is a LEL detector?
A combustible-gas monitor, single-gas or as the LEL channel of a four-gas unit, with a catalytic or infrared sensor; the band on this page lists both kinds with their prices.
Which sensor is used to detect LPG gas?
A catalytic bead combustible sensor, reading propane and butane as percent LEL through the manual’s correction factor or calibrated to isobutane (an option on RKI’s GX-Force), or an infrared hydrocarbon sensor.
Which sensor detects methane gas?
A catalytic bead calibrated to methane, RKI’s standard, or an infrared CH4 sensor, which RKI uses where there is little oxygen, in landfill and digester gas.
How is LEL determined?
By test: the limits are empirically determined, RKI’s FAQ notes, so different authorities quote slightly different figures (hydrogen 75 percent UEL in the NIOSH Pocket Guide, 75.6 in RKI’s chart). The monitor is then calibrated with a certified gas at a known fraction of the LEL, 50 percent LEL methane in RKI’s kits.
How do I convert LEL to ppm?
LEL in percent by volume × 10,000 is ppm at 100 percent LEL; multiply by the reading over 100. Methane: 10 percent LEL is 5,000 ppm. Propane: 10 percent LEL is 2,100 ppm.
What does an LEL sensor detect or measure?
Any flammable gas or vapor that reaches it and burns on the bead (catalytic) or absorbs at its wavelength (infrared), reported as percent of the calibration gas’s LEL. It does not identify the gas and does not read toxic gases at ppm levels.
Why does my LEL sensor read zero in a tank full of fuel vapor?
Above the UEL the mixture is too rich to burn on the bead, and without oxygen a catalytic sensor cannot read at all; RKI’s FAQ describes a dilution fitting or an infrared sensor for those spaces, with the oxygen channel confirming the purge.
What is the working principle of combustible gas detectors?
Catalytic combustion on a heated bead measured as a resistance change, or infrared absorption measured as a light loss, each calibrated to a reference gas and displayed as percent LEL; the four-sensor guide on this site sets them beside the electrochemical and PID sensors.
Related guides
What is LEL?And what 10 percent LEL means on a monitorLEL and UEL by gasMethane, propane, hydrogenWhat does 20 percent LEL mean?The 3, 10, 25 and 50 percent readingsThe four types of gas detector sensorsAnd how to read a 4-gas meterGas detectors for landfills and wastewaterMethane, H2S and oxygenBump test vs calibrationWhat each does and how oftenThe ratings, test results and standards shown here are the manufacturers' published values, reproduced as published. Nothing on this page is a recommendation by Bayshore Safety that any product is suitable for a particular task, hazard or workplace. Selecting safety products and protective equipment is the responsibility of the end user and the employer, based on their own hazard assessment and the regulations that apply to their work. Confirm every value on the manufacturer's own documentation before use.