Guide · Gas detection · OSHA 1910.146(b) · NIOSH Pocket Guide · RKI Instruments data
What does 20 percent LEL mean, why does a detector read 3 percent LEL in clean air, and where do the 10, 25 and 50 percent alarms come from?
A reading of 20 percent LEL means the air holds one fifth of the gas it would need to ignite: for methane, whose lower explosive limit is 5 percent by volume, that is 1 percent methane in air. It is twice OSHA’s hazardous-atmosphere line, which 29 CFR 1910.146(b) sets at anything above 10 percent of the lower flammable limit, and it sits between the first and second alarms RKI Instruments presets on its monitors, 10 and 25 percent LEL, with a third at 50 percent and an over-range alarm at 100 percent. A steady 3 percent LEL in air you believe is clean is either a small amount of combustible gas or a zero that has drifted: RKI’s GX-Force suppresses readings below 2 percent LEL to zero, so 3 percent is above the suppression band and the manual’s answer is a fresh-air adjustment (AIR CAL) in a place with 20.9 percent oxygen and no gas, followed by a bump test if the reading returns. Fifty percent LEL is half way to an explosive mixture and is RKI’s third alarm; nothing in the OSHA rule allows work in a space above 10 percent.
Bayshore Safety. Industrial safety and PPE supplier since 2017. · Current as of November 2026 · 29 CFR 1910.146(b), definition of hazardous atmosphere (current e-CFR); NIOSH Pocket Guide to Chemical Hazards (current online edition), lower and upper explosive limits for methane, propane, butane, hydrogen and hydrogen sulfide; RKI Instruments FAQ page (rkiinstruments.com/resources/faqs, rkiinstruments.com, fetched Nov 8 2026), "What is %LEL and %UEL" and "Why is the LEL important in combustible gas detection?"; RKI Instruments GX-3R Pro product page and specification table (rkiinstruments.com/product/gx-3r-pro-gas-detector, rkiinstruments.com, fetched Nov 8 2026); RKI Instruments GX-Force product page and specification table (rkiinstruments.com/product/gx-force-portable-4-gas-monitor, rkiinstruments.com, fetched Nov 8 2026); RKI Instruments GX-Force operator's manual (PDF, fetched Nov 8 2026), Chapter 5, User Mode and Calibration (zero suppression values and the AIR CAL procedure)
Why the percent sign confuses people. A combustible-gas monitor does not read percent gas in air; it reads percent of the lower explosive limit, a scale that runs 0 to 100 for every gas and is only converted back to a gas concentration by the gas’s own LEL. RKI puts it plainly in its FAQ: methane has an LEL of 5 percent by volume, so an alarm at 10 percent LEL trips at 0.5 percent methane, and the detector is calibrated over 0 to 5 percent gas while displaying 0 to 100 percent LEL. The same arithmetic gives 20 percent LEL as 1.0 percent methane, 50 percent LEL as 2.5 percent, and for propane (LEL 2.1 percent) 20 percent LEL is 0.42 percent propane. The alarm levels are the manufacturer’s factory presets and the employer’s to adjust; the 10 percent line is OSHA’s and is not adjustable, because 1910.146(b) defines a hazardous atmosphere as a flammable gas, vapor or mist above 10 percent of its LFL and the permit-space rules turn on that definition.
What each reading means on the scale
Methane is the gas RKI calibrates to as standard; the gas-in-air figures below use its 5 percent LEL from the NIOSH Pocket Guide.
0 to 2 percent LELClean air as the monitor sees it. RKI’s GX-Force suppresses combustible readings below 2 percent LEL (and oxygen within 0.5 percent of 20.9, H2S below 0.3 ppm, CO below 2 ppm) to zero, so the display sits at 0 in fresh air.
3 percent LELAbove the suppression band, so the instrument is reporting something: a trace of gas, a drifted zero or a sensor that is ageing. The GX-Force manual’s fresh air adjustment (AIR CAL) resets the zero in an environment of 20.9 percent oxygen free of toxic and combustible gas; a reading that comes back is gas or a sensor to bump test.
10 percent LEL0.5 percent methane in air. OSHA’s hazardous-atmosphere threshold in 1910.146(b) and RKI’s first alarm. Above it a permit space may not be entered or worked in; the standard’s alternate-entry procedure requires ventilation to keep the space below it.
20 percent LEL1.0 percent methane in air, twice the OSHA line. Not an RKI preset, but RKI’s FAQ notes that typical safety standards have a detector warn at 10 to 20 percent of the LEL; many hot-work permits are written to it.
25 percent LEL1.25 percent methane. RKI’s second alarm on the GX-3R, GX-3R Pro and GX-Force.
50 percent LEL2.5 percent methane, half way to an ignitable mixture. RKI’s third alarm.
100 percent LEL and the over-range alarmThe LEL itself: 5 percent methane. RKI sets an over-range alarm here. Above the UEL (15 percent for methane) the mixture is too rich to burn and a catalytic sensor can read low or zero; RKI’s FAQ notes a catalytic sensor flooded with a very high concentration may show an erroneously low or no response and may be damaged.
What RKI Instruments states about its LEL channel
From the RKI pages and manuals named in the byline:
The combustible sensor in the GX-3R, GX-3R Pro and GX-Force is catalytic, reads 0 to 100 percent LEL in 1 percent steps within ± 5 percent of reading or ± 2 percent LEL, and is calibrated to methane as standard; RKI's manuals carry conversion factors for other hydrocarbons when the instrument is methane-calibrated, and the GX-Force can be set up and calibrated to isobutane instead.
Preset alarms: first 10 percent LEL, second 25 percent, third 50 percent, over-range 100 percent, user adjustable in User Mode.
Response time (T90): within 30 seconds for methane on all three, 40 seconds for hydrocarbons on the GX-Force.
The GX-Force's Leak Check mode reads combustibles in ppm on the same catalytic sensor, at a display range of 500, 1,000, 2,000 or 5,000 ppm, for finding small leaks at fittings and valves; RKI states the GX-Force has no percent-volume sensor.
RKI's FAQ: explosion needs fuel, oxygen and ignition; each gas has its own LEL and UEL, with the most explosive energy near the midpoint; the figures are empirically determined and authorities quote slightly different values.
RKI Instruments GX-3R Pro product page and specification table (rkiinstruments.com/product/gx-3r-pro-gas-detector, rkiinstruments.com, fetched Nov 8 2026); RKI Instruments GX-Force product page and specification table (rkiinstruments.com/product/gx-force-portable-4-gas-monitor, rkiinstruments.com, fetched Nov 8 2026); RKI Instruments GX-Force operator's manual (PDF, fetched Nov 8 2026), Chapter 5, User Mode and Calibration; RKI Instruments FAQ page (rkiinstruments.com/resources/faqs, rkiinstruments.com, fetched Nov 8 2026).
LEL and UEL of the gases people ask about, and what 10, 20 and 50 percent LEL mean in gas
Gas
LEL (% in air)
UEL (% in air)
10% LEL
20% LEL
50% LEL
Methane (natural gas)
5.0
15.0
0.5% gas
1.0% gas
2.5% gas
Propane
2.1
9.5
0.21%
0.42%
1.05%
Butane
1.6
8.4
0.16%
0.32%
0.8%
LPG (propane and butane blends)
Between the two, by blend: about 1.6 to 2.1
About 8.4 to 9.5
About 0.16 to 0.21%
About 0.32 to 0.42%
About 0.8 to 1.05%
Hydrogen
4.0
75.0
0.4%
0.8%
2.0%
Hydrogen sulfide
4.0
44.0
0.4% (4,000 ppm; the toxic ceiling is 20 ppm)
0.8%
2.0%
Pentane (a common calibration gas)
1.5
7.8
0.15%
0.3%
0.75%
RKI prints hydrogen at 4.0 to 75.6 percent and isobutane at 1.8 to 8.5 percent in its own chart; the small differences are the different test procedures RKI’s FAQ mentions.
Source: NIOSH Pocket Guide to Chemical Hazards (current online edition) (methane, propane, n-butane, hydrogen, hydrogen sulfide, n-pentane). LPG has no single LEL: it is a blend, so the limits sit between those of its propane and butane. Percent LEL to percent gas is arithmetic, not a measurement.
Questions people ask
What is 20% LEL?
One fifth of the gas needed to ignite: for methane, whose LEL is 5 percent by volume, 20 percent LEL is 1 percent methane in air. It is twice OSHA’s 10 percent hazardous-atmosphere line in 1910.146(b) and sits between RKI Instruments’ first and second LEL alarms (10 and 25 percent).
What does 3% LEL mean on a gas detector?
Either a small amount of combustible gas or a zero that has drifted. RKI’s GX-Force suppresses LEL readings below 2 percent to zero, so 3 percent is above the suppression band; the manual’s answer is a fresh air adjustment (AIR CAL) in clean air with 20.9 percent oxygen, and a bump test if the reading comes back.
My gas detector says 3 LEL. Is this normal?
Not in clean air. A monitor zeroed in clean air reads 0 percent LEL; 3 percent is a reading. Zero it in fresh air as the manual describes; if it returns there is gas present or the sensor needs a bump test and calibration.
What is the lower explosive limit (LEL) for propane?
2.1 percent by volume in air (NIOSH Pocket Guide), UEL 9.5 percent. On a propane-calibrated monitor 20 percent LEL is 0.42 percent propane; on a methane-calibrated monitor a correction factor from the manual applies.
What is a normal LEL?
Zero. Clean air holds no combustible gas, so a working monitor reads 0 percent LEL; RKI suppresses readings under 2 percent to zero. Anything above zero is gas or drift, and 10 percent is the OSHA line.
How much gas is 20% LEL?
For methane 1.0 percent by volume (10,000 ppm); for propane 0.42 percent (4,200 ppm); for hydrogen 0.8 percent (8,000 ppm); for pentane 0.3 percent. The table on this page gives 10, 20 and 50 percent LEL in percent gas for each.
What does 50% LEL mean?
Half way to an ignitable mixture: 2.5 percent methane. It is RKI’s third LEL alarm. Nothing in the OSHA rule allows work in a confined space above 10 percent, so 50 percent is an evacuation reading, not a working level.
What does "4 LEL" mean on a gas detector?
4 percent of the lower explosive limit, 0.2 percent methane: below every alarm and below OSHA’s line, but combustible gas is present or the zero has drifted, and the reading is checked the same way as 3 percent.
What are the LEL and UEL of hydrogen?
4.0 percent and 75 percent by volume (RKI’s chart prints 75.6). Hydrogen is read on a catalytic sensor; an infrared combustible sensor cannot see it.
What LEL percentage must work be stopped?
In a permit-required confined space, 10 percent of the LEL: above it the atmosphere is hazardous under 1910.146(b) and entrants exit on the alarm (h)(5). Hot work permits commonly stop work at 10 percent as well; RKI’s first alarm is set there.
Why 10, 25 and 50 percent?
They are RKI Instruments’ factory presets, adjustable in User Mode: the first at OSHA’s hazardous-atmosphere line, the second and third as escalating warnings before the mixture can ignite at 100 percent. RKI’s FAQ notes typical standards have a detector warn at 10 to 20 percent of the LEL.
The 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.
Need monitors matched to the gases on your permit?Call (925) 215-7337 or email us the gases in your hazard assessment and where people work, and we will quote the monitors, calibration gas and docks with the manufacturer figures listed. We service and calibrate portable gas instruments.
RKI Instruments 72-PAX-D GX-3R Pro 4-Gas Monitor, LEL/O2/H2S/CO, Li-ionFour-gas monitor RKI presets at 10, 25 and 50 percent LEL (RKI data)