Guide by Vistec Gas Detection. Last updated October 2026.
LEL, the lower explosive limit, is the number behind every flammable gas reading on a detector. This guide explains what LEL and %LEL mean, gives the limits for common gases, and shows how alarm levels and sensor types fit together, for anyone who manages or specifies flammable gas detection.
A flammable gas only burns or explodes when it is mixed with air in the right proportion. The lowest concentration that can ignite is the lower explosive limit (LEL). The highest is the upper explosive limit (UEL). Below the LEL the mixture is too lean to burn. Above the UEL there is not enough oxygen, so the mixture is too rich. Between the two is the flammable range.
You will also see the terms LFL and UFL, lower and upper flammable limit. For gas detection purposes they mean the same thing. Honeywell’s technical guide notes that higher pressure, higher temperature or extra oxygen will generally widen the flammable range, so the published limits apply at normal conditions.
Flammable gas detectors rarely show percent by volume. They show percent of the lower explosive limit, written %LEL. A reading of 100% LEL means the gas has reached its lower explosive limit. A reading of 10% LEL means the gas is at one tenth of that level.
Take methane, the main component of natural gas. The European data in EN 60079-20-1 gives methane an LEL of 4.4% by volume, so 10% LEL of methane is 0.44% by volume. The HSA’s Code of Practice for Working in Confined Spaces refers to exactly this kind of instrument, a portable device designed to measure flammable atmospheres as a percentage of the LEL.
Not every instrument uses the same basis. The manual for one widely used portable detector, written to North American conventions, equates 50% LEL with 2.5% methane by volume, which assumes an LEL of 5%. When you compare readings or set alarms, check which LEL value your instrument and its calibration are based on.
The figures below are taken from the flammable gas data in Honeywell’s technical guide, which references EN 60079-20-1. Values can differ slightly between data sources, so always use the safety data sheet and the figures your detector is set up for.
In a normal workplace there should be no flammable gas in the air, or only a trace. Detection is therefore set up to cover the range from zero to the LEL, and to act well before the LEL is reached. Honeywell’s guide says shut-down or site clearance typically happens at under 50% LEL, to leave a safety margin.
For fixed systems with two alarm levels, the same guide gives typical settings of 20 to 25% LEL for the first alarm and 50 to 55% LEL for the second. Portable detectors are supplied with factory setpoints that vary by region and can be changed. The right settings for your site come from your risk assessment, the gas involved and what each alarm is meant to trigger, such as ventilation, a gas shut-off valve or evacuation.
Most %LEL detectors use one of two technologies. A catalytic sensor, often called a pellistor, burns a tiny amount of gas on a heated bead. The heat changes the electrical resistance of the bead, and that change is read as %LEL. Catalytic sensors respond to a wide range of flammable gases but can be poisoned by silicones, sulphur compounds and lead compounds, which is why regular checks matter. The HSA confined spaces code also advises testing oxygen first, because other gas monitoring equipment may only be accurate in a normal oxygen atmosphere.
An infrared sensor measures how much infrared light the gas absorbs. Infrared detectors respond quickly, can be designed to be unaffected by catalyst poisons and work in inert atmospheres. They cannot detect hydrogen, so hydrogen needs a different sensor type. Our fixed gas detectors range covers both technologies, and portable %LEL monitoring for entry work is covered on our confined space gas monitors page.
Natural gas is mostly methane. The European data in EN 60079-20-1 gives methane an LEL of 4.4% by volume. Some instruments, particularly those set up to North American conventions, use 5%.
It means the flammable sensor sees no flammable gas. It tells you nothing about oxygen levels or toxic gases such as carbon monoxide or hydrogen sulphide, which need their own sensors. A multi-gas monitor checks all of these together.
No. It will not ignite while it stays too rich, but as soon as fresh air gets in, for example when a hatch or door is opened, it can be diluted back into the flammable range. Treat any reading above the LEL as dangerous.
LPG is commercial propane, commercial butane or a mix of the two. The European data gives propane an LEL of 1.7% by volume, well below methane. Propane and butane are normally heavier than air, so sensors for LPG usually sit low, although gas released hot or under pressure can rise at first.
A catalytic or infrared sensor responds differently to different gases. BW Technologies by Honeywell, for example, factory calibrates its combustible sensor to methane and advises calibrating with the appropriate gas if a different flammable gas is being monitored.
A reading that rises sharply and then falls or becomes erratic can mean the gas concentration is beyond the top of the sensor’s range. BW Technologies by Honeywell warns that high off-scale readings may indicate an explosive concentration. Leave the area and act on it.
We supply, commission and calibrate flammable gas detection, with alarm levels set to suit the gas and your site. Call 01 289 7720 or email info@vistec.ie.
© 2026 Vistec Gas Detection Ltd
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