LEL stands for Lower Explosive Limit — the lowest concentration of a combustible gas or vapor in air that can ignite. Understanding LEL (and how it relates to ppm and % volume) is essential for anyone working around flammable atmospheres. This guide explains LEL in plain English.
What does LEL mean?
Every flammable gas has a range in which it will burn. Below the Lower Explosive Limit (LEL) there is too little fuel to ignite; above the Upper Explosive Limit (UEL) there is too little oxygen. For methane, the LEL is about 5% by volume in air — so 5% volume methane equals 100% LEL.
%LEL vs % volume vs ppm
These three units describe the same gas at different scales:
- % volume — the actual fraction of the air that is the gas (methane LEL = 5% vol).
- %LEL — the fraction of the way to the explosive limit. 100% LEL methane = 5% vol; 10% LEL methane = 0.5% vol.
- ppm — parts per million, used for low concentrations (10,000 ppm = 1% vol).
Gas detectors display combustibles in %LEL because it tells you directly how close you are to an explosive atmosphere.
Why detectors alarm at 10% LEL
A typical first alarm is set at 10% LEL — only one-tenth of the way to an explosive mixture. That conservative margin gives workers time to evacuate and ventilate long before the atmosphere becomes dangerous.
Methane vs pentane LEL scales (correlation factors)
Combustible sensors are calibrated to one reference gas (often methane or pentane) but respond differently to other gases. Manufacturers publish correlation factors to convert the reading for the actual gas present. This is why the calibration gas you use must match what your detector and procedure specify.
LEL calibration gas
LEL sensors are calibrated with a combustible gas at a known %LEL — commonly 2.5% methane (50% LEL) or a pentane mix. Trigas USA offers methane / LEL calibration gas and full 4-gas mixes with the LEL component included. Learn more in what gases a 4-gas monitor detects.
Shop LEL / methane calibration gas →
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Frequently asked questions
What does LEL mean?
Lower Explosive Limit — the lowest concentration of a combustible gas in air that can ignite. For methane it is about 5% by volume.
What is the difference between %LEL and % volume?
%LEL is the fraction of the way to the explosive limit; % volume is the actual gas fraction. For methane, 100% LEL = 5% volume.
Why do gas detectors alarm at 10% LEL?
To give a large safety margin — 10% LEL is only one-tenth of the explosive concentration, so workers can act well before danger.