Meaning of 0–100% LEL Measuring Range of Combustible Gas Detector
LEL stands for Lower Explosive Limit. It refers to the minimum concentration at which a flammable gas can explode when exposed to an open flame in air, abbreviated as %LEL. The full English term is Lower Explosive Limit.
The maximum concentration at which a flammable gas can explode in air with an ignition source is called the Upper Explosive Limit, abbreviated as %UEL. The full English term is Upper Explosive Limit.
Definition of Lower Explosive Limit
A flammable gas poses no explosion risk when its concentration is too low or too high. Combustion or explosion only occurs when it mixes with air (more precisely, with oxygen) in a specific proportional mixture. Combustion is a violent oxidation reaction accompanied by light and heat, which requires three essential conditions: a. Combustible substance (fuel gas); b. Oxidizer (oxygen); c. Ignition source (temperature).
Combustion of flammable gases is divided into two types:
Diffusion combustion: Volatilized or leaked flammable gas from equipment mixes with an ignition source and burns.
Premixed combustion: Flammable gas fully mixes with air before ignition. This reaction is violent and rapid, usually generating tremendous pressure and loud noise, also defined as explosion. There is no rigid dividing line between combustion and explosion.
Authoritative institutions and experts have conducted combustion and explosion analysis on known flammable gases and formulated their explosion limits, which are split into Upper Explosive Limit (UEL) and Lower Explosive Limit (LEL). Below the LEL, the proportion of combustible gas in the mixture is insufficient to sustain combustion or explosion. Above the UEL, the oxygen content in the mixture is inadequate to trigger a reaction. In addition, combustion and explosion of flammable gases are also affected by factors such as gas pressure, temperature and ignition energy. Explosion limits are generally expressed as volume percentage concentrations.
Explosion limit is the general term for LEL and UEL. A flammable gas can only explode when its airborne concentration falls between the LEL and UEL. No explosion will take place if the concentration is below LEL or above UEL. For safety monitoring, the alarm threshold is normally set below 25% LEL.
Fixed flammable gas detectors are commonly equipped with two alarm setpoints (depending on the model of the alarm controller):
10% LEL for the first-level alarm
25% LEL for the second-level alarm
