# Lower Explosive Limit (LEL) #fundamental #gas-detection #safety The Lower Explosive Limit (LEL) is the minimum concentration of a flammable gas or vapor in air that can ignite when exposed to an ignition source. Below the LEL, the mixture is too lean to burn. ## Key Points - Each flammable gas has its own unique LEL value - Methane has an LEL of approximately 5% by volume - Gas detectors measure concentrations as a percentage of LEL (%LEL) - Alarm thresholds are typically set at 20% LEL (Alarm Range 1) and 40% LEL (Alarm Range 2) - At 20% LEL, countermeasures like ventilation are activated - At 40% LEL, emergency measures like shutdown are triggered ## Measurement Gas detectors indicate LEL concentrations as: - %LEL (percentage of Lower Explosive Limit) - PPM (parts per million) - %Vol (percentage by volume) ## Safety Significance Monitoring LEL concentrations is critical for preventing explosions in industrial environments. Early detection allows for preventive actions before dangerous levels are reached. ## Diagram  *Diagram showing flammable range (LEL to UEL) and alarm threshold levels (20% LEL and 40% LEL)* ## Related Concepts - [[Upper Explosive Limit - UEL]] - [[Alarm Thresholds]] - [[Methane]] - [[Catalytic Bead Sensor]]