Google is your friend!!!!!!! :smartass: :smartass: :smartass:
Lower Explosive Limit
Lower Explosive Limit (LEL): The lowest concentration (percentage) of a gas or a vapor in air capable of producing a flash of fire in presence of an ignition source (arc, flame, heat). At a concentration in air below the LEL there is not enough fuel to continue an explosion. Concentrations lower than the LEL are "too lean" to explode but may still deflagrate. Methane gas has a LEL of 4.4% (at 138 degrees C) by volume, meaning 4.4% of the total volume of the air consists of methane. At 20 degrees C the LEL is 5.1 % by volume. If the atmosphere has less than 5.1% methane, an explosion cannot occur even if a source of ignition is present. When methane (CH4) concentration reaches 5.1% an explosion can occur if there is an ignition source. LEL concentrations vary greatly between combustible gases.
Percentage reading on combustible air monitors should not be confused with the LEL concentrations. Explosimeters designed and calibrated to a specific gas may show the relative concentration of the atmosphere to the LEL - the LEL being 100%. A 5% displayed LEL reading for methane, for example, would be equivalent to 5.1% multiplied by 5%, or approximately 0.25% methane by volume at 20 degrees C. Control of the explosion hazard is usually achieved by sufficient natural or mechanical ventilation, to limit the concentration of flammable gases or vapors to a maximum level of 25% of their Lower Explosive or Flammable Limit.
[edit] Upper Explosive Limit
Upper Explosive Limit (UEL): Highest concentration (percentage) of a gas or a vapor in air capable of producing a flash of fire in presence of an ignition source (arc, flame, heat). Concentration higher than UFL or UEL are "too rich" to burn.