A: The lower explosive limit (LEL) of a gas or a vapour, is the lowest concentration (in air) that is needed for the gas to ignite and explode. For example, propane can explode when it reaches 2.1 per cent of the air, by volume. At 2.1%v/v, propane has reached 100% of its lower explosive level. 50% LEL propane is 0.6%v/v.
Most flammable gas detectors measure the percent of LEL present. For example, the MicroClip XL uses a flammable sensor that reports the percent of the LEL present in the atmosphere being monitored.
Some sensors are capable of measuring methane by volume, as well as percent LEL.
There are two explosive limits for any gas or vapor, the lower explosive limit (LEL) and the upper explosive limit (UEL). At concentrations in air below the LEL there is not enough fuel to continue an explosion; at concentrations above the UEL the fuel has displaced so much air that there is not enough oxygen to begin a reaction. Concentrations of explosive gases are often given in terms of percent of lower explosive limit (%LEL).