Lower Explosive LimitLEL

The Lower Explosive Limit — LEL — is the minimum concentration of a gas or vapour in air at which the mixture will ignite and propagate a flame, and below it the mixture is too lean to burn. The companion is the Upper Explosive Limit, the UEL, above which the mixture is too rich to burn, and the band between the LEL and the UEL is the flammable range the gas detector watches for. The detector alarms in per cent of the LEL, not in per cent of the gas, so the reading is the fraction of the way to the ignitable mixture, and the alarm points are typically 10 per cent LEL as the action level and 25 per cent LEL as the evacuation, set below the ignition threshold to give the worker time to leave.

The LEL is the number that governs the confined-space entry and the hot-work permit, because the atmosphere a worker puts a torch or a grinder into has to be below the LEL or the spark closes the circuit. The hazard that catches the worker is the oxygen-displaced atmosphere — the inert gas purge that drops the oxygen below the 19.5 per cent the body needs, which suppresses combustion and reads as a safe LEL while it asphyxiates the entrant — so the LEL reading is necessary but not sufficient, and the confined-space monitor reads the LEL, the oxygen, and the carbon monoxide and the hydrogen sulfide together. The calibration and the bump-test of the monitor are the controls that keep the reading honest. See hot work, confined space, and SDS.