What are LEL and UEL? Understand the explosion range

Flammable gases and vapours ignite in air only inside a concentration window. The lower edge is LEL (lower explosive limit), the upper edge UEL (upper explosive limit). Between them is the explosive (flammable) range. Field detectors usually display %LEL; calibration documents that scale.

LEL — lower explosive limit

• Below LEL the mix is too lean to burn.
• Above LEL (inside the range) a mix can burn if an ignition source exists.
• Concentration is expressed as % by volume in air.

Methane example: LEL ≈ 5% vol. Below 5% it will not burn; from 5% up to UEL it can.

UEL — upper explosive limit

• Above UEL the mix is too rich to burn (not enough oxygen).
• Below UEL (inside the range) it can burn with an ignition source.

Methane example: UEL ≈ 15% vol. Above 15% it will not burn; down to LEL it can.

Safety effect

Below LEL a flammable atmosphere is not expected — that is not a licence to skip ventilation, hot-work controls or monitoring. Between LEL and UEL any gas-air mix can explode if ignited. Above UEL the cloud is “too rich”, but dilution can bring it back through the range; do not treat UEL-plus as safe.

Common gases (% vol)

Figures are literature summaries; site procedures and SDS prevail. Detector calibration keeps displayed %LEL close to the real gas. ESA calibrates flammable-gas cells under TURKAK AB-0319-K.

Request a calibration quote · TURKAK gas calibration · 0312 385 52 81 · 0533 659 91 60