Reference

ppm and %volume tell you how much gas is actually present. %LEL tells you how close the atmosphere is to igniting. Confusing the two understates a methane hazard twentyfold. This guide covers the conversion arithmetic, why each channel uses the unit it does, and how to cross-check a mixed-unit display for consistency before entry.

A 4-gas monitor shows four channels in three different units, and they are not comparable to each other. This guide reads the display field by field: the clean-air baseline, what an oxygen drop implies, why 10% LEL is not 10% gas, what the LOW, HIGH, TWA, STEL and PEAK labels mean, and which hazards the four channels cannot see at all.