Reference

A catalytic bead burns the gas and measures the heat released; an infrared sensor measures the light the gas absorbs. Catalytic needs oxygen and can be poisoned but detects hydrogen. Infrared cannot be poisoned but is blind to hydrogen. This guide covers both principles and the specification decision that follows from them.

Cross-sensitivity is a sensor responding to a gas it was not built to measure, and it is not a defect but a property of the sensing chemistry itself. This guide covers the mechanism, the interference patterns crews actually meet, the under-reporting direction nobody expects, and how to diagnose a suspect alarm from clean air.

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.