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.

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.