Oxygen (O2) depletion monitors are devices capable of measuring the ambient concentration of atmospheric oxygen and signalling when Ievels have fallen. This can enable action to be taken before an evacuation is required and levels become dangerous. In addition to oxygen depletion, problems arise when oxygen levels are too high. This oxygen “enrichment” (levels above 23%) can be dangerous; increasing the chances of combustion and the intensity of fires. In summary, oxygen detectors can signal all dangerous levels of oxygen and are important instruments for a range of applications.