Monitor for Combustible Gases

Instruments Usually Monitor for Combustible Gases Below the Lower Explosive Limit.
A gas-air mixture becomes explosive at the lower explosive limit (LEL), but the monitoring range of most instruments (0-100% of the LEL) is below this limit to provide advanced warning. Alarm levels may be set at 10% (and 40% for a high alarm) of the LEL to forewarn workers of a potentially hazardous combustible atmosphere.

Hazardous atmospheres in a confined space can be created by may conditions.