Methane or natural gas (CH4) is a colorless, odorless and lighter than air gas. It is extremely flammable (R12) and explosive. Use of a methane detector allows the detection and measurement of this gas to prevent possible danger.
An LEL Detector simply detects, indicates and alarms for levels between 0-100% LEL of materials it is calibrated to monitor and detect. Using methane in air as an example, the LEL of methane is 5% by volume.
We generally consider its LEL (lower explosive level) to be 5% by volume (5% bv). The mixture is too lean to burn if less than 5% methane is present. But at 5%, we can burn or explode if there gas detector is an ignition source. So we call 5% bv methane the same as 100% LEL
20% LEL is the High Alarm, a distress signal. Not explosive yet. 100% LEL means the gas concentration has reached the lower explosive limit and became explosive.