LEL means the lower explosion limited. The lowest concentration of the combustible gas in the air comes in contact with an open flame then explodes. Combustion is an intense oxidation reaction with and heat. It must have three elements: a, fuel(gas) b, comburent; c, ignition source(temperature). The measuring range of all gas detector is 0~100%LEL. The fixed combustible gas detector is usually provided two warning points(related to the type of alarm host). The frist alarm point is 10%LEL; the second alarm point is 25%LEL. Portable combustible gas detector usually provide one alarm point: 25%LEL.

PPM is a representation of solution concentration (solute mass fraction), and PPM represents one part per million.For the concentration (solute mass fraction) of a solution: 1/1000 milliliters in 1 liter of water, so the concentration is 1 PPM. For gases: one of the ways to represent the concentration of pollutants in the atmosphere. Volume concentration representation: the number of air pollutants in a million volume is PPM. Most gas instruments measure volume concentration (PPM). According to China's regulations, especially the environmental protection department, the gas concentration is required to be expressed in the unit of mass concentration (such as mg/m3), and the standard specifications of our country also adopt the unit of mass concentration (such as mg/m cubed;) 

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