Lower Explosive Limit
The first thing that you need to understand is that gases have different LEL from each other. Also what is LEL? OK so the LEL for Isobutane is 1.8%. If you combine 1.8% Isobutane with regular air and light a match, Boom. Next thing I want you to memorize is that 10,000 PPM is equal to 1%...... Read that again. And again. We normall will calibrate an analyzer to 50% LEL so what is 50% of 1.8% = .9% or 9,000 PPM

The Flammable Range (Explosive Range) is the range of a concentration of a gas or vapor that will burn (or explode) if an ignition source is introduced.

Below the explosive or flammable range the mixture is too lean to burn and above the upper explosive or flammable limit the mixture is too rich to burn. The limits are commonly called the "Lower Explosive or Flammable Limit" (LEL/LFL) and the "Upper Explosive or Flammable Limit" (UEL/UFL).

The lower and upper explosion concentration limits for some common gases are indicated in the table below. Some of the gases are commonly used as fuel in combustion processes.

Fuel Gas "Lower Explosive or Flammable Limit"
(%) "Upper Explosive or Flammable Limit"

Acetaldehyde       4                 60
Acetic acid           4                 19.9
Acetone                2.6              12.8
Acetyl chloride    7.3              19
Acetylene             2.5              81
Acrolein               2.8              31
Acrylonitrile         3.0             17
Allyl chloride       2.9             11.1
Ammonia              15              28
Arsine                   5.1             78
Benzene                 1.35          6.65
1,3-Butadiene        2.0            12
n-Butane                1.86          8.41
iso-Butane             1.80          8.44