PPM unit can be converted to LEL as follows:
ppm=%LEL * LEL (vol%) *100
For example: 35% LEL of methane, its LEL is 2 vol%, that is, ppm=35(%LEL)*2(vol%)*100=7000ppm methane
%LEL=ppm/(LEL(vol%)*100)
300ppm of propane, its LEL is 1.2vol5, that is, %LEL=300ppm/(1.2 vol%＊100)=2.5% LEL propane

Meaning
ppm means one millionth
Vol% means one hundredth
That is, 10000ppm = 1Vol%
% LEL means the lower limit of explosion, that is, in which concentration will the combustible gas explode when it meets fire. It is worth noticing that this unit is not a natural unit, but the lower limit of the explosion is divided into one hundred parts, one part for 1% LEL.

The relationship between LEL and VOL is defined according to the standard, for example, the lower limit of methane concentration in China is defined as 5Vol%, while the lower limit of explosion in Germany is set to be more conservative and rigorous, that is 4.4Vol%.

If you cannot understand from above, please see the following specific explanation:

The meaning and conversion of ppm, LEL and VOL
I. Meaning of ppm, LEL and VOL

1. ppm: gas volume percentage per million, it is a dimensionless unit.
i.e.: 5ppm carbon monoxide means that the air contains 5 per million of carbon monoxide.
2. LEL: the minimum volume percentage concentration of combustible gas in air that can detonate, that is, the lower limit of gas explosion concentration. (UEL: upper limit concentration of gas explosion.)
LEL% Percentage of lower limit of explosion, that is, the lower limit of the explosion is divided into one hundred parts, one unit is 1LEL%.
i.e.: 25LEL% means 25% of lower limit of explosion
50LEL% means 50% of the lower limit of explosion
3.VOL: gas volume percentage, it is a physical unit.
i.e.: 5% VOL means the specific gas occupies 5% volume of air.

The relationship between the three units: In normal cases, ppm is used for accurate measurement; LEL is used for explosion test; magnitude of VOL is the largest of the three units. For example, lower limit of explosion of methane is 5% VOL, so 10% LEL methane gas has the following correspondence: 10% LEL = 5000ppm = 0.5% VOL

II. Conversion of ppm and LEL
ppm unit can be converted to LEL as follows:
ppm=%LEL×LEL（vol%）*100
i.e.: 35% LEL of methane, its LEL is 2vol%
That is, ppm = 35 (% LEL) * 2 (vol%) * 100 = 7000 ppm methane
％LEL=ppm/（LEL（vol%）*100）
ppm is the volume concentration.

Conversion relationship of gas detection concentration unit: ppm and milligrams / cubic meter
There are two ways to represent pollutant concentrations in ambient air (air):
Mass concentration representation: the mass per ton of air contained in the air mg / m3 volume concentration represents: one million volumes of air contained in the volume of pollutants, that is, concentration results of most of the gas detection equipment is volume concentration (ppm).
According to our regulations, especially Environmental Protection Dept., gas concentration is required to be expressed in units of mass concentration (eg mg / m3). Our national standards are also expressed in mass concentration units (eg mg / m3).

What is the relationship between these two gas concentration units in mg / m3 and ppm? How to convert them?
The use of mass concentration unit (mg / m3) as the air pollutant representation method, we can easily calculate the true amount of pollutants.
But the quality concentration has some relationship with gas temperature, pressure and environmental conditions, and its value will be different with temperature, air pressure and other environmental conditions variation; the actual measurement needs simultaneously measure the gas temperature and atmospheric pressure.
In the case of using ppm as the concentration of contaminants, this problem does not occur for it is volume ratio.

Concentration unit ppm and mg / m3 conversion:
calculated by the following formula:
mg/m3=M/22.4·ppm·[273/(273+T)]*（Ba/101325）
M---- Gas molecular weight
ppm---- Measured volume concentration value
T----Temperature
Ba----Pressure
mass - volume concentration
The concentration expressed in terms of the mass of the contaminants per cubic meter of the atmosphere is called the mass-volume concentration, its unit is milligrams per cubic meter or gram / cubic meter.
Its conversion relationship with ppm is:
X=M.C/22.4
C=22.4X/M
From the formula: X-contaminant is expressed as the concentration value in milligrams per cubic meter; C-concentration value of contaminants is expressed by ppm; M-the molecular weight of contaminants.

Note: Since the concentration of methane collected in pipeline field is generally above 90% VOL, in order to prevent the instrument sensor poisoning, it is not recommended to test the instrument by sample gas which is collected directly with the pipeline, if necessary, it is a must to decrease dilution and test time, thus to extend instrument sensor’s lifespan.