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What is a Lower Explosive Limit?

Boilers Room

by | Sep 13, 2022

What is LEL?

LEL stands for Lower Explosive Limit. Put simply this is the lowest concentration of a gas with potential to burn in air or explode, this changes from gas to gas.

There is also the Upper Explosive Limit . After this concentration of a gas is exceeded the gas too rich to burn and is non explosive.

Explosive conditions

Three things needed for an explosion to occur. A fuel (combustible gas such a natural gas or methane) and oxygen (air) must mix in certain proportions, along with an ignition source, such as a spark or flame.

The range between the LEL (100%) and UEL is known as the flammable or explosive range.

Methane Lower Explosive Limit visual

An LEL example

Each gas has different percent volume in air which in the case of Methane is 5%

So 5% volume Methane would display as 100% LEL on a gas detector and 1% volume Methane as 20% LEL

Gas detection controller

For Methane a Duomo controller with SGM595 sensor is designed to go into alarm at 20% of the LEL. As shown above this is 1% volume Methane, this acts as an early warning to rising gas levels.

So if the LEL is 5% by volume of Methane then the alarm level is set to 1% by volume (20% LEL).

Test GasCalculating LEL from gas concentration

Say you are using calibration gas of 1.75% Methane volume and you want to know the LEL percent. Divide the concentration by the gas 100% LEL volume (Methane 5%)

1.75 / 5 = 0.35 and then multiply this by 100 which is 35. So this is 35% LEL Methane concentration gas.

Calculating gas concentration from LEL

What if you wanted to calculate the gas concentration from the LEL? Firstly you take the LEL level of the gas being used and multiply it by the 100% LEL

So using Methane as an example again:

35 x 5= 175 and then divide this by 100 which is 1.75. So the gas concentration is 1.75% volume Methane

Need more information about anything you’ve read? Contact our gas detection experts now

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