Fire & Gas

Understanding Lower Explosive Limit (LEL)

  • Lower Explosive Limit (LEL) is the lowest or minimum concentration of flammable gas or vapour that will burn in air below which combustion or explosion is not possible.
  • The Lower Explosive Limit (LEL) is expressed as a percentage of the gas or vapour in the air.
  • The Lower Explosive Limit (LEL) range varies for different gases.
  • Generally, the Lower Explosive Limit (LEL) for most flammable gases is very less than 5% by volume.
  • At concentrations below the Lower Explosive Limit, there is insufficient fuel to sustain a fire or explosion. 
  • For example, if the Lower Explosive Limit for any given flammable gas is about 5%.
  • If the concentration of flammable gas in the air is lower than the Lower Explosive Limit or below 5% is too lean to burn or support combustion. 
  • If the concentration of flammable gas in the air is higher than the Upper Explosive Limit or more than 5% is too rich to burn or support combustion. 
  • However, once the concentration of the gas reaches or exceeds its Lower Explosive Limit, there is a potential for ignition and combustion in the presence of an ignition source.
  • It’s important to consider that the Lower Explosive Limit is just one part of the flammability characteristics of a substance. 
  • In other words, the Upper Explosive Limit (UEL) is the maximum concentration of a gas or vapor in the air and combustion is not possible because there is too much fuel. 
  • In working with gas detection systems, the concentration of flammable gas present is specified as a percentage of LEL (%LEL). 
  1. Zero percentage Lower Explosive Limit (0% LEL) represents a combustible gas-free atmosphere. 
  2. One hundred percent Lower Explosive Limit (100% LEL) represents an atmosphere in which combustible gas is at its lower flammable limit. 
  • The relationship between the percentage Lower Explosive Limit (0% LEL to 100% LEL) and percentage by volume varies for various gases.

Let us consider methane gas as a general example 

This example represents the flammability of Methane or Natural Gas in Air. 

  1. If the concentration of Methane gas in the air is about 0% to 5%, the mixture is too lean to ignite or burn. 
  2. If the concentration of Methane gas in the air is about 5% to 18% the mixture supports ignition and is considered highly flammable. 
  3. If the concentration of Methane gas in air is above 18% the mixture in the atmosphere is too rich for the methane to ignite.
  • Flammable Range or Explosive Range is defined as the range between the Lower Explosive Limit and Upper Explosive Limit.
  • Various process Industries deal with flammable gases or vapours often to measure and monitor Lower Explosive Limit levels to ensure safety and take precautions to prevent the accumulation of concentrations within the flammable range. 
  • Instruments such as gas detectors are commonly used to measure and monitor Lower Explosive Limit levels in various industrial settings.

The Lower Explosive Limit for a given gas in the air can be calculated by dividing the unknown concentration by the 100% LEL shown in the table above.

Take about 2.5% of Methane in the air and divide it by 100% LEL of methane (5%), the result is 50% LEL. 

  (2.5% ÷ 5% = 50%).

Conversely, you can multiply the %LEL of the unknown concentration by the 100% LEL to obtain the % by volume. 

(50% LEL methane x 5% = 2.5% volume).

Let us assume that      we are using methane as a calibration gas of 1.75% by Volume.

Now to calculate the %LEL, divide the concentration by 100% LEL volume 5%.

(5% for methane gas)


now multiply this by 100 which gives 35. 

So, this is 35% LEL Methane concentration gas

To determine the gas concentration from the Lower Explosive Limit,

Initially, take the LEL level of the gas being used and multiply it by the 100% LEL.

Considering Methane gas as an example


Now dividing this by 100 will give 1.75. 

So, the gas concentration is 1.75% volume of Methane.

Early Gas Leak Detection: Prevents the accumulation of flammable concentrations.

Preventing Explosive Atmospheres: Minimizes the risk of creating explosive conditions.(To prevent catastrophic gas explosions)

Worker Safety: Provides early warnings for timely evacuation or action.

Protection of Facilities and Equipment: Safeguards against damage from fires or explosions.

Regulatory Compliance: Ensures adherence to safety regulations.

Optimized Emergency Response: Activates predefined protocols when concentrations approach unsafe levels.

Process Optimization: Informed decision-making for balancing safety and operational efficiency.

Reduced Insurance Costs: Shows a dedication to safety, which could result in a premium reduction.

Maintenance of Equipment: Early identification of malfunctions for timely maintenance

  • LEL is the Lower Explosive Limit
  • UEL is the Upper Explosive Limit

The Lower Explosive Limit of a gas can be determined as a percentage of volume.

An LEL Detector is an integral part of an entire gas detection system, it is also known as Gas Detector.

An LEL Detector or LEL Meter is used to detect the hazardous levels of a combustible gas or solvent vapour present in the air.

It is expressed in percent Lower Explosive Limit.

If a gas concentration reduces below the lower limit, there is insufficient gas for the mixture to ignite. 

For example, 

Methane or Natural Gas has a 5% by-volume LEL and 18% by-volume UEL.

LFL and UFL of flammable gas mixtures can be determined by using Le Chatelier’s equation,

The minimum concentration of a particular combustible gas or vapour required for combustion in air is known as the Lower Explosive Limit (LEL) for that gas the amount of fuel and oxygen required differs for various combustible gas or vapour.

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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