Measurements · Gases & VOCs
Combustible (LEL)
Also known as combustible gas, explosive gas, flammable gas
Combustible-gas concentration as a percentage of the Lower Explosive Limit, a key flammability and explosion-safety metric.
What it is
What is combustible gas (LEL)?
Every flammable gas or vapor has a lower explosive limit (LEL): the minimum concentration in air at which it will ignite. Below the LEL the mixture is too lean to burn; above it (up to the upper explosive limit) it is flammable, and beyond that too rich. Combustible-gas monitoring expresses the measured concentration as a percentage of the LEL, so 0 % LEL is clean air and 100 % LEL is the threshold at which the atmosphere becomes explosive. Methane, propane, solvents, fuels, and many process vapors all contribute. Because reaching the LEL turns an ordinary space into a potential bomb, combustible-gas detection is a cornerstone of process and confined-space safety.
Health and operational effects
The hazard here is not primarily toxicity but catastrophic fire and explosion. An atmosphere allowed to build toward its LEL can be ignited by a spark, a hot surface, static, or a tool, with devastating consequences for people and plant. This is why combustible-gas monitors alarm at a small fraction of the LEL (commonly around 10 % LEL) to force ventilation and work stoppage long before the mixture becomes explosive. Leaks, evaporating solvents, purged fuel lines, and decomposition gases such as methane are the usual sources.
For confined-space entry, refueling, hot work, and any operation involving flammable substances, LEL monitoring is a fundamental safeguard.
Limits
Combustible (LEL) Limit Values
The published values, by averaging period; each links to the standard that sets it.
Action level
Combustible gas is reported as % LEL. The safety convention is not to approach 100 % LEL but to act at a small fraction of it: confined-space and process-safety practice commonly alarms at around 10 % LEL and evacuates or ventilates well below the explosive threshold. Interpret any sustained rise in % LEL as a leak or accumulation to be found and controlled. The number is a margin-to-explosion indicator, and the goal is to keep it near zero, not merely below 100 %.
How it is measured
How Combustible (LEL) Is Measured
Two technologies dominate. Catalytic bead (pellistor) sensors burn the gas on a heated catalytic element and measure the resulting temperature change; they respond to a broad range of combustibles but need oxygen to work and can be poisoned by certain compounds. Infrared (IR) sensors measure absorption of infrared light by hydrocarbon gas; they work without oxygen, resist poisoning, and are favored for methane and inert or oxygen-deficient atmospheres. LEL is a standard channel of the confined-space multi-gas monitor, tested after oxygen because catalytic sensors depend on adequate oxygen.
An instrument on Thiamis1
A third-party instrument connected to a Thiamis gateway, reporting into the same Environet account.
Placing LEL monitoring
In confined-space entry, combustible gas is tested at multiple depths before entry and then monitored continuously during occupancy, since flammable vapors may be heavier or lighter than air and collect high or low. In process areas, fixed gas detectors watch around potential leak sources and ignition points. Sensor choice matters with location: infrared is preferred where oxygen may be low or catalyst poisons are present, catalytic where a broad combustible response is needed.
Every device and option that measures Combustible (LEL) 1
| Device / option | Type | Base platform | Measures |
|---|---|---|---|
 RAE · MultiRAE Lite, MultiRAE, MultiRAE Pro… | Gas Detectors | Thiamis |
Where it is monitored
Solutions That Monitor Combustible (LEL)
Each one links to the solution page showing what that industry measures it for.
Occupational Safety & Industrial Hygiene
Core parameterCombustible-gas monitoring, expressed as a percentage of the lower explosive limit (LEL), guards against the flammable atmospheres that cause catastrophic industrial explosions.
Oil & Gas / Petrochemical
Core parameterFlammable hydrocarbon vapors make combustible-gas monitoring essential across oil and gas operations, where a leak building toward the lower explosive limit turns a routine area into an explosion risk.
Cabin Air Quality
Often addedRespirable dust, gases, and blast monitoring
Related
More in Gases & VOCs
Questions
Combustible (LEL) FAQ
What does "% LEL" actually mean?
It is the measured combustible-gas concentration expressed as a percentage of that gas's lower explosive limit, the level at which the air becomes flammable. 0 % LEL is clean air; 100 % LEL is the explosive threshold. Monitors alarm at a small fraction, often around 10 % LEL, to force action long before the atmosphere can ignite.
Why alarm at 10 % LEL instead of 100 %?
At 100 % LEL the atmosphere is already explosive, far too late to react safely. Alarming at around 10 % gives a large safety margin to ventilate, stop work, and find the source before there is any risk of ignition. The number is managed as a margin to explosion, not a limit to approach.
Catalytic or infrared LEL sensor: which should I use?
Catalytic bead sensors respond to a broad range of combustibles but need oxygen and can be poisoned by some compounds. Infrared sensors work without oxygen, resist poisoning, and excel for methane and oxygen-deficient or inert atmospheres. The choice depends on the gases present and whether oxygen may be low.
How do I monitor combustible gas?
Use a multi-gas monitor with an LEL channel alongside oxygen, CO, and H₂S for confined-space entry, or fixed gas detectors around leak and ignition points in process areas. Use the configurator to build a confined-space or flammable-atmosphere panel.
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