Introduction to Combustible Gases

Combustible gases are gases that will burn when mixed with oxygen and ignited. Typically, most combustion reactions involve the burning of organic materials containing carbon and hydrogen. Combustion reactions are essential to life, and are exploited to generate power, to provide heat, to run motors, and in many other ways.

Combustible gases can be released into a facility naturally as a byproduct or leaked unintentionally. This presents the hazard of combustion within a facility if the concentration reaches a certain level.

Some combustible gases include:

  • Methane (CH4)
  • Pentane (C5H12)
  • Propane (C3H8)
  • Butane (C4H10)
  • Hydrogen (H2)


Combustible gas detectors are required whenever there is a possibility of a hazard to life or property caused by the accumulation of combustible gases. Each type of combustible gas has three important ranges to consider and each of these ranges differ for specific gases, but utilize the same definitions. Below the Lower Explosive Limit (LEL, also known as the Lower Flammability Limit (LFL)), the combustible gas concentration is too lean for combustion. This is the range in which most combustible gas detectors operate.

The Upper Explosive Limit (UEL), or Upper Flammability Limit (UFL) is the point where the gas concentration is too rich for combustion, or the oxygen level is too low to support combustion. In between the LEL and UEL, the concentration (measured as a percent of volume in air) of combustible gas will support combustion if exposed to a source of ignition. The flammability of many gases lies in a very limited, concentrated range.

Concentration Limits

When researching combustible gas detection solutions, facility managers should be aware of the following combustible gas concentration limits:

UEL/UFL LEL/LFL Combustible Gas
15.0% 5.0% Methane
7.8% 1.5% Pentane
9.5% 2.1% Propane
8.4% 1.8% Butane
75.0% 4.0% Hydrogen

Technologies for Combustible Gas Detection

Catalytic Bead Sensors

A catalytic bead sensor consists of two alumina beads, each surrounding a platinum wire operating at approximately 450 oC. One bead is passivated to not allow it to react with combustible gas, while the other bead is catalyzed to promote a reaction with combustible gas. Effects of changes in ambient temperature and relative humidity are nullified by placing the two beads in separate legs of a Wheatstone bridge circuit. When the catalyzed bead reacts with combustible gas, it heats up and increases its resistance and, in turn, increases the output of the Wheatstone bridge circuit.

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Infrared (IR) Sensors

Infrared sensors offer a large range of measurement, providing accurate and stable detection. No matter how high a concentration of a gas is, it cannot oversaturate a sensor. Infrared technology is made to detect light hydrocarbons, but does not respond to hydrogen. Although sturdy and reliable, gas sensors come at a higher upfront cost.

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Solid State (Semiconductor) Sensors

Solid state (semiconductor) sensors have a resistance that is affected by oxygen adsorbed on the surface of the sensor. Oxygen atoms capture electrons on the semiconductor surface, thereby increasing its resistance. The sensors can be impregnated with dopants such that the sensor's resistance changes when specific gases displace the adsorbed oxygen.

Learn more about the advantages of using Sierra Monitor's solid state gas sensor modules »