Certain conditions must exist for a fire or explosion to occur involving flammable gases. The gas and oxygen must exist in specific proportions to create an optimal ratio. With an ignition source, such as a spark or flame, the gas will ignite or combust. However, the fuel and oxygen ratio will vary depending upon the gas or vapor in the area.

The minimum concentration of a specific gas or vapor that will support combustion is defined as the Lower Explosive Limit (LEL) for that gas. Below this level, the mixture is considered too lean to burn. This means that there is too much air and not enough gas present.

The maximum concentration of a gas or vapor that will burn is defined as the Upper Explosive Limit (UEL). Anything above this level is considered too rich to burn. There is too much gas present and not enough air. But between the range of the LEL and the UEL, combustible and flammable gases will burn or explode.

Lower Explosive Limit and Upper Explosive Limit Values

The flammable range is considered the values between the LEL and UEL. At these ratios, the combustible and flammable gases and vapors will burn or explode if an ignition source is present. The list below includes the LEL and UEL of some of the most commonly used fuel gases. LEL and UEL figures are defined as the percentage by volume of the air.


This list includes just a few of the combustible or flammable gases that may be present in a workplace. For additional gases, you may want to consult a more comprehensive list.

Lower Explosive Limits and Calibration

In gas detection, the amount of gas or vapor present is denoted as a percentage of LEL. Zero percent means that the atmosphere is entirely free of explosive and flammable gases. One hundred percent means that the gas concentration is at its lower explosive limit. A 50% reading indicates that gas or vapor concentration is half what is needed to combust. The relationship between LEl percent and percent by volume varies from gas to gas.

It’s important not to confuse the percentage reading on a gas sensor device with the actual LEL concentration. For instance, a 50% reading for methane means that the concentration is actually 2.2. The LEL concentration for this gas is 4.4, making a 50% reading half this amount.

Lower Explosive Limits and Occupational Safety

Accurate calibration of LEL percentages is crucial in upholding occupational safety in the workplace. Controlling gas and vapor concentrations outside of the flammable limits is a significant theme in occupational safety and health. The best way to ensure that a sensor is working is to expose it to a known gas concentration. Then you can compare the reading to the known data. This process is known as a bump test. Calibration of the sensor is deemed necessary if the reading varies from the known value by more than 10%.

If the sensor reaches 100% LEL, combustion is imminent under the right conditions. These sensors are a vital way to alert workers to dangerous conditions. As a general rule of thumb, factory default alarms on gas monitors are conservative. They may alert an employee to a problem with a concentration level as low as 10% LEL. Prompt alerts ensure a fast response and that workers can take appropriate actions to mitigate a dangerous situation.

It’s also crucial to understand that confined spaces present unique dangers when explosive or flammable gases may be present. Generally, the oxygen content in these rooms varies only slightly. And the confinement traps the gases and vapors. A slow leak can quickly hit the LEL required for combustion or to start a fire. Gas detection should occur before any worker enters a confined space where explosive or flammable gases may be present.

NevadaNano’s LEL Sensors and the Lower Explosive Limit

NevadaNano’s Molecular property Spectrometer (MPS) is the next generation of gas detection. These LEL sensors enhance worker safety by providing accurate leak detection in many industries. These industries include drilling, transportation, and the production of oil, gas, and chemical products. Users can trust that these sensors will deliver an accurate LEL reading.

These smart LEL sensors have built-in environmental compensation. They can detect and report on over a dozen gases and gas mixtures. The reading can accurately provide this information and concentration to the user in an easy-to-understand format. The LEL values for a mixture of gases achieve the same accuracy as single gas detection. Additional intelligent algorithms enable the determination of the class of gas present.

As the world’s first intelligent flammable gas LEL sensors, they provide accurate readings. This is true even when exposed to rapid temperature and humidity changes. Other sensors may temporarily shut down when faced with common environmental changes. These new capabilities eliminate false positives, nuisance alarms, and false negatives, providing improved ease of use and user safety. Equipped with built-in environmental monitoring, the advanced algorithms of the MPS enable accurate results. These results are achieved even in temperatures ranging from -40˚C to 75˚C.

The MPS LEL sensor uses a micro-electromechanical system (MEMS) transducer. This transducer consists of an inert, micrometer-scale membrane with an embedded heater and thermometer. The MEMS transducer measures changes in the thermal properties of the surrounding air and gases in its proximity. The output reading contains multiple measurements, similar to a thermal spectrum. And environmental data that can be used to identify the type and concentration of any flammable gases present.

The MPS Flammable Gas Sensor is intrinsically safe, robust, and highly poison-resistant. It is calibrated for all gases by performing a single calibration with methane. The MPS sensor is the best option for accurate readings of Lower Explosive Limits of gas and vapor concentrations.