In the field of industrial safety and environmental protection, accurate detection of gas concentration is crucial. LEL (lower explosion limit), VOL (volume percentage) and ppm (parts per million concentration) are the core parameters of the gas sensor industry, which correspond to safety thresholds and detection requirements in different scenarios. This article will analyze the internal connection between the three from the perspectives of definition, conversion relationship, application scenarios and sensor selection, and provide technical reference for industrial users.

1. Analysis of basic concepts

1.1 LEL (Lower Explosive Limit)

Definition: The lowest concentration of combustible gas that explodes when it encounters fire in the air, 

expressed as volume percentage (VOL%).

Example: The LEL of methane is 5%VOL, that is, when the methane concentration in the air is ≥5%VOL, it 

may explode when it encounters open flames.

Importance: LEL is the core indicator of industrial explosion safety. Sensors usually use 0-100%LEL as the 

range to monitor the risk level in real time.

1.2 VOL (Volume Percentage)

Definition: The percentage of gas volume to the total volume of the mixed gas, used for direct 

characterization of high-concentration gases (such as pure gas and pipeline transmission gas).

Example: The methane content in natural gas is usually above 90%VOL.

1.3 ppm (Parts Per Million)

Definition: The gas volume accounts for one millionth of the total volume of the mixed gas, suitable for 

accurate measurement of low-concentration gases (especially toxic gases).

Example: The occupational exposure limit (OEL) for hydrogen sulfide (H₂S) is 10ppm, or 0.001%VOL.

2. Conversion between LEL, VOL and ppm

2.1 Unit conversion formula

VOL and ppm:

    1%VOL=10,000ppm

Example: 0.5%VOL = 5,000ppm.

LEL and VOL/ppm:

The output of the combustible gas sensor is in %LEL, which needs to be converted to the actual 

concentration in combination with the LEL value of the specific gas:

Actual VOL% = LEL sensor reading (%LEL) * LEL of the gas (%vol) / 100

Example: When the methane sensor displays 20%LEL, the actual concentration is:20%*5%VOL/100

=1%VOL=10,000ppm.

2.2 Typical gas parameter comparison table

3. Application differences and sensor selection in industrial scenarios

3.1 LEL: The first line of defense for explosion-proof safety

Applicable scenarios: areas with flammable gases such as petrochemicals, coal mines, and gas stations.

Sensor technology:

Catalytic combustion sensor: low cost, fast response, but susceptible to sulfide/silane poisoning.

Infrared sensor (IR): strong resistance to poisoning, suitable for complex environments, but high cost.

3.2 ppm: precise control of toxic gas exposure

Applicable scenarios: chemical workshops, sewage treatment plants, laboratories, etc., where H₂S, CO, and VOCs need to be monitored.

Sensor technology:

Electrochemical sensor: high sensitivity, low power consumption, suitable for portable devices.

Photoionization sensor (PID): can detect ppb-level VOCs, but cannot distinguish specific gas types.

3.3 VOL: direct monitoring of high-concentration gases

Applicable scenarios: purity or high-pressure environments such as natural gas pipelines and industrial gas storage tanks.

Sensor technology:

Thermal conductivity sensor (TCD): suitable for high-concentration gases (such as pure hydrogen and pure nitrogen).

Ultrasonic sensor: resistant to high pressure, used for pipeline flow monitoring.

4. Common problems and solutions in practical applications

4.1 Cross-interference in mixed gas environment

Problem: When multiple gases coexist, the sensor may misreport (such as the impact of methane on propane LEL detection).

Solution:

Select infrared sensors with selective filtering.

Compensate through algorithms (such as multi-sensor data fusion).

4.2 Error control of unit conversion

Problem: Ignore temperature and pressure corrections when converting LEL to ppm.

Solution:

Smart sensor with built-in temperature and pressure compensation module.

Calibrate regularly (such as verification with standard gas).

4.3 Performance stability in extreme environments

Problem: High temperature, high humidity or corrosive environment causes sensor drift.

Solution:

Select infrared or laser sensors with industrial-grade packaging (IP67 protection).

Design redundant detection nodes to improve system reliability.

    LEL, VOL and ppm are the three cornerstones of gas detection. Understanding their relationship and applicable scenarios is the key to ensuring industrial safety and efficient operation. As a gas sensor manufacturer, we are committed to providing high-precision and high-reliability detection solutions to help customers cope with challenges in complex environments. If you need further technical exchanges or customized product requirements, please contact our engineering team!

ShanXi TengXing Sensor TechnologyCo.,Ltd

Website: www.tensensor.com

Technology support: [email protected]