Limiting Oxygen Concentration

Limiting Oxygen Concentration (LOC)

The Limiting Oxygen Concentration (LOC) for combustible dust refers to the minimum oxygen level in the air at which a dust explosion or combustion involving the dust cannot be sustained, even in the presence of an ignition source or sufficient fuel. It represents the lower boundary below which the dust-air mixture is unable to support combustion or explosion.

Testing Principle and Methodology

A quantity of the material to be tested is dispersed in a 20L Sphere pressurized with compressed air-inert air mixture using partial pressures and an ignition source, typically, 2.5KJ chemical ignitor is activated to attempt to ignite the material.  ASTM recommends the particles size of the material to be tested to be at least 95% less than 75 microns and less than 5% moisture.

In certain unique cases where there is no chance of segregation of the material during normal operation, the material may be tested as received. In case of uncertainty on how to perform the testing please contact Prime Process Safety Center for guidance.  In determining the LOC, it is recommended by the standard to have an idea of the Kst/Pmax concentration of the material.

This concentration represents the most easily ignitable concentration around which the LOC may be determined. The concentration of the material is varied around the most easily ignitable concentration of the dust while decreasing the oxygen concentration until the pressure ratio in the 20L Sphere is less than 2.0 In each of the cases where the oxygen concentration is decreased, the fraction of the oxygen is replaced by Nitrogen inert gas to attempt to render the material non ignitable.


Typical Dust Explosion Graph for LOC determination in a 20L Sphere

Applicable Standard

The LOC test is conducted in accordance with the American Standard Testing Method (ASTM) E1291. ‘’Standard Test Method for Limiting Oxygen (Oxidant) Concentration of Combustible Dust Clouds’’ and BS EN 14034 part 4.

Data interpretation

The LOC of the material is determined from the pressure ratio generated in the Sphere. When the pressure ratio in the sphere at any dust-air mixture is 2.0 or greater or the Pmax is greater than 0.5 barG, the dust is deemed to have ignited. The lowest concentration of oxygen below which ignition of the dust is not possible is deemed the limiting oxygen concentration. The result of this test is used in the design of nitrogen inerting systems to ensure that concentration of oxygen during the process is kept below the threshold that can allow deflagration to occur. The Limiting Oxygen Concentration (LOC) of combustible dust holds significant importance in assessing and managing the risks associated with dust-related fire and explosion hazards in various industries. The test is particularly important during Fire and Explosion Prevention, Risk Assessment and Hazard Analysis, Safety Protocol Development, Process and Equipment Design, Regulatory Compliance, Incident Prevention and Worker Safety and Emergency Preparedness.

When to perform (LOC)

Determining the Limiting Oxygen Concentration (LOC) is essential in various scenarios, especially in environments where flammable materials, including combustible dust, are present.  When inerting is used as a measure to prevent dust explosions, it is recommended to establish the limiting oxygen concentration to understand the highest oxygen concentration that can be allowed in the process.  Performing Limiting Oxygen Concentration (LOC) assessments at appropriate intervals, especially during new material introductions, regulatory compliance checks, and incident reviews, helps in ensuring workplace safety and preventing potential fire or explosion hazards associated with combustible materials.

Why work with Prime Process Safety Center

  • Prime Process Safety Center is a leader in process safety testing with very experienced laboratory personnel. At Prime Process Safety Center our goal is to provide accurate, reliable and defensible data that meets industry and regulatory standards. We understand the need for the quality of your data, and we work assiduously to achieve just that.
  • We are knowledgeable and experienced in performing dust LOC tests, ensuring accurate and reliable results.
  • We have state-of-the-art explosibility testing equipment, providing precise and sensitive measurements.
  • We follow strict testing protocols and quality control measures to ensure consistent and reliable test results.
  • Our team can interpret and analyze the data obtained from the tests, providing valuable insights and recommendations for your specific application or research.


What is the Limiting Oxygen Concentration (LOC) of combustible dust?

Answer: The LOC refers to the minimum oxygen concentration in the air below which a dust explosion or combustion involving combustible dust cannot be sustained, even in the presence of an ignition source or sufficient fuel.

Why is understanding the Limiting Oxygen Concentration (LOC) important in industries dealing with combustible dust?

Answer: Knowledge of the LOC is critical for assessing fire and explosion risks associated with combustible dust. It assists in establishing safe oxygen concentration limits, preventing dust-related incidents, and ensuring workplace safety.

How is the Limiting Oxygen Concentration (LOC) determined for combustible dust?

Answer: LOC assessments involve testing methods that gradually reduce the oxygen concentration in controlled environments while observing the ability of combustible dust to sustain combustion or support an explosion.

What factors influence the Limiting Oxygen Concentration (LOC) of combustible dust?

Answer: Factors such as the type of dust, particle size distribution, moisture content, chemical composition, and specific characteristics of the dust significantly impact the LOC for combustible dust.

How is knowledge of the Limiting Oxygen Concentration (LOC) utilized in workplace safety and prevention of dust-related incidents?

Answer: Understanding the LOC assists in developing safety protocols, inerting systems, compliance with safety regulations, incident prevention, emergency preparedness, and ensuring safe handling practices concerning oxygen concentrations in workplaces dealing with combustible dust.