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Gas Mixture Flammability Calculator

LEL/UEL for mixed gases with O2 adjustment, dilution analysis, and safety warnings

Calculate the Lower and Upper Explosive Limits for mixtures of flammable gases using Le Chatelier's mixing rule. Adjusts the flammable range for actual oxygen concentration using Minimum Oxygen Concentration (MOC) interpolation. Supports multi-gas blends with 20 common industrial gases, preset scenarios (digester gas, landfill gas, natural gas, coke oven gas), and a dilution analysis slider that shows exactly where a rich mixture will pass through the explosive range during air ingress. Built for confined space entry teams, safety officers, hot work planners, and wastewater/digester operators.

Pro Tip: A gas mixture that reads "above UEL" on your detector is not safe. It means the atmosphere is too fuel-rich to ignite right now, but any air leak, door opening, or ventilation change will dilute the mixture through the flammable range on its way to becoming lean enough to be safe. The dilution slider on this calculator shows you exactly where that danger window is. If your boss says "it won't light, we're fine," show them the dilution chart.

Read the practical field guide for above-UEL and oxygen-deficient atmospheres

Gas Mixture Flammability Guide →

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Gas Mixture Flammability Calculator
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How It Works

  1. Add Your Gas Readings

    Select fuel gases from the dropdown (20 common industrial gases) and enter their concentrations in % volume. Add multiple gases for mixture calculations. Or load a preset scenario to start with a common gas mixture.

  2. Set the Atmosphere

    Enter the O2 reading from your gas detector. N2 auto-balances to 100% unless you override it. Add CO2 and other inerts if known. The composition summary shows your full atmosphere breakdown.

  3. Read the Status Banner

    The status banner shows whether your atmosphere is below LEL (lean), in the flammable range, above UEL (rich), or below MOC (inert). The gas bar visualization shows exactly where your fuel concentration sits relative to the explosive envelope.

  4. Run the Dilution Analysis

    Open the dilution panel and drag the slider to see what happens when air mixes into your atmosphere. If the mixture is above UEL, this shows the exact dilution range where it passes through the flammable zone. This is the feature you show your boss.

Built For

  • Confined space entry pre-planning: verify gas readings against LEL/UEL before entry under OSHA 1910.146
  • Hot work permit atmospheric assessment: document that the work area is below 10% LEL per API RP 2009
  • Digester and landfill gas monitoring: understand biogas mixture flammability with CO2 dilution
  • Tank and vessel purging verification: confirm atmosphere is below MOC or below LEL after N2 purge
  • Incident investigation: reconstruct atmospheric conditions at the time of an ignition event
  • Safety training: demonstrate why "above UEL" is not the same as "safe" using the dilution slider
  • Wastewater treatment plant operations: monitor headspace gas in wet wells, lift stations, and covered lagoons

References

  • NFPA 497: Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous Locations for Electrical Installations
  • Zabetakis, M.G. "Flammability Characteristics of Combustible Gases and Vapors" Bureau of Mines Bulletin 627, 1965
  • Crowl, D.A. and Louvar, J.F. "Chemical Process Safety: Fundamentals with Applications" 4th Edition
  • Matheson Gas Data Book, 7th Edition
  • OSHA 29 CFR 1910.146: Permit-Required Confined Spaces
  • OSHA 29 CFR 1910.146 Appendix D: Confined Space Pre-Entry Check List
  • OSHA confined-space atmospheric testing guidance: oxygen first, then combustibles, then toxics
  • OSHA 29 CFR 1910.1000 Table Z-1: Permissible Exposure Limits (toxic gas TWA values)
  • NIOSH Pocket Guide to Chemical Hazards (IDLH values)
  • API RP 2009: Safe Welding, Cutting, and Hot Work Practices in the Petroleum and Petrochemical Industries

Frequently Asked Questions

A gas concentration above the Upper Explosive Limit means there is too much fuel and not enough oxygen to sustain combustion right now. But if anything changes, like opening a hatch, starting a fan, or a seal leaking, air will mix in and dilute the gas concentration. As it dilutes, it must pass through the flammable range before it becomes lean enough to be safe. The dilution slider on this calculator shows you exactly where that danger window is. This is the most misunderstood concept in gas monitoring.
MOC is the Minimum Oxygen Concentration required to support combustion of a specific gas. For methane, the MOC is 12.0% O2. If the oxygen level in your space is below the MOC, the atmosphere cannot support combustion regardless of how much fuel is present. This is why N2 inerting works: it displaces oxygen below the MOC. Note that while the atmosphere may be inert (non-flammable), it is also immediately dangerous to life from oxygen deficiency. SCBA is required.
Le Chatelier's mixing rule provides a reasonable approximation for most hydrocarbon mixtures at standard conditions (25C, 1 atm). It assumes ideal gas behavior and that each gas contributes independently to the flammable limits. It is less accurate for mixtures containing hydrogen with heavy hydrocarbons, or for gases at elevated temperatures and pressures. For critical safety decisions involving unusual mixtures, laboratory testing of the specific mixture composition is recommended.
As oxygen decreases below the normal 20.9% in air, the flammable range narrows. The LEL increases slightly (harder to ignite a lean mixture) and the UEL decreases (less oxygen available to sustain combustion of rich mixtures). At the MOC, the flammable range collapses to zero. This calculator uses linear interpolation between normal air and MOC, which is a conservative approximation. The real flammable envelope is curved, but the linear model errs on the side of caution.
Disclaimer: This calculator uses Le Chatelier's mixing rule with linear O2 interpolation to MOC. This is a conservative approximation at standard conditions (25C, 1 atm). It does not account for temperature, pressure, humidity, mist, or dust effects on flammable limits. It does not replace calibrated gas detection instruments, professional hazard analysis, or facility-specific confined space and hot work procedures. Always verify calculations independently before making safety decisions.

Learn More

Safety

Gas Mixture Flammability Guide

Why 17% oxygen and 15% methane is still a serious problem, how LEL and UEL really behave in mixed atmospheres, and what OSHA requires before entry or hot work.

Read Guide →

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