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Gas Mixture Flammability Calculator

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

Screen mixed flammable-gas atmospheres with Le Chatelier's mixing rule, local oxygen-adjusted limits, and an air-ingress dilution check. Supports multi-gas blends with 20 local planning rows, preset scenarios (digester gas, landfill gas, natural gas, coke oven gas), and visible source/safety warnings. Built for confined space entry teams, safety officers, hot work planners, and wastewater/digester operators as planning support only, not as an entry permit, detector calibration record, or hot-work authorization.

Pro Tip: A gas mixture that reads "above UEL" is not a green light. It means the atmosphere is too fuel-rich to ignite under that snapshot, but air leaks, door openings, or ventilation changes can move the mixture through a flammable range. Treat the dilution chart as a planning prompt and control ignition sources, ventilation, monitoring, rescue, and permit decisions through qualified site procedures.

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

Gas Mixture Flammability Guide →

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Screen oxygen-displacement prompts before entry and detector review

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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 the local screen places your atmosphere below LEL (lean), in the flammable range, above UEL (rich), or below MOC (inert). The gas bar visualization shows where the entered fuel concentration sits relative to the modeled envelope.

  4. Run the Dilution Analysis

    Open the dilution panel and drag the slider to screen what can happen when air mixes into your atmosphere. If the mixture is above UEL, the model highlights a possible flammable transition range under the local assumptions.

Built For

  • Confined space entry pre-planning: compare gas readings against local LEL/UEL screening before formal OSHA 1910.146 permit review
  • Hot work permit pre-screening: flag combustible-gas and oxygen issues before the site hot-work authorization process
  • Digester and landfill gas monitoring: understand biogas mixture flammability with CO2 dilution
  • Tank and vessel purging planning: screen whether a modeled atmosphere is below MOC or below LEL before instrumented verification
  • 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)
  • Current site hot-work, confined-space, SDS, detector, ventilation, rescue, AHJ, and employer procedures

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 is a screening approximation for idealized mixtures at standard conditions. It can be materially wrong for unusual blends, hydrogen-rich mixtures, elevated temperature or pressure, oxygen enrichment, nonideal gases, mist, dust, or reactive systems. Critical safety work needs current source data, calibrated instruments, site procedures, and qualified review.
As oxygen decreases below normal air, many flammable ranges narrow. This calculator uses a local linear screen between normal air and MOC. Real flammable envelopes are curved and depend on gas identity, inert gas, temperature, pressure, humidity, and oxygen enrichment, so the result is a planning warning, not a lab diagram.
Disclaimer: This calculator is a source-aware planning screen. Local gas-property rows are not a complete current NFPA 497, IEC, SDS, manufacturer, or laboratory table. The model does not account for temperature, pressure, humidity, mist, dust, oxygen enrichment, nonideal gas behavior, detector condition, or site-specific mixing. It does not replace calibrated gas detection instruments, an entry permit, hot-work authorization, ventilation/inerting procedure, rescue plan, SDS review, or qualified safety review.

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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