How do I convert ppm to %LEL?

%LEL = (ppm / LEL_ppm) × 100, where LEL_ppm is the lower explosive limit of that gas in ppm. For methane LEL = 50000 ppm (5% by volume), so 5000 ppm methane = 10% LEL. The tool handles the gas-specific lookup automatically.

Why is mg/m³ different from ppm for the same gas?

ppm is a volume ratio; mg/m³ is a mass concentration. The conversion factor depends on the gas's molar mass: mg/m³ = ppm × MW / 24.45 at 25 °C and 1 atm. Heavy gases (Cl₂, SO₂) have larger conversion factors than light gases (CH₄, H₂).

What's the action level for %LEL?

Industry-standard alarms: 10% LEL = first alarm (evacuate from confined space); 20% LEL = high alarm (all personnel evacuate, stop hot work). For continuous monitoring, OSHA 1910.146 confined-space entry requires < 10% LEL before entry.

How long should the reserve be?

NFPA 1404 specifies a 33% reserve for escape — i.e. one-third of the cylinder must remain when the working phase ends. The tool reports both working duration and total duration, and warns if work-phase + escape leaves under 33% reserve.

What RMV should I use?

Light work: 20 L/min. Moderate: 30 L/min. Heavy work / escape under stress: 40 L/min. NIOSH bases SCBA ratings on 40 L/min — heavy work shortens duration considerably.

Why does the cylinder pressure matter so much?

Service life ≈ (P_cyl − P_reserve) × V_cyl / P_atm / RMV. Halving the usable pressure halves the duration linearly. Always start with a fully charged cylinder; ≥ 90% rated pressure is industry practice.

How many air changes per hour are recommended?

Per OSHA 1910.146 confined-space entry: minimum 20 ACH before entry to clear residual atmosphere, and continuous ventilation while work is underway. Hot work (welding, cutting) typically requires ≥ 30 ACH and continuous gas monitoring.

What blower size do I need?

Required airflow Q = volume × ACH. A 10 m³ vessel needing 20 ACH requires Q = 200 m³/h ≈ 118 CFM. Add 25–50% for duct losses and account for back-pressure if the space is congested.

Can I rely on natural ventilation?

Only for spaces with multiple open hatches at different heights AND no IDLH potential. Per most HSE standards, mechanical ventilation is required for any space classified as a confined space, both pre-entry and continuous.

What's the difference between TWA and STEL?

TWA (Time-Weighted Average) is the average exposure over an 8-hour shift. STEL (Short-Term Exposure Limit) is a 15-minute TWA that must not be exceeded at any point during the shift, regardless of the 8-hour average.

How do I sum exposures with varying concentrations?

TWA = Σ(C_i × t_i) / 8h. Each segment's concentration is weighted by its duration. The tool does this automatically when you add multiple exposure rows.

Are ACGIH TLVs legally binding?

TLVs are recommended limits, not regulations. Legally enforceable limits come from OSHA (PEL in the US), HSE WELs (UK), or national authority equivalents. Many TLVs are tighter than PELs; using TLVs gives a safety margin.

Gas Safety Calculators – LEL, SCBA, TWA

JavaScript is disabled in your browser. The calculators on this page require JavaScript to compute results. The tool descriptions, formulas and reference standards remain readable below — useful for AI assistants and search engines that index without executing scripts.

5Gas Unit Converter

Convert between ppm ↔ %LEL ↔ mg/m³ ↔ %Volume for the most common industrial gases. Conversion uses the molar volume at the selected T & P; defaults are 25 °C and 101.325 kPa per NIOSH.

Gas i

Temperature (°C)

Pressure (kPa)

Input value

Converted values

ppm—

% LEL—

mg/m³—

% Volume—

6SCBA Duration Calculator

Service life ≈ [(Pᶜʸˡ − Pʳᵉˢ) × Vᶜʸˡ / Pₐₜₘ] / RMV, with breathing rate scaled by work intensity. Output includes the recommended one-third safety reserve for escape (NFPA 1404).

Cylinder pressure

Cylinder water volume

Common SCBA cylinders: 6.0 L (~1200 L air @ 200 bar), 6.8 L (~2050 L air @ 300 bar), 9.0 L.

Reserve / alarm pressure

Work intensity i

Air available

Free air in cylinder—

Usable air (above reserve)—

Working duration

— min total

— min safe (after reserve)

—

7Confined Space Ventilation Calculator

Compute the airflow required to achieve the target air-changes-per-hour (ACH) for a confined space: Q = (V × ACH) / 60 in CFM, also expressed in m³/h.

Length

Width

Height

Dimension unit

Target air changes per hour (ACH) i

Ventilation type

Required airflow

Volume—

Required Q—

Recommendation—

—

20TWA / STEL Exposure Calculator

Add up to 6 exposure intervals during a shift, then the tool computes the 8-hour Time-Weighted Average and compares against the OEL/STEL for the selected gas. Formula: TWA₈ = Σ(Cᵢ·tᵢ) / 8 h.

Gas / agent

Exposure intervals

Activity	Conc (ppm)	Duration (min)

Exposure summary

— ppm · 8-h TWA

—

Total weighted exposure (ppm·h)—

Total exposed time—

Peak short-term concentration—

vs TWA limit—

vs STEL (15-min limit)—

—

For training purposes only. Defer to NIOSH, OSHA 1910.146, NFPA and the gas-detector manufacturer's calibration data for actual entry decisions.

44Welding Fume Exposure (TLV / PEL screen)

Estimates breathing-zone metal-fume concentration from welding process + duty cycle and checks against ACGIH TLV-TWA limits. Indicative; site-specific air monitoring required for confirmation.

Process

Material (governs TLV)

Local exhaust ventilation (LEV) efficiency

Duty cycle (% arc-on)

Shift length (hours)

Result

— mg/m³ TWA

—

TLV-TWA (this metal)—

% of TLV—

Recommendation—

Reference: ACGIH TLV Book 2024, OSHA 1910.252. IARC classifies welding fume as Group 1 carcinogen.

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Gas Safety Calculators

5Gas Unit Converter

Converted values

Exposure status

6SCBA Duration Calculator

Air available

Working duration

7Confined Space Ventilation Calculator

Required airflow

20TWA / STEL Exposure Calculator

Exposure intervals

Exposure summary

44Welding Fume Exposure (TLV / PEL screen)

Result

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