Why model residual oxygen?
Inerting, purging, or displacement with nitrogen (or other gases) can drive oxygen below the safe entry band. A simple mixing model estimates residual O₂% from space volume and inert volume added.
Typical workplace band for entry is often cited as about 19.5–23.5% O₂ — always confirm with calibrated meters and the permit.
Oxygen Deficiency Calculator
Confined space · residual oxygen
Estimate O₂% after adding inert gas. Flag deficiency (<19.5%) or enrichment (>23.5%). Always verify with meters.
Formula: O₂% ≈ 20.9 × e^(−V_inert ÷ V) or O₂% = 20.9 × (1 − V_displaced ÷ V)
- Ideal mixing only — stratify and dead zones are common.
- Continuous ventilation and monitoring beat one-shot purge maths.
- Permit and rescue plan remain mandatory.
Frequently asked questions
Is the exponential model exact?
No. It assumes well-mixed ideal behaviour. Dead zones and stratification make real O₂ lower in places — gas-test.
Enrichment risk?
Oxygen above ~23.5% increases fire intensity. Do not assume “more air is safer”.
Replace or dilute?
Use the mode that matches your purge method. Displacement toward empty then refill is different from continuous dilution.
Does this replace a permit?
Never. It is a planning aid only.
