What is confined space purge time?
Before entry, teams often ventilate or purge a confined space to lower a contaminant (or raise oxygen) toward a safe target. Under a simple perfect-mixing dilution model:
t = (V ÷ Q) × ln(C₀ ÷ C)
where V is space volume, Q is clean-air ventilation rate, C₀ is initial concentration, and C is the target. Air changes needed ≈ ln(C₀ ÷ C).
This is a planning aid only. Real spaces have dead zones, adsorption, and stratification — always gas-test, follow the permit, and use a competent person.
Confined Space Purge Calculator
Confined space · dilution purge
Estimate ventilation time to dilute from C₀ to target C under a perfect-mixing model. Planning aid only — always gas-test and follow the permit.
Formula: t = (V ÷ Q) × ln(C₀ ÷ C) · Air changes = ln(C₀ ÷ C)
- Not a substitute for atmospheric testing or rescue planning.
- Exhaust-only systems may draw contaminated make-up air.
- Increase time for poor mixing, long ducts, or porous residues.
Frequently asked questions
Does this guarantee safe entry?
No. It estimates dilution time under ideal mixing. Permits, isolation, continuous monitoring, and rescue plans remain mandatory.
What about oxygen deficiency?
The same dilution form can approximate raising oxygen if you treat “contaminant” as nitrogen enrichment carefully — in practice use oxygen-specific procedures and meters.
Should Q be supply or exhaust?
Use the effective clean-air exchange rate for the space. Exhaust-only systems may pull contaminated make-up air — design matters.
Why is measured clearance slower?
Poor mixing, long ducts, porous deposits, and temperature gradients all extend real purge times beyond the ideal model.
