Confined-Space Pre-Entry Ventilation (OSHA 1910.146)
L x W x H volume, contaminant-driven target ACH (combustible / O2-deficient / H2S / CO / general).
Example
You enter
- Length (ft) 10
- Width (ft) 10
- Height (ft) 10
- Blower CFM (Q) 200
- Target contaminant general
You get
- Volume (ft³) 1000
- Minutes to purge 35 min
- Steady-state ACH 12
Checked against published worked examples from: OSHA / NIOSH.
Details, formula, and sources
ScopeMinutes-to-purge and steady-state ACH, plus the 1910.146(d)(5) 4-gas-meter reminder. Companion to a confined-space purge calculation.
Formula or tableV = L * W * H (or operator-supplied volume). minutes_to_purge = V * N / Q. steady_ACH = Q * 60 / V. Default N (target air-changes) keyed to contaminant class: combustible-gas / oxygen-deficient / general = 7; H2S / CO = 10.
Source and editionOSHA 29 CFR 1910.146 (Permit-Required Confined Spaces); NIOSH 80-106 (Working in Confined Spaces).
Free accessecfr.gov for 1910.146; cdc.gov/niosh for NIOSH 80-106.
What governsEstimate. AHJ and licensed fire protection engineer govern.
API field nameslength_ft, width_ft, height_ft, blower_cfm, contaminant, volume_ft3, minutes_to_purge, steady_ACH
- Default ACH targets combustible-gas / oxygen-deficient / general 7 ACH; H2S / CO 10 ACHNIOSH 80-106 typical and engineering practice for denser-than-air contaminants
- Pre-entry monitoring calibrated 4-gas meter readings before and during entry are required regardless of ventilation resultOSHA 29 CFR 1910.146(d)(5)
- Oxygen range O2 must be maintained between 19.5% and 23.5%OSHA 29 CFR 1910.146
- Purge-time warning purge time above 60 minutes surfaces a higher-capacity-blower hintengineering practice
- Steady-ACH floor steady-state ACH below 6 surfaces a 'verify blower placement / path length' warning per NIOSH 80-106 typical minimumspec-v9 §C.6