Atmospheric Monitoring Gap During a Confined Space Entry
A 12 m diameter floating-roof gasoline storage tank has been emptied and isolated. The entry supervisor arrives at 06:00 for a planned internal inspection. The night-shift technician's gas test, logged at 22:00 the previous evening, recorded 19.8% oxygen, 0 ppm LEL, and 0 ppm H2S. The supervisor's portable meter is due for calibration; its last bump test was 30 hours ago and the calibration certificate expired 11 days ago. The tank has been under forced ventilation through a single 300 mm manway since 23:00. Ambient temperature dropped from 18 C at 22:00 to 4 C overnight. The supervisor wants to begin entry immediately to stay on schedule, arguing the tank was 'clean and stable all night.' The entrant and attendant are ready at the manway.
1. Evaluate whether entry should proceed. Identify the specific deficiencies and what must happen before entry.
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Entry must not proceed. Three separate failures compound:
- Stale pre-entry test. A gas test from 8 hours earlier does not satisfy pre-entry testing requirements; conditions change. Testing must be repeated immediately before entry, at the entry point and at representative depths.
- Unreliable instrument. The meter is out of calibration and its bump test is overdue. An uncalibrated meter's readings cannot be trusted to authorize entry. A calibrated, bump-tested meter must be used.
- Temperature-driven change. A 14 C overnight drop can shift vapor behavior and stratification; the 'stable all night' assumption is unsupported. Ventilation through one manway may also be inadequate to keep the atmosphere uniform.
Before entry: obtain a calibrated instrument, perform a fresh bump test and full pre-entry test (O2, LEL, H2S, plus any other required contaminants), confirm ventilation effectiveness, and document results. Entry is authorized only on current, valid data.
2. The supervisor proposes entering briefly 'just to look' while a new meter is fetched. Explain why this is unacceptable and what supervisory controls apply.
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A 'quick look' entry is still a confined space entry and carries the same hazards: oxygen deficiency, flammable atmosphere, and toxic exposure. A short duration does not reduce risk, and without valid atmospheric data the entrant could be exposed to an unknown, potentially fatal atmosphere. The attendant could not monitor effectively and rescue provisions are not confirmed.
Controls: the entry supervisor has authority and responsibility to deny entry when conditions are not verified. No entry without a current gas test, a functioning calibrated meter, an entry permit, a standby attendant, and rescue arrangements. The supervisor should document the denial and the reason, and reschedule. Pressure to meet schedule never overrides atmospheric verification.