Pre-entry testing and monitoring of confined spaces
Pre-entry testing and monitoring of confined spaces
This domain tests the order and technique of testing before entry, and the monitoring that continues during occupancy.
Core rules
- Test from outside. Use a pumped instrument and a long probe or sample line; never lean into the space to take a reading.
- Sequence: oxygen first (flammable and toxic sensors can be affected by oxygen, and oxygen deficiency or enrichment changes the hazard), then flammable gas (%LEL), then toxic gas. Allow the sample line to purge fully before reading.
- Test at multiple levels and locations: top, middle and bottom, plus the work area itself and adjacent connected spaces. Heavy gases such as H2S, propane and CO2 collect low; methane and hydrogen collect high. Open drains, sumps and ducts can carry gas into the space.
- Entry criteria are site-specific but commonly: O2 19.5–23.5%, flammable gas below 10% LEL (many permits require lower, and hot work requires 0% LEL), and toxics below the WEL and any permit-specific limit (for example H2S below 5 ppm, CO below 30 ppm 8-hour TWA).
- Continuous monitoring during occupancy, with periodic re-tests at defined intervals, after breaks, after ventilation changes and whenever the work changes. Conditions are not static.
Worked example
A 3 m pit containing sludge reads 20.9% O2 and 0% LEL at the top, but 15% O2 and 12% LEL at the bottom. Reasoning: the heavier components are at the bottom and the sludge is releasing gas. The space is not safe to enter. Ventilate, re-test at all levels, and only enter when all criteria are met with continuous monitoring in place.
Traps
Testing only at the entrance; testing before isolation or purging; lowering a diffusion meter on a rope; skipping re-tests after a break; assuming mechanical ventilation is still running; using oxygen to ventilate.