Atmospheric monitoring (confined space)
Atmospheric monitoring is continuous measurement of O2, LEL, CO and H2S throughout a confined-space entry. AS/NZS 2865 mandates it where hazards may change.
Ask Chalkline about this →Atmospheric monitoring in confined space work is the continuous measurement of the atmosphere throughout the duration of an entry, using a worn or stationed multi-gas detector that alarms if any monitored parameter moves outside safe limits. It is distinguished from atmospheric testing, which is a discrete pre-entry measurement to confirm the atmosphere is safe before anyone enters. Both are required steps under AS/NZS 2865:2009, in different roles.
Standard four-gas parameters monitored:
| Parameter | Safe range | Alarm trigger | Why it matters |
|---|---|---|---|
| Oxygen (O₂) | 19.5%-23.5% | Below 19.5% or above 23.5% | Below: asphyxiation. Above: combustibility risk |
| Lower Explosive Limit (LEL) for flammable gas | 0%-10% LEL | At or above 10% LEL | Approaching combustible mixture; ignition risk |
| Carbon monoxide (CO) | 0-30 ppm | At 30 ppm typically | Asphyxiant, accumulates from combustion |
| Hydrogen sulphide (H₂S) | 0-10 ppm | At 10 ppm typically | Highly toxic, smells of rotten eggs at low ppm but is paralyzing at higher |
Some sites monitor additional gases: ammonia (NH₃), volatile organic compounds (VOCs), specific contaminants relevant to the work. Sewage pits commonly need H₂S + methane; agricultural pits often need NH₃ + CO₂; building basements need CO + LEL.
Atmospheric monitoring vs atmospheric testing:
| Activity | When | Equipment |
|---|---|---|
| Atmospheric testing (pre-entry) | Before each entry, after re-opening, after any break | Same multi-gas detector, snapshot reading |
| Atmospheric monitoring (continuous) | Throughout the entry, every minute or continuously | Worn or stationed detector, continuous logging |
The same multi-gas detector typically does both jobs. The key distinction is time: testing is one moment, monitoring is throughout.
When continuous monitoring is required (AS/NZS 2865):
- The atmosphere may change during the entry (e.g. work activity releases gas, ventilation may fail, source of contamination is not eliminated).
- The space contains hot work (welding, oxy-acetylene cutting) that introduces ignition or oxygen displacement risks.
- Workers are using respirators or supplied-air breathing apparatus that depends on the atmosphere remaining stable.
- The space cannot be visually inspected from the entry point.
Effectively, most confined space entries in construction require continuous monitoring, not just pre-entry testing.
Detector specifications:
- Calibrated to manufacturer schedule (typically zero-cal daily, bump-test daily, full-cal every 6 months).
- Logged: continuous readings stored and downloadable to satisfy WHS recordkeeping.
- Alarm types: visual (LED), audible (>85 dB), vibration (in noisy environments).
- Battery life: at least the duration of the shift, usually 12+ hours.
- Worn on the body in the breathing zone, or stationed in the air the worker breathes.
Common defects:
- Detector zero’d inside a contaminated space (zero values misread as “clean” when contamination is present). Always zero in clean air.
- Bump-test skipped (alarm not actually verified before entry). Detector may have a failed sensor.
- Reading the detector on the wrong worker (one detector on a standby person, multiple workers in space). Every worker in a confined space needs their own detector or one shared in their breathing zone.
- Failure to evacuate on alarm: the alarm is the trigger to leave, not to “check what’s happening”. Always evacuate first, investigate after.
Also known as: continuous gas monitoring; continuous atmospheric testing; multi-gas monitoring; CSE gas monitoring.
Category: WHS.
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Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.