Atmospheric Monitoring and Gas Testing

Atmospheric monitoring is the process of testing air quality before and during confined space entry to detect invisible hazards.
A multi-gas detector typically monitors oxygen levels, flammable gases (LEL), carbon monoxide, and hydrogen sulphide simultaneously.
Normal oxygen concentration is 20.9% — below 19.5% is oxygen-deficient and above 23.5% is oxygen-enriched, both dangerous.
Flammable gas readings above 10% of the lower explosive limit require immediate evacuation and investigation.
Gas monitors must be calibrated at the manufacturer's specified intervals and bump-tested before every shift.
Pre-entry testing must be carried out from outside the confined space using extension probes at multiple levels.
Continuous monitoring must be maintained throughout the entire duration of the entry, not just at the start.
Gas conditions can change rapidly due to disturbance of sludge, chemical reactions, or changes in ventilation.
Only trained persons may interpret gas readings and make decisions about safe entry based on the results.
The gas monitoring results must be recorded on the confined space entry permit as evidence of safe conditions.

Invisible killers	Toxic gases and oxygen depletion are invisible and odourless — gas monitors are the only way to detect them.
Conditions change	An atmosphere that tested safe at entry can become lethal within minutes — continuous monitoring catches rapid deterioration.
Correct calibration	An uncalibrated gas monitor gives false readings that provide dangerous false reassurance — bump testing is essential every shift.

Do

Don't

Bump-test your gas monitor against known gas concentrations before every shift.
Test the atmosphere at multiple levels before entry using an extension sampling probe.
Check oxygen is between 19.5% and 23.5% before permitting confined space entry.
Evacuate immediately if flammable gas readings exceed 10% of the lower explosive limit.
Maintain continuous atmospheric monitoring throughout the duration of every entry.
Record all gas test results on the confined space entry permit before authorising entry.
Ensure your gas monitor is within its calibration date and serviced at required intervals.
Understand the alarm set points on your monitor and what each alarm level means.
Re-test the atmosphere after any interruption, disturbance, or change in ventilation.
Keep the gas monitor on your person, not left at the entry point during entry.

DON'T enter any confined space without testing the atmosphere with a calibrated monitor.
DON'T test from only one position — sample at high, mid, and low levels in the space.
DON'T enter if oxygen readings are outside the 19.5% to 23.5% safe range.
DON'T ignore flammable gas readings — even low levels indicate a hazard that may increase.
DON'T rely on pre-entry testing alone — conditions change and need continuous monitoring.
DON'T issue an entry permit without documenting the gas test results on the permit form.
DON'T use a gas monitor that is overdue for calibration or has failed its bump test.
DON'T dismiss an alarm as a false positive — evacuate first, then investigate from outside.
DON'T assume the atmosphere is still safe after a break — re-test before re-entering.
DON'T leave your gas monitor at the entrance — wear it on your person throughout entry.

See also: Confined Space Awareness | Confined Space Entry Procedures

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