Atmospheric monitoring is a critical safety requirement before and during entry into any confined space. The atmosphere inside confined spaces can contain insufficient oxygen, toxic gases such as hydrogen sulphide and carbon monoxide, or flammable gases such as methane. These conditions can change rapidly and without warning. Gas detection equipment must be used by trained personnel, properly calibrated, and the readings must be understood and acted upon before any person enters the space.
Key Hazards
Oxygen depletion below 19.5% causing impaired judgement and loss of consciousness
Toxic gas concentrations exceeding safe limits causing rapid incapacitation
Flammable gas accumulation creating an explosion risk from ignition sources
Inaccurate readings from poorly calibrated or incorrectly used gas detectors
Control Measures
Test the atmosphere before every confined space entry using a calibrated multi-gas detector.
Monitor for oxygen (O2), flammable gases (LEL), carbon monoxide (CO), and hydrogen sulphide (H2S).
Ensure the gas detector has been bump-tested at the start of each shift before use.
Lower the detector into the space on a line to test at different levels before anyone enters.
Continue monitoring the atmosphere continuously throughout the entire duration of the entry.
Ensure the person operating the gas detector is trained in its use and understands the alarm levels.
Record all gas readings on the confined space entry permit before authorising entry.
Evacuate the space immediately if any gas alarm activates and do not re-enter until investigated.
Calibrate gas detectors at the intervals specified by the manufacturer and maintain records.
Emergency / Rescue
If the gas detector alarms during entry, evacuate the confined space immediately using the nearest exit. Raise the alarm and inform the entry controller. Do not re-enter until the atmosphere has been retested and confirmed safe.
Remember
Gas detection is mandatory before and during every confined space entry without exception.
Test at multiple levels because gases stratify — heavy gases sink and light gases rise.
Bump test the detector at the start of every shift to confirm the sensors are responding correctly.
Oxygen levels below 19.5% are dangerous and levels below 16% can cause loss of consciousness.
If any alarm sounds during entry, evacuate immediately and do not wait to investigate the cause.
Gas conditions in confined spaces can change rapidly so continuous monitoring is essential throughout.
Applicable Legislation: Confined Spaces Regulations 1997 · COSHH Regulations 2002 · Health and Safety at Work etc. Act 1974 · BSIF Guide to Gas Detection