Atmospheric monitoring is mandatory before and during every confined space entry. The four-gas monitor checks oxygen, lower explosive limit, carbon monoxide and hydrogen sulphide.

Parameter	Acceptable range	Alarm action
Oxygen (O&sub2;)	19.5%–23.5%	Below 19.5% = oxygen deficient (asphyxiation risk); above 23.5% = oxygen enriched (fire risk). Evacuate, ventilate, re-test
Lower Explosive Limit (LEL)	< 10% of LEL	10–25% LEL: evacuate, ventilate; above 25% LEL: do not enter, treat as explosive atmosphere
Carbon monoxide (CO)	< 30 ppm (long-term); short-term limit 200 ppm	Above 30 ppm sustained or 200 ppm peak: evacuate; ventilate; investigate source
Hydrogen sulphide (H&sub2;S)	< 5 ppm (long-term); short-term limit 10 ppm	Above 5 ppm sustained or 10 ppm peak: evacuate immediately; H&sub2;S is fatal at 100 ppm and dulls smell at low concentration

AMWS confined space categories: Category O (low-risk) — chambers and pits below ground level with permanent ventilation, no chemical hazard, free egress (e.g. shallow valve chamber). Category A (medium-risk) — deeper chambers with restricted ventilation but no specified hazard substance (e.g. deep meter chambers). Category B / C (high-risk) — sewers, manholes connected to live foul/storm system, raw-water structures, any structure with potential for atmospheric / engulfment / asphyxiation hazard. Operational rules: all confined space entry requires a minimum of two operatives (entrant + attendant) per SOP 8.8; rescue plan must be in place before entry — do not enter without a means of recovery without entrant rescue. Tripod and winch where appropriate. RPE selection per SOP 8.4: BA (breathing apparatus) for IDLH atmospheres, never filtering RPE in oxygen-deficient atmospheres.