The order the standard requires
Under §1910.146(d)(5) and the accompanying non-mandatory appendices, atmospheric testing before entry follows a fixed sequence: oxygen first, flammable gases and vapours second, toxic contaminants last. Test this order every time — pre-entry and any time monitoring is re-verified.
This is the atmospheric leg of the entry programme summarised in our [confined space entry permits](/resources/confined-space-entry-permits) pillar; the permit itself has to record the initial and periodic readings with the tester's initials and time.
Step 1: Oxygen (19.5%–23.5%)
Acceptable atmosphere is 19.5% to 23.5% O₂. Below 19.5% is oxygen-deficient; above 23.5% is oxygen-enriched and dramatically raises the flammability of any combustibles present.
Oxygen goes first because most catalytic-bead LEL sensors require normal ambient oxygen (~20.9%) to give an accurate flammability reading. In an oxygen-deficient atmosphere, the sensor under-reads flammables — sometimes by 50% or more.
Step 2: Flammable gases and vapours (< 10% LEL)
Test for combustibles second, expressed as a percentage of the Lower Explosive Limit. Entry is prohibited at or above 10% LEL. Some sectors — hot work, welding — impose stricter limits at 5% LEL.
If oxygen is outside the acceptable range, do not trust flammability readings. Ventilate to normal oxygen levels first, then retest for combustibles.
Step 3: Toxic contaminants (below PEL)
Toxic sensors — carbon monoxide, hydrogen sulphide, and any process-specific contaminants — go last. Each must read below its OSHA Permissible Exposure Limit. Note that H₂S is deadly at concentrations that don't produce a smell; never trust nose-detection over an instrument.
Process-specific toxics require you to know what the space contains or has contained. A gas detector calibrated for CO and H₂S will silently miss benzene, ammonia, or nitrogen dioxide unless you specifically sensor for them.
Continuous monitoring during entry
Testing isn't only pre-entry. Entrants carry a personal 4-gas monitor (O₂, LEL, CO, H₂S at minimum) that alarms if any parameter drifts outside acceptable ranges. Attendants log periodic readings on the permit.
Bump-test each monitor before every shift; calibrate at the manufacturer's interval (typically monthly). See our full [confined space entry permits](/resources/confined-space-entry-permits) pillar for how the atmospheric log ties into the permit lifecycle, or [book a demo](/book-demo) to see gas-detector integration in the permit-to-work module.