What Cal/OSHA Requires You to Test For

Three atmospheric conditions must be evaluated before entry, in this order:

The order is not optional. A four-gas monitor reads all three categories at once but the reading sequence inside the instrument follows the same logic: oxygen first because every other sensor depends on it. When we audit a confined space program and find LEL readings recorded without a matching O₂ reading on the same line, that is a deficiency. The entrant who signed the permit had no way to know whether the LEL reading was meaningful.

Test Before Entry, Then Keep Testing

Pre-entry testing alone is not enough. §5157 requires that the atmosphere be evaluated as often as needed to confirm acceptable entry conditions are maintained throughout the entry. In practice, that means one of two patterns:

• Continuous monitoring. The four-gas monitor stays with the entrant inside the space and alarms if any reading moves out of the acceptable range. This is the default for any space with ongoing atmospheric risk (sewers, vessels with chemical residue, spaces near operating equipment).
• Periodic monitoring. Readings are taken at the intervals defined in the entry permit (commonly every 15 to 30 minutes in practice) and recorded. Periodic is acceptable when the hazard is stable and well-characterized; it is not acceptable when conditions are likely to change.

Conditions that require an immediate re-test, regardless of the monitoring pattern:

• Work activity introduces new contaminants (welding, cutting, painting, solvent use).
• Ventilation stops or changes.
• The entrant moves to a different elevation in the space.
• Any alarm triggers, even briefly.
• Entry resumes after an interruption.

Test Every Level of the Space

Gases stratify by density. Methane and hydrogen rise. Hydrogen sulfide and propane sink. Carbon monoxide is close to the density of air but pools where ventilation is weak. A reading taken at the manhole rim says nothing about the bottom of the tank.

Cal/OSHA §5157 requires testing at the levels where work will occur and at any level a person could occupy. When we walk a facility, the most common atmospheric deficiency we see, after blank permit fields, is a single reading taken at the top of the space and used to clear the entire entry. That is not a valid test. A vertical entry into a tank, vault, or pit requires readings at the top, middle, and bottom, typically with a probe extension or a lowered sensor.

Equipment, Calibration, and Who Owns It

A confined space program is only as reliable as the monitor on the entrant’s belt. Cal/OSHA does not specify a meter brand, but it does require that the equipment be appropriate for the contaminants present, in working order, and used by a person trained to interpret the readings.

Practical equipment expectations:

• Four-gas monitor (O₂, LEL, CO, H₂S) is the baseline for general industry. Spaces with chemical-specific hazards may require additional sensors (ammonia, chlorine, volatile organic compounds) or a single-gas instrument for the specific contaminant.
• Bump test before each shift. A bump test exposes the sensors to a known concentration of gas to confirm the alarms work. This is a daily check, not an annual one.
• Calibration on the schedule the manufacturer specifies, typically every 30 to 180 days, with documentation kept on file.
• Sample draw vs. diffusion. Pre-entry testing of an unopened space requires a sample-draw pump and a probe long enough to reach the work area. A diffusion-mode reading from outside the manhole is not a pre-entry test.

CDMS provides the written program, the training, and the entry permit forms that document monitoring. The facility provides the monitor, the calibration records, and the day-to-day operation of the equipment during entries. If equipment availability is a question during program development, we flag it in writing and recommend a path forward.

What Inspectors Look For in Your Test Records

When Cal/OSHA reviews a confined space program, the inspector reads the entry permits and the test logs together. Specific items they look for:

• Real numbers, not initials. “✓” or “OK” in the oxygen field is not a recorded test. The percentage value is.
• A tester’s name next to the reading. The person who took the test is documented and trained.
• A timestamp that matches the entry duration on the permit. Pre-entry tests dated hours before the entry started do not satisfy §5157.
• Re-tests after interruptions. If the entry log shows the team left for lunch and returned, the inspector expects a new atmospheric reading on the return.
• Periodic readings recorded on the same permit, not in a separate book that no one can find.
• Alarm events documented. If the monitor alarmed, the response is on the permit.

Interpreting atmospheric data takes training and judgment. A reading that is technically within range can still be a warning sign (oxygen at 19.6% with a downward trend, LEL climbing from 2% to 6% over ten minutes). The training piece of the program is where that judgment gets built. For frequency and content requirements, see confined space training requirements in California.

Where Atmospheric Testing Connects to the Rest of the Program

Atmospheric data is the trigger for nearly every other decision during an entry. The acceptable entry conditions on the permit are atmospheric values. The rescue plan assumes the monitor will alarm in time to retrieve a worker. The entry permit’s “isolation” field exists partly to prevent the atmosphere from being disturbed by upstream equipment. None of these work in isolation.

For how the test results get captured on the permit itself, see confined space entry permits explained. For how monitoring data informs rescue response, see rescue planning for permit-required confined spaces. For air-monitoring programs that go beyond confined space entry (engineering controls, exposure assessments, hexavalent chromium, silica, lead), see our industrial hygiene services.

Cal/OSHA §5157 vs federal §1910.146 for California employers →

Get Your Atmospheric Testing Procedures Reviewed