A confined space gas detector is the portable instrument that keeps workers alive in tanks, vaults, sewers, silos and other confined spaces. This complete guide covers the four atmospheric hazards you must test for, the OSHA testing order, how to choose a detector, and how to calibrate it correctly.
The four atmospheric hazards in a confined space
- Oxygen deficiency or enrichment — the number-one killer in confined spaces. Normal is 20.9%; alarms typically trigger below 19.5% and above 23.5%.
- Combustible gases (LEL) — flammable vapors that can ignite. Measured in %LEL.
- Carbon monoxide (CO) — toxic byproduct of combustion.
- Hydrogen sulfide (H2S) — toxic gas common in sewers, wastewater and oil & gas.
OSHA testing order: O2 → combustibles → toxics
OSHA's confined space standard (29 CFR 1910.146) requires testing the atmosphere in this order: first oxygen, then combustible gases and vapors, then toxic gases and vapors. The order matters because oxygen level affects combustible-sensor readings, and you must confirm a safe, breathable, non-explosive atmosphere before entry. Re-test or monitor continuously as conditions can change.
Choosing a confined space gas detector
Most confined-space work uses a 4-gas monitor (O2, LEL, CO, H2S). Leading models include the MSA Altair 4X, Honeywell BW GasAlert Quattro, Industrial Scientific Ventis MX4, RAE MultiRAE, Draeger X-am and RKI GX series. Choose based on the gases present, sampling method (diffusion vs. pump for remote sampling), and your fleet's standard.
Calibration and bump testing for confined space
A confined-space detector must be bump tested before each day's use and fully calibrated on schedule with NIST-traceable gas. A failed or uncalibrated monitor gives false confidence — the most dangerous condition of all.
Recommended calibration gas
Use a 4-gas confined-space mix matched to your detector — for example 100 ppm CO / 25 ppm H2S / 50% LEL CH4 / 18% O2. Match an OEM part number on our cross-reference.
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Frequently asked questions
What gases do you test for in a confined space?
Oxygen, combustible gases (LEL), and toxic gases — most commonly carbon monoxide (CO) and hydrogen sulfide (H2S).
What order does OSHA require for confined space testing?
Oxygen first, then combustible gases and vapors, then toxic gases and vapors (29 CFR 1910.146).
What detector is used for confined space?
A 4-gas monitor such as the MSA Altair 4X, BW GasAlert Quattro, or Industrial Scientific Ventis MX4, calibrated with a 4-gas mix.