A standard 4-gas monitor detects four atmospheric hazards: oxygen (O2), combustible gases (LEL), carbon monoxide (CO) and hydrogen sulfide (H2S). These are the four hazards most likely to injure or kill workers in confined spaces and industrial environments, which is why they became the industry-standard package. This guide explains each gas, why these four, the 5-gas options, and how to calibrate a 4-gas monitor.
The standard four gases
- Oxygen (O2) — normal air is 20.9% O2. Monitors warn for oxygen deficiency (typically below 19.5%) and enrichment (above 23.5%). Low O2 is the leading cause of confined-space deaths.
- Combustibles / LEL — flammable gases and vapors, measured as a percentage of the Lower Explosive Limit (LEL). Detectors usually alarm at 10% LEL, well before an explosive atmosphere forms.
- Carbon monoxide (CO) — a toxic, odorless gas from combustion. Calibrated in ppm; common alarm points are 35 ppm (TWA) and higher for short-term exposure.
- Hydrogen sulfide (H2S) — toxic "rotten egg" gas common in oil & gas, wastewater, and refining. Extremely dangerous; sensors are calibrated at low ppm levels.
Why these four gases?
Together they cover the three categories OSHA requires you to test for in a confined space: oxygen level, flammability, and toxicity. CO and H2S are the two most commonly encountered toxic gases across industry, so the "4-gas" package (O2 + LEL + CO + H2S) protects against the widest set of real-world hazards with one instrument.
5-gas and expanded options
Some applications add a fifth sensor — commonly nitrogen dioxide (NO2), sulfur dioxide (SO2), or a photoionization detector (PID) for volatile organics. Trigas USA offers 4-gas and 5-gas mixes matched to these configurations.
How a 4-gas calibration mix is built
A 4-gas calibration cylinder contains all four components at certified concentrations in a single balance gas — for example 100 ppm CO / 25 ppm H2S / 50% LEL methane / 18% O2, balance nitrogen. One cylinder calibrates all four sensors at once. The exact recipe varies slightly by detector brand; find yours on our calibration gas by detector page.
Calibrating a 4-gas monitor
4-gas monitors need a daily bump test and periodic full calibration with NIST-traceable gas. See our full calibration guide for the step-by-step process, or jump to the 4-gas mixes.
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Direct drop-in for MSA, BW, Industrial Scientific, RAE, Draeger & RKI — NIST-traceable, 15–30% below OEM, ships in 48 hours from Florida.
Frequently asked questions
What 4 gases does a 4-gas monitor detect?
Oxygen (O2), combustible gases (LEL), carbon monoxide (CO), and hydrogen sulfide (H2S).
What is a 5-gas monitor?
A 4-gas monitor plus a fifth sensor — commonly NO2, SO2, or a PID for volatile organic compounds.
What gas calibrates a 4-gas monitor?
A 4-gas mix containing CO, H2S, a combustible (methane or pentane), and O2 in one cylinder, e.g. 100 ppm CO / 25 ppm H2S / 50% LEL CH4 / 18% O2.