A standard 4-gas monitor detects four atmospheric hazards: oxygen levels, carbon monoxide (CO), hydrogen sulfide (H₂S), and combustible gases. These are the most common threats in confined spaces, industrial sites, and anywhere workers might encounter dangerous air quality. Each sensor watches for a different type of danger, from toxic exposure to explosion risk to suffocation.
The Four Standard Sensors
Nearly every 4-gas monitor ships with the same default sensor configuration. Here’s what each one does and why it matters.
Oxygen (O₂)
Normal air contains about 20.9% oxygen. The oxygen sensor continuously measures that percentage and alerts you when it drifts outside the safe range. OSHA defines a hazardous atmosphere as anything below 19.5% or above 23.5% oxygen by volume. Too little oxygen causes dizziness, confusion, and eventually unconsciousness. Too much creates a fire and explosion hazard, since materials that wouldn’t normally ignite can burn aggressively in oxygen-enriched air. Most monitors set their low alarm at 19.5% and their high alarm at 23.5% to match these OSHA thresholds.
Carbon Monoxide (CO)
Carbon monoxide is colorless and odorless, which makes it impossible to detect without a sensor. It’s produced by engines, generators, furnaces, and any incomplete combustion. The CO sensor uses an electrochemical cell that generates a tiny electrical signal proportional to the gas concentration. OSHA’s permissible exposure limit for an 8-hour workday is 50 ppm. Default alarm settings on most monitors are more conservative: a low alarm at 35 ppm and a high alarm at 70 ppm, with a short-term exposure limit of 200 ppm.
Hydrogen Sulfide (H₂S)
Hydrogen sulfide smells like rotten eggs at low concentrations, but at dangerous levels it paralyzes your sense of smell entirely. That’s what makes it so deadly: by the time you can’t smell it anymore, you’re already in serious trouble. H₂S is common in sewers, oil and gas operations, wastewater treatment plants, and agricultural settings. Most monitors trigger a low alarm at 10 ppm and a high alarm at 20 ppm. Concentrations above 100 ppm can be immediately life-threatening.
Combustible Gases (LEL)
The fourth sensor detects flammable gases and vapors, including methane, propane, butane, and hydrogen. Rather than measuring a specific gas, it reports how close the atmosphere is to being explosive, expressed as a percentage of the Lower Explosive Limit (LEL). An LEL reading of 0% means no combustible gas is present. A reading of 100% LEL means the air has reached the minimum concentration needed to ignite. Most monitors set a low alarm at 10% LEL and a high alarm at 20% LEL, giving you time to evacuate well before conditions become explosive.
The combustible gas sensor works using a catalytic bead: a small platinum coil coated with a catalyst that heats to roughly 500-550°C. When flammable gas passes over the bead, it oxidizes and raises the temperature further, changing the electrical resistance. The sensor compares this to a reference bead that can’t oxidize gas, and the difference produces the reading. One important limitation is that this sensor needs at least 12% oxygen in the atmosphere to function, since it relies on oxidation. In oxygen-deficient environments, the LEL reading may be unreliable.
Cross-Sensitivity: When Sensors React to the Wrong Gas
Every electrochemical sensor has some tendency to respond to gases other than its target. This is called cross-sensitivity, and understanding it helps you interpret readings correctly.
The CO sensor is mostly specific to carbon monoxide, but it picks up hydrogen gas at about 20% of its true concentration. More dramatically, acetylene triggers a CO sensor at roughly 202% of the actual concentration, meaning a small amount of acetylene can produce a large false CO reading. Ethylene also causes interference at around 77%. If you’re working near welding operations or in environments with these gases, a CO alarm doesn’t necessarily mean carbon monoxide is present.
The H₂S sensor has its own quirks. It responds strongly to hydrochloric acid, reading at about 300% of the actual concentration. Methyl mercaptan (the odorant added to natural gas) triggers an H₂S sensor at about 77% of its concentration, and ethyl mercaptan at 34%. Nitrogen dioxide actually causes a negative reading on H₂S sensors, suppressing the display by roughly 40%. That means in environments with both H₂S and NO₂, the monitor could underreport the actual hydrogen sulfide level.
Cross-sensitivity values apply to new sensors and can shift as sensors age. They also vary between sensor manufacturers and models.
Daily Testing and Calibration
A 4-gas monitor is only useful if its sensors are responding accurately. The International Safety Equipment Association recommends verifying that the instrument works before each day’s use. The simplest check is a bump test: you expose the sensors briefly to a known concentration of test gas and confirm the alarms trigger. If the monitor fails a bump test, a full calibration is needed before using it in the field.
Full calibration involves exposing each sensor to a certified concentration of gas and adjusting the readings to match. How often you need a full calibration depends on the manufacturer’s recommendations, but it’s always required after a failed bump test and typically at regular intervals (often monthly, though some workplaces calibrate weekly). Sensors degrade over time and eventually need replacement, usually every two to three years for electrochemical cells.
Where 4-Gas Monitors Are Used
The standard 4-gas configuration covers the hazards that show up most often across a wide range of industries. Confined space entry is the most common application: tanks, manholes, tunnels, silos, and any enclosed area where dangerous gases can accumulate or oxygen can be displaced. Oil and gas workers carry them on drilling platforms and around wellheads. Firefighters use them during overhaul to check for CO in structures. Utility workers bring them into underground vaults. Wastewater crews rely on them every time they open a manhole cover.
Some manufacturers offer 4-gas monitors with swappable sensors, letting you replace one of the standard four with a sensor for sulfur dioxide, nitrogen dioxide, chlorine, or other gases specific to your industry. The monitor still reads four gases at a time, but the configuration matches the actual hazards at your worksite rather than the default setup. If your workplace has a known hazard beyond the standard four, talk to your safety team about whether a custom sensor configuration makes more sense.