What are the 5 gases a portable detector must measure onboard?

A portable gas detector onboard must measure five gases: oxygen (O₂), flammable gases (expressed as a percentage of the Lower Explosive Limit), carbon monoxide (CO), hydrogen sulfide (H₂S), and carbon dioxide (CO₂). This fifth gas, CO₂, became a mandatory testing parameter under IMO Resolution MSC 581(110), which was endorsed in December 2025 and supersedes the older Resolution A.1050(27). The sections below explain why each gas matters, what the safe limits are, and what to look for when choosing compliant equipment.

Why must ships carry a portable multi-gas detector?

Ships must carry portable multi-gas detectors because SOLAS Regulation XI-1/7 requires vessels to have the means to test the atmosphere of enclosed spaces before crew members enter them. Without confirmed atmospheric data, entry into a cargo hold, tank, or any confined space is a life-threatening act based on guesswork rather than evidence.

The reason this requirement exists is straightforward: enclosed spaces on ships can become lethal without any visible warning. Oxygen can be consumed by rusting steel, CO₂ can accumulate from decomposing cargo, and flammable or toxic gases can build up from residual cargo vapors or fuel leaks. None of these hazards are visible or smell strong enough to warn a crew member before they become incapacitated.

IMO Resolution MSC 581(110), titled the Revised Recommendations for Entering Enclosed Spaces on Board Ships, significantly strengthened these requirements. The resolution introduces a “Closed-Loop Management” philosophy, meaning atmospheric safety must be continuously monitored, verifiable, and backed by a documented rescue framework. Entry permits now carry a maximum validity of 8 hours, after which a full reassessment and re-test are required before work may continue. An unrecorded test is treated by Port State Control inspectors as a test that never happened.

For Fleet Engineers and Technical Superintendents, this means the detector your crew carries directly determines whether an entry permit is valid and whether a Port State Control inspection passes. Investing in the right gas detection equipment is not a paperwork exercise; it is the last line of defense before a crew member steps into a potentially fatal atmosphere.

What are the 5 gases a portable detector must measure?

Under MSC 581(110), a compliant portable gas detector must be capable of measuring oxygen (O₂), flammable gases (LEL), carbon monoxide (CO), hydrogen sulfide (H₂S), and carbon dioxide (CO₂). The first four were already the industry standard in so-called “4-in-1” detectors, but CO₂ is now explicitly required as the fifth mandatory parameter for pre-entry testing.

Here is a summary of each gas and its regulatory limit:

Oxygen (O₂): Must be at or above 20.9%. Some flag states permit a minimum of 19.5%, but 20.9% is the standard reference point.
Flammable gases (LEL): Must remain below 1% of the Lower Explosive Limit.
Carbon monoxide (CO): Must stay below 50% of the applicable Occupational Exposure Limit (OEL).
Hydrogen sulfide (H₂S): Must stay below 50% of the applicable OEL.
Carbon dioxide (CO₂): Must be confirmed below 0.5%, which equals 5,000 parts per million (ppm).

The addition of CO₂ is the most significant technical change MSC 581(110) introduces. Most legacy 4-in-1 detectors use electrochemical sensors that cannot accurately measure CO₂ at the ppm resolution needed to confirm the 5,000 ppm limit. The benchmark technology for CO₂ detection is now Non-Dispersive Infrared (NDIR), which provides the stability and resolution required for compliant testing. If your current fleet of detectors does not include NDIR-based CO₂ measurement, the recommended compliance path is to supplement existing equipment with a dedicated standalone CO₂ detector rather than replacing every unit immediately.

What happens if oxygen levels fall below safe limits onboard?

If oxygen levels fall below safe limits onboard, crew members face rapid incapacitation and potentially fatal asphyxiation. The human body requires oxygen concentrations close to the normal atmospheric level of 20.9% to function safely. A drop to around 17% causes impaired judgment and coordination; below 16%, symptoms become severe; and at concentrations below 10%, unconsciousness and death can follow within minutes.

What makes oxygen depletion particularly dangerous in a maritime context is that it is invisible and odorless. A crew member entering a depleted space may feel only slightly dizzy before losing consciousness, with no time to self-rescue. This is why MSC 581(110) explicitly addresses the oxygen-depleting properties of iron in damp environments. Ordinary rusting in a confined, damp space, whether from the ship’s own steel structure or from a cargo of scrap metal, can rapidly consume oxygen and elevate CO₂ levels within days. A space does not need to contain inherently hazardous cargo to become lethal.

The regulation also highlights that more than half of enclosed space fatalities are would-be rescuers who entered without confirmed atmospheric data. MSC 581(110) strictly prohibits unplanned rescue attempts and requires a documented, ship-specific Emergency Response Plan before any entry permit is issued.

What is LEL and how does it apply to ship gas detection?

LEL stands for Lower Explosive Limit, which is the minimum concentration of a flammable gas in air at which that gas can ignite. Below the LEL, a gas mixture is too lean to combust. Above the Upper Explosive Limit (UEL), it is too rich. The dangerous explosive range sits between these two thresholds, and the LEL is the first warning boundary a gas detector monitors.

On ships, LEL monitoring is relevant wherever flammable vapors can accumulate: cargo tanks, pump rooms, engine room bilges, and any space that has held fuel, solvents, or flammable cargo residues. Portable gas detectors express LEL readings as a percentage of the limit, so a reading of 1% LEL means the atmosphere contains 1% of the concentration needed for ignition. MSC 581(110) requires that flammable gas levels remain below 1% LEL before entry is permitted.

It is worth noting that LEL sensors in portable detectors are typically calibrated to a specific reference gas, often methane or pentane. When the actual gas present differs from the calibration gas, readings may underestimate the real concentration. Regular calibration and correct sensor selection for the expected gas type are therefore important parts of maintaining a reliable gas detection system. Our calibration and repair service can help ensure your detectors remain accurate and fit for purpose.

How do CO and H₂S endanger crew in enclosed ship spaces?

Carbon monoxide (CO) and hydrogen sulfide (H₂S) are both toxic gases that can reach dangerous concentrations in enclosed ship spaces and incapacitate or kill crew members at relatively low levels. Both are particularly hazardous because they can impair the senses that would otherwise warn a person to leave a dangerous area.

Carbon monoxide in ship environments

CO is produced by incomplete combustion, which means it can be present near engine rooms, incinerators, or any space where fuel-burning equipment operates. It binds to hemoglobin in the blood far more effectively than oxygen, meaning the body is deprived of oxygen even when the surrounding air appears normal. Symptoms of CO poisoning, including headache and dizziness, are easy to mistake for fatigue, which makes it especially dangerous in working environments. The regulatory limit is below 50% of the applicable OEL, and portable detectors must alert crew well before that threshold is reached.

Hydrogen sulfide in cargo and tank spaces

H₂S is produced by the decomposition of organic material and is commonly encountered in cargo holds carrying bulk organic goods, in sewage spaces, and in tanks that previously held crude oil or similar products. At low concentrations, it has a distinctive rotten egg smell, but at higher concentrations, it rapidly paralyzes the olfactory nerves, removing that warning signal entirely. A crew member can lose consciousness within seconds at high H₂S concentrations. Like CO, the regulatory limit is below 50% of the applicable OEL, and continuous monitoring during enclosed space entry is the only reliable safeguard.

Which certifications should a portable gas detector have for maritime use?

A portable gas detector for maritime use should carry ATEX or IECEx certification for use in explosive atmospheres, and it should be approved by recognized classification societies such as DNV, Lloyd’s Register, Bureau Veritas, or equivalent bodies. These certifications confirm that the device has been independently tested and meets the safety and performance standards required for shipboard environments.

Beyond explosion protection, the detector should be certified or verified against the specific measurement requirements of MSC 581(110), including the ability to measure CO₂ at ppm resolution in the 0 to 5% volume range. Standard percent-range CO₂ sensors lack the resolution to accurately confirm the 5,000 ppm regulatory limit, which is why NDIR-based CO₂ sensors have become the benchmark technology for compliance.

When evaluating a detector for your fleet, check the following:

ATEX or IECEx certification for the relevant equipment group and category
Classification society type approval from a recognized body
CO₂ measurement capability using NDIR technology, confirmed to the 0 to 5,000 ppm range
Calibration traceability to recognized standards, with a documented calibration interval
Compatibility with your existing onboard systems, including alarm panels and data logging requirements

Compatibility with existing installations is a point worth emphasizing. If your vessel already carries calibrated 4-in-1 detectors, replacing the entire fleet is not always necessary. A dedicated standalone CO₂ detector with NDIR technology can bridge the regulatory gap introduced by MSC 581(110) without the capital expenditure of a full fleet replacement, provided your existing detectors remain in calibration and cover the other four gases correctly.

How Lavastica helps with portable gas detection onboard

Lavastica specializes in maritime fire and gas detection and understands the pressure of keeping vessels compliant and moving. Whether you need a standalone CO₂ detector to supplement your existing 4-in-1 units, a fully certified multi-gas detector for a new vessel, or advice on which equipment is compatible with your current onboard installation, we can help quickly and practically. Here is what we offer:

Supply of portable gas detectors from more than 100 brands, including NDIR-based CO₂ detectors that meet MSC 581(110) requirements
Advice on compatibility with your existing detection systems and panels
Calibration, repair, and overhaul of portable detectors in our in-house workshop
Fast worldwide delivery from our Rotterdam warehouse to minimize port days
Technical support on regulatory compliance, certification requirements, and product selection

Get in touch with our team to find the right solution for your fleet. Learn more about who we are or contact us directly for fast, practical advice. You can reach us by phone at +31 (0) 10 265 5070 or by email at [email protected].