What gases must a portable five gas detector measure?

A portable five gas detector typically measures oxygen (O2), flammable gases (LEL), carbon monoxide (CO), hydrogen sulfide (H2S), and carbon dioxide (CO2). That fifth gas, CO2, is the key difference from the older four-gas standard and is now required under updated maritime safety regulations. Below, we answer the most common questions about five-gas detectors so you know exactly what to look for before your next enclosed space entry.

Which five gases does a 5-gas detector typically measure?

A standard portable five gas detector measures oxygen (O2), flammable gases expressed as a percentage of the Lower Explosive Limit (LEL), carbon monoxide (CO), hydrogen sulfide (H2S), and carbon dioxide (CO2). These five gases cover the most common atmospheric hazards found in enclosed spaces on board ships, from cargo holds and ballast tanks to pipe tunnels and void spaces.

The first four gases have been the industry standard for many years. The addition of CO2 as a fifth parameter is what separates a five-gas detector from the traditional four-in-one units that most fleets have relied on. CO2 is odorless, colorless, and can accumulate rapidly in confined spaces, particularly where iron oxidation is occurring or where organic cargo is present. Without a dedicated CO2 sensor, a space can appear safe while actually containing lethal concentrations of the gas.

Each of the five gases represents a distinct type of hazard: oxygen depletion, explosion risk, toxic gas poisoning, and asphyxiation. A single instrument covering all five gives the entry team a complete atmospheric picture before anyone steps through a manhole.

Why is oxygen the most critical gas to monitor?

Oxygen is the most important gas to monitor because its depletion is the fastest route to unconsciousness and death in an enclosed space. A person entering a space where O2 has dropped below safe levels can lose consciousness within seconds, with no warning and no smell. Under current maritime safety guidelines, entry is only permitted when O2 is at or above 20.9%, though certain flag states may accept a minimum of 19.5%.

What makes O2 monitoring particularly important is that oxygen can be consumed by processes that have nothing to do with dangerous cargo. Ordinary rusting, which is iron reacting with moisture, actively consumes oxygen. A ballast tank or void space that contains nothing more than bare steel and humidity can become oxygen-deficient within days. This passive chemical process is easy to overlook during a risk assessment, which is exactly why regulators now require it to be explicitly accounted for.

Because oxygen depletion and CO2 buildup often happen simultaneously, a five-gas detector gives you both readings at once, which is far more reliable than relying on a single parameter to confirm atmospheric safety.

What makes LEL measurement different from the other gas readings?

LEL measurement is different because it does not identify a specific gas. Instead, it measures the total concentration of all flammable gases present and expresses that as a percentage of the Lower Explosive Limit, which is the minimum concentration at which a gas-air mixture can ignite. Entry is only permitted when flammable gas readings remain below 1% LEL.

In practice, this means a single LEL sensor can detect methane, propane, butane, and dozens of other flammable vapors without needing a dedicated sensor for each one. That is a practical advantage in maritime environments where cargo type can vary significantly between voyages.

The important limitation to understand is that LEL sensors cannot tell you which gas is present or at what exact concentration. They give you a combined flammability risk score. For spaces where a specific flammable gas is known to be present, such as a tank that previously held LPG, additional targeted testing may be appropriate alongside the LEL reading.

When should a portable five gas detector be used on a vessel?

A portable five gas detector should be used before and continuously during any entry into an enclosed space on board a vessel. This includes cargo holds, ballast tanks, fuel tanks, void spaces, pipe tunnels, cofferdams, and any space that is not naturally ventilated and has been closed for a period of time.

Under current regulations, atmospheric testing is not a one-time pre-entry check. If the work team takes a break, exits the space, or if ventilation stops for any reason, the permit is void and the atmosphere must be re-tested before re-entry. Continuous monitoring throughout the work period is the expected standard.

It is also worth remembering that connected and adjacent spaces need their own independent assessment. A space next to a hazardous area, sharing a bulkhead or connected via a door or pipe, can accumulate gas through structural gaps or seal failure. Each connection point requires its own atmospheric test, not an assumption that the neighboring space is safe because the primary space has been cleared.

For crews looking to understand how fire and gas detection equipment fits into a broader onboard safety setup, it helps to think of the portable detector as the first line of defense, with fixed detection systems providing continuous background monitoring.

How do you know if a 5-gas detector is approved for maritime use?

A five gas detector is approved for maritime use when it carries certification from a recognized classification society or meets the technical requirements referenced in applicable SOLAS regulations. Look for approval marks from bodies such as DNV, Lloyd’s Register, Bureau Veritas, or similar classification societies. These approvals confirm the instrument has been tested for reliability, accuracy, and performance in marine environments.

Beyond classification society approval, check that the CO2 sensor uses Non-Dispersive Infrared (NDIR) technology. Standard electrochemical sensors cannot accurately measure CO2 at the 5,000 ppm regulatory threshold because they lack the resolution needed at that concentration range. NDIR sensors are now the benchmark for compliance because they provide stable, high-resolution ppm readings that electrochemical alternatives cannot match.

When in doubt, request the product’s calibration certificate and confirm that the sensor ranges cover the regulatory limits: O2 down to at least 19.5%, CO2 up to at least 5,000 ppm with ppm-level resolution, LEL from 0 to 100%, and toxic gas readings calibrated against the relevant Occupational Exposure Limit for the gases in question.

What’s the difference between a 4-gas and a 5-gas detector?

The key difference is that a four-gas detector measures O2, LEL, CO, and H2S, while a five-gas detector adds a dedicated CO2 sensor. That single addition has significant compliance implications. Most legacy four-in-one units cannot detect CO2, which is now a mandatory pre-entry testing parameter under updated maritime safety guidelines.

The table below summarizes the difference at a glance:

Gas Parameter	4-Gas Detector	5-Gas Detector
Oxygen (O2)	Yes	Yes
Flammable gases (LEL)	Yes	Yes
Carbon monoxide (CO)	Yes	Yes
Hydrogen sulfide (H2S)	Yes	Yes
Carbon dioxide (CO2)	No	Yes

The practical implication for fleet managers is that replacing an entire fleet of functional, calibrated four-gas detectors is not always necessary. A common compliance path is to supplement existing four-in-one units with a dedicated standalone CO2 detector. This bridges the regulatory gap without the capital expenditure of a full fleet replacement, provided the standalone unit uses NDIR technology and delivers ppm-level resolution.

If your existing detectors are still within calibration and performing reliably, our service and repair team can advise on whether supplementing rather than replacing makes sense for your specific equipment and fleet setup.

How Lavastica helps with portable gas detection for maritime use

At Lavastica, we work with fleet engineers and technical superintendents who need the right equipment quickly, without the guesswork. Whether you are looking to upgrade from a four-gas to a five-gas detector, source a standalone CO2 unit to supplement existing equipment, or verify that your current instruments meet the latest regulatory requirements, we can help.

Broad product range: We supply gas detection equipment from more than 100 brands, so we can match products to your existing onboard systems rather than forcing a full replacement.
Compliance advice: We help you identify which detectors meet current maritime safety requirements, including CO2 sensor technology and classification society approvals.
Fast delivery: We maintain a large stock in Rotterdam and can arrange worldwide delivery to minimize port time.
Sensor audits and calibration support: Our team can advise on sensor audits to confirm your portable and personal gas detectors cover all required parameters at the right resolution.

Get in touch with us directly. Learn more about who we are or contact our team for fast, practical advice on your gas detection needs.

Phone: +31 (0) 10 265 5070
Email: [email protected]