What does a five gas detector measure on vessels?

A five gas detector on a vessel measures oxygen (O2), flammable gases (expressed as a percentage of the Lower Explosive Limit, or LEL), carbon monoxide (CO), hydrogen sulfide (H2S), and carbon dioxide (CO2). That fifth gas, CO2, is what sets a five gas monitor apart from the older four gas standard, and under the updated IMO Resolution MSC 581(110), monitoring it has become a mandatory requirement for enclosed space entry on ships. Below, we walk through everything you need to know about how these detectors work, why each gas matters, and how to keep your instrument ready for use.

Which five gases does a marine detector typically monitor?

A marine five gas detector monitors oxygen (O2), flammable gases (LEL), carbon monoxide (CO), hydrogen sulfide (H2S), and carbon dioxide (CO2). These five parameters together cover the most common atmospheric hazards found in enclosed spaces on board ships, from cargo holds and ballast tanks to pump rooms and void spaces.

O2 confirms that breathable air is present (safe range: 20.9% by volume)
LEL detects combustible gas concentrations before they reach explosive levels
CO warns of incomplete combustion or engine exhaust ingress
H2S detects the toxic gas produced by organic decay and certain cargoes
CO2 identifies dangerous concentrations from biological processes, rusting, or inert gas systems

The inclusion of CO2 is the defining difference between a four gas and a five gas detector. Most legacy instruments measure CO2 as a rough percentage, which is not precise enough to catch the 5,000 ppm (0.5%) threshold that modern regulations now require. A proper five gas detector uses Non-Dispersive Infrared (NDIR) technology to measure CO2 at the parts-per-million level, giving you the resolution you actually need for compliant pre-entry testing.

Why is each gas dangerous on a vessel?

Each of the five gases presents a distinct hazard, and understanding what makes each one dangerous helps you interpret your detector’s readings correctly rather than just reacting to an alarm.

Oxygen depletion and enrichment

Oxygen deficiency below 19.5% by volume impairs judgment and physical coordination, and below 16% it becomes life-threatening within minutes. Oxygen enrichment above 23.5% dramatically increases the risk of fire and explosion. Both extremes are common in enclosed spaces, which is why O2 is always the first reading to check before entry.

Flammable gases (LEL)

Combustible gases such as methane, propane, or hydrocarbon vapors become explosive when they reach their Lower Explosive Limit. Detectors display readings as a percentage of LEL, and evacuation is typically required at 10% LEL, well before the mixture becomes ignitable. In cargo holds, pump rooms, and tank spaces, flammable gas buildup can occur rapidly.

Carbon monoxide

CO is produced by incomplete combustion and can accumulate in engine rooms, generator spaces, or anywhere exhaust gases are present. It is colorless and odorless, making detection impossible without an instrument. Even short-term exposure to elevated concentrations causes incapacitation before the person realizes they are in danger.

Hydrogen sulfide

H2S is generated by the decomposition of organic material and is present in sewage spaces, fish cargo holds, and certain bulk cargoes. It is acutely toxic and, critically, causes olfactory fatigue at low concentrations, meaning your sense of smell stops detecting it just as levels become dangerous.

Carbon dioxide

CO2 is the gas that IMO Resolution MSC 581(110) specifically elevated to mandatory testing status. It builds up in enclosed spaces through biological processes, the oxidation of iron (ordinary rusting in damp conditions), and discharge from CO2 firefighting systems. A space with bare steel and moisture can become lethal within days, even without any inherently dangerous cargo. The regulatory limit for entry is CO2 below 0.5% (5,000 ppm).

How does a five gas detector take its readings?

A five gas detector uses a combination of sensor technologies to monitor each gas simultaneously. The instrument draws in a sample of the surrounding atmosphere, passes it through the sensor array, and displays real-time readings on a screen, triggering audible and visual alarms when a preset threshold is exceeded.

The sensor technologies typically used are:

Electrochemical sensors for O2, CO, and H2S: these generate a small electrical current proportional to the gas concentration, making them accurate and responsive for these specific gases
Catalytic bead (pellistor) sensors for LEL: a heated bead oxidizes flammable gas, and the change in resistance corresponds to the concentration
NDIR (Non-Dispersive Infrared) sensors for CO2: infrared light passes through the gas sample, and CO2 absorbs a specific wavelength; the degree of absorption indicates the concentration at ppm resolution

The NDIR sensor is the reason a five gas detector is physically larger and more expensive than a standard four gas unit. It is also why it is more reliable for CO2 detection. Traditional electrochemical CO2 sensors drift over time and cannot accurately resolve the low concentrations that matter for regulatory compliance. NDIR sensors are stable, long-lived, and provide the precision that maritime gas detection now demands.

What is the difference between a four gas and a five gas detector?

The key difference between a four gas and a five gas detector is the addition of a dedicated CO2 sensor. A standard four gas monitor measures O2, LEL, CO, and H2S. A five gas detector adds CO2 monitoring, typically using NDIR technology, which is the only sensor type capable of detecting CO2 at the ppm resolution required by current maritime regulations.

This distinction has become operationally significant since IMO Resolution MSC 581(110) came into effect. The resolution explicitly requires CO2 to be tested as part of every enclosed space pre-entry assessment, and it sets a threshold of 5,000 ppm (0.5%). Most four gas detectors either cannot measure CO2 at all, or include a basic sensor that only reads in rough percentage increments, making them non-compliant for this specific requirement.

From a practical standpoint, if your vessel still relies on a legacy four gas monitor, it is worth checking whether it meets the requirements of MSC 581(110). The regulation is not a minor administrative update; it represents a meaningful shift in what constitutes a safe pre-entry procedure. Replacing or supplementing a four gas unit with a five gas detector is the most straightforward way to close that gap.

When should a five gas detector be used during vessel operations?

A five gas detector should be used any time crew members need to enter an enclosed or confined space on board. This includes cargo holds, ballast tanks, void spaces, pump rooms, cofferdams, duct keels, and any space that is not continuously ventilated and occupied. Under MSC 581(110), pre-entry atmospheric testing is mandatory, and the atmosphere must be confirmed safe before any person enters.

Beyond pre-entry testing, a five gas detector should be used in these situations:

During cargo operations involving materials that can deplete oxygen or emit gases, such as grain, scrap metal, or coal
After CO2 system discharge in any protected space, before re-entry for inspection or reset
In adjacent and connected spaces, not just the primary space being entered, since gas can migrate through bulkheads, doors, and pipe penetrations
During continuous monitoring while crew are working inside an enclosed space, particularly when the work is expected to take longer than a few minutes
After breaks or permit renewal, since MSC 581(110) requires the atmosphere to be re-tested if the entry permit expires or the work team leaves the space

The regulation also draws attention to connected and adjacent spaces. A space sharing a bulkhead with a hazardous area, or linked to it through a door, manhole, or pipe, must be treated as potentially hazardous until independently tested and confirmed safe.

How do you maintain a five gas detector on board?

Maintaining a five gas detector on board requires a routine that covers bump testing, calibration, sensor replacement, and battery management. A detector that has not been properly maintained can give false readings or fail to alarm, which is more dangerous than having no detector at all.

A practical maintenance routine looks like this:

Bump test before every use: expose the detector to a known concentration of test gas to confirm that all sensors respond and alarms activate. This is a functional check, not a calibration.
Full calibration at regular intervals: typically every 3 to 6 months, depending on manufacturer guidance and usage intensity. Calibration adjusts the sensor output to match a certified reference gas.
Check and replace sensors on schedule: electrochemical sensors for O2, CO, and H2S have a typical lifespan of 2 to 3 years. NDIR sensors for CO2 last longer but still require periodic verification.
Inspect the sampling pump and filters: a blocked filter or failing pump will prevent the instrument from drawing in a representative sample, making all readings unreliable.
Store correctly: keep the detector in a clean, dry location at a moderate temperature. Extreme heat or cold affects sensor performance and battery life.
Keep calibration records on board: MSC 581(110) requires that atmospheric test results be recorded and verifiable. Port State Control inspectors treat an unrecorded test as a test that did not happen.

Our service and repair workshop handles calibration, sensor replacement, and overhaul for a wide range of gas detector brands and models, including units that are no longer in current production.

How Lavastica helps with gas detection on board

We supply, service, and advise on five gas detectors and broader gas detection systems for vessels of all types. Whether you need a single replacement instrument, a stock of calibration gas, or advice on upgrading from a four gas to a five gas setup that integrates with your existing onboard systems, we can help. Our team understands the urgency of port schedules and the complexity of keeping detection equipment compliant with evolving regulations like MSC 581(110).

Large stock of five gas detectors and spare sensors for fast worldwide delivery
In-house calibration and repair, including obsolete and end-of-life models
Technical advice on compatibility with existing fire and gas detection panels
Support for regulatory compliance, including MSC 581(110) requirements
Emergency service available when a vessel is in port and time is short

Get in touch with us to find the right gas detector for your fleet. Learn more about our expertise or contact our team directly for fast, practical advice.

Phone: +31 (0) 10 265 5070Email: info@lavastica.com