What gases does a five gas detector measure?

A five gas detector measures oxygen (O₂), flammable gases (LEL), carbon monoxide (CO), carbon dioxide (CO₂), and hydrogen sulphide (H₂S). This combination gives you a complete picture of the atmosphere in an enclosed space before anyone steps inside. Since IMO Resolution MSC.581(110) came into force in December 2025, CO₂ is now a mandatory measurement parameter, making five gas detectors the new standard for enclosed space entry on board ships. Below, we cover how each gas is measured, why it matters, and what this means for your vessel.

Which five gases does a five gas detector typically measure?

A five gas detector typically measures oxygen (O₂), flammable gases expressed as a percentage of the Lower Explosive Limit (LEL), carbon monoxide (CO), carbon dioxide (CO₂), and hydrogen sulphide (H₂S). These five parameters together cover the most common atmospheric hazards found in enclosed spaces on board ships, from oxygen deficiency and explosive atmospheres to toxic gas buildup.

Here is a quick overview of what each measurement tells you:

O₂: Confirms there is enough oxygen to breathe safely. Entry is only permitted at or above 20.9% by volume.
LEL: Detects flammable gases or vapours. Must stay below 1% of the Lower Explosive Limit before entry.
CO: Detects carbon monoxide, a colourless, odourless gas produced by combustion and certain cargo types.
CO₂: Now mandatory under MSC.581(110). Must be below 0.5% by volume (5,000 ppm) before entry is permitted.
H₂S: Detects hydrogen sulphide, which is common in cargo holds, sewage spaces, and tank environments.

The H₂S channel is the fifth gas that fulfils the requirement for at least one additional toxic gas, determined by risk assessment. On most vessels, H₂S is the obvious choice given how frequently it appears in maritime confined space environments.

Why are these specific gases dangerous in confined spaces?

These five gases are dangerous in confined spaces because they are either invisible, odourless, or fast-acting, and enclosed spaces give them nowhere to go. A person entering a space with depleted oxygen or elevated CO₂ can lose consciousness within seconds, often without any warning.

Each gas presents a distinct type of risk:

Oxygen depletion can happen passively. Rusting steel in a damp hold consumes oxygen and raises CO₂ levels without any dangerous cargo being present. A space that was safe yesterday may not be safe today.
CO₂ is heavier than air and accumulates at the bottom of tanks and holds. At concentrations above 5,000 ppm, it causes headaches and disorientation. At higher levels, it is rapidly fatal.
CO binds to haemoglobin in the blood far more effectively than oxygen, meaning even low concentrations cause serious harm before a person realises anything is wrong.
H₂S is detectable by smell at low concentrations, but at higher levels, it paralyses the sense of smell entirely. People have walked into H₂S-rich spaces convinced the air was clear.
Flammable gases create an explosion risk. A single ignition source in a space above the LEL threshold can be catastrophic.

IMO Resolution MSC.581(110) specifically highlights that more than half of enclosed space fatalities are would-be rescuers, not the initial entrants. This is exactly why proper atmospheric testing with a compliant gas detection system is not optional.

How does a five gas detector measure each gas?

A five gas detector uses different sensor technologies depending on the gas being measured. Electrochemical sensors handle O₂, CO, and H₂S, a catalytic bead sensor measures flammable gases, and Non-Dispersive Infrared (NDIR) technology is used for CO₂. Each sensor type is matched to the specific physical or chemical properties of the target gas.

Electrochemical sensors (O₂, CO, H₂S)

Electrochemical sensors work by allowing the target gas to react at an electrode surface, generating a small electrical current proportional to the gas concentration. They are accurate, relatively low-cost, and well-suited to gases like oxygen, carbon monoxide, and hydrogen sulphide. Their main limitation is that they degrade over time and require regular calibration and eventual replacement.

Catalytic bead sensor (LEL)

The LEL sensor contains a heated catalytic bead. When a flammable gas contacts the bead, it oxidises and raises the temperature, which changes the electrical resistance in a measurable way. This sensor is effective across a wide range of flammable gases but requires the presence of sufficient oxygen to function correctly. In oxygen-deficient atmospheres, LEL readings can be unreliable, which is one reason why O₂ is always measured simultaneously.

NDIR sensor (CO₂)

Non-Dispersive Infrared sensors measure CO₂ by passing infrared light through a sample of air. CO₂ molecules absorb infrared energy at a specific wavelength, and the detector measures how much light is absorbed to calculate the concentration. NDIR technology provides the high-resolution ppm-level accuracy needed to reliably detect CO₂ at the 5,000 ppm regulatory threshold. Standard electrochemical sensors cannot achieve this resolution, which is why NDIR is now the benchmark for CO₂ compliance.

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

The difference between a four gas and five gas detector is the addition of a dedicated CO₂ sensor. Traditional four gas detectors measure O₂, LEL, CO, and H₂S. A five gas detector adds CO₂ as a fifth parameter, which is now a mandatory measurement requirement under IMO Resolution MSC.581(110) for enclosed space entry on board ships.

This distinction matters practically. Most legacy four gas detectors use electrochemical technology across all channels, and electrochemical sensors lack the resolution to accurately detect CO₂ at the 5,000 ppm regulatory limit. A five gas detector incorporates an NDIR sensor specifically for CO₂, which is a fundamentally different technology that cannot simply be added to an existing four gas unit.

For fleets currently operating four gas detectors, the recommended compliance path is to supplement existing equipment with a dedicated standalone CO₂ detector rather than replacing entire fleets of functional units. This approach bridges the regulatory gap without unnecessary capital expenditure, provided the standalone unit meets the required ppm resolution. Our service and repair team can help you assess which of your existing detectors are still fit for purpose and what supplementary equipment you need.

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

A five gas detector must be used before and during any entry into an enclosed space on board a ship. Under MSC.581(110), atmospheric testing is mandatory prior to entry, and a compliant detector must be capable of measuring at least five gases, including CO₂. No permit to work may be issued without confirmed atmospheric readings within acceptable limits.

Beyond the mandatory pre-entry test, there are several situations where continuous or repeated testing is required:

When work is ongoing inside an enclosed space and ventilation is interrupted for any reason, the permit is immediately void and re-testing is required before re-entry.
Entry permits carry a maximum validity of 8 hours under the new regulations. After 8 hours, a full re-assessment and re-test are required.
Connected spaces (linked by doors, trunks, or manholes) and adjacent spaces (sharing a common bulkhead) must each be tested independently. Gas does not respect physical boundaries.
Any time cargo changes, the medium in a tank changes, or the purpose of a space is altered, the atmospheric risk profile changes and re-testing is required.

It is worth remembering that passive chemical processes, such as iron oxidation (ordinary rusting) in a damp hold, can deplete oxygen and raise CO₂ levels within days. A space that tested safe last week is not guaranteed to be safe today.

How do you calibrate and maintain a five gas detector?

A five gas detector must be calibrated regularly using certified calibration gas mixtures that match each sensor channel. Bump testing before each use confirms the sensors respond correctly, while full calibration at the manufacturer-recommended interval verifies accuracy against known concentrations. Maintenance includes sensor replacement on schedule, battery checks, and keeping the instrument clean and dry.

Here is a practical maintenance checklist for five gas detectors on board:

Bump test before every use. Expose the detector to a known concentration of test gas and confirm each sensor triggers an alarm. This takes less than a minute and confirms the unit is working.
Full calibration at regular intervals. Follow the manufacturer’s recommended schedule, typically every one to six months depending on usage and sensor type. Use certified calibration gas mixtures for all five channels.
Replace sensors on schedule. Electrochemical sensors have a finite lifespan, usually two to three years. NDIR sensors for CO₂ last longer but still require periodic verification. Do not wait for a sensor to fail in the field.
Record all calibrations and tests. Under MSC.581(110), an unrecorded test is treated by Port State Control as a test that did not occur. Keep a calibration log for every instrument on board.
Store correctly. Keep detectors in a clean, dry environment within the temperature range specified by the manufacturer. Exposure to extreme temperatures or humidity accelerates sensor degradation.
Train the crew. Enclosed space drills must now be conducted at least once every two months and must include practical use of atmospheric testing instruments. A detector is only as reliable as the person using it.

How Lavastica helps with gas detection compliance

At Lavastica, we supply and support portable gas detectors for maritime enclosed space entry, including five gas detectors that comply with the requirements of IMO Resolution MSC.581(110). Whether you need to upgrade your existing four gas detectors, source a standalone CO₂ unit to supplement your current fleet, or get advice on which equipment works with your existing onboard systems, we can help. Our team understands the urgency of port turnarounds and the pressure of keeping your fleet compliant.

Supply of five gas detectors and standalone CO₂ detectors from our Rotterdam warehouse stock
Expert advice on compatibility with your existing gas detection equipment on board
Calibration and repair services through our in-house workshop
Fast worldwide delivery to minimise vessel downtime in port
Technical support on MSC.581(110) compliance, sensor selection, and documentation requirements

Want to know which detector fits your fleet? Learn more about us or get in touch directly and we will help you find the right solution quickly.

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