When is a portable gas detector mandatory before entering confined spaces?

A portable gas detector is mandatory before entering any confined space on a ship whenever there is a risk of oxygen deficiency, flammable gas, or a toxic atmosphere. Under SOLAS XI-1/7, every vessel must carry at least one portable gas detector, and international regulations require that the atmosphere inside a confined space is tested and recorded before any crew member enters. This applies to cargo holds, ballast tanks, pump rooms, void spaces, and many other enclosed areas onboard. The questions below break down exactly what to test, who is responsible, and what your detector needs to be able to measure.

What gases must be tested before entering a confined space on a ship?

Before entering a confined space on a ship, you must test for oxygen (O2), flammable gases (LEL), carbon monoxide (CO), and hydrogen sulfide (H2S) as a minimum. Following IMO Resolution MSC 581(110), which was endorsed in December 2025, carbon dioxide (CO2) at parts per million (ppm) level is now also a mandatory pre-entry parameter for vessels carrying cargo that depletes oxygen or emits toxins.

The traditional four-gas monitor covering O2, LEL, CO, and H2S has been the industry standard for years. However, MSC 581(110) closes a significant gap by requiring CO2 monitoring at a resolution that standard percentage-range sensors simply cannot deliver. The regulatory limit for CO2 is 5,000 ppm, and only sensors using Non-Dispersive Infrared (NDIR) technology can reliably detect concentrations at that level. Electrochemical sensors used in older detectors often miss dangerous CO2 buildup until it is already life-threatening.

One thing that catches many crews off guard: a space does not need to contain hazardous cargo to become dangerous. Ordinary rusting in a damp, enclosed space, such as a ballast tank or a hold carrying scrap metal, can rapidly consume oxygen and raise CO2 to lethal levels within days. MSC 581(110) specifically requires that these passive chemical changes are accounted for in every pre-entry risk assessment.

Which regulations make portable gas detection mandatory for confined spaces?

Portable gas detection for confined space entry on ships is made mandatory primarily by SOLAS XI-1/7, which requires vessels to carry portable atmosphere testing instruments. SOLAS Regulation III/19 mandates enclosed space entry and rescue drills. IMO Resolution MSC 581(110), endorsed in December 2025, replaces the previously recommended Resolution A.1050(27) and introduces binding technical requirements that go significantly further than the older framework.

For a long time, the industry relied on A.1050(27) as guidance rather than a hard mandate. MSC 581(110) changes that. It is not a routine administrative update; it is a high-priority safety mandate that shipowners and masters need to treat accordingly. Classification societies and Port State Control inspectors are now using this resolution as the benchmark during inspections, and an unrecorded atmospheric test is treated as a test that did not happen at all.

If you want to understand how gas detection systems need to be configured to meet these requirements, it is worth reviewing the specific technical demands of MSC 581(110) alongside your current equipment.

What types of spaces on a vessel are classified as confined spaces?

A confined space on a vessel is any enclosed or partially enclosed space that has limited openings for entry and exit, is not designed for continuous human occupancy, and may have a hazardous or potentially hazardous atmosphere. This includes cargo holds, ballast tanks, void spaces, fuel tanks, pump rooms, cofferdams, chain lockers, and duct keels, among others.

MSC 581(110) expands the scope further by introducing clearer definitions for connected spaces and adjacent spaces:

Connected spaces are any areas linked by doors, hatches, or trunks. Even a manual watertight door counts as a connection because an airtight seal cannot be verified from the outside.
Adjacent spaces share a common boundary with a potentially hazardous atmosphere. A bulkhead affected by corrosion or a failed seal can allow gas to migrate, making the adjacent space equally dangerous.

Both connected and adjacent spaces must be treated as hazardous until atmospheric testing proves otherwise. Paragraph 4.3 of the resolution specifically notes that ventilation requirements in an adjacent space may differ significantly from those in the source space, so each entry point requires its own risk assessment.

What are the minimum requirements for a compliant portable gas detector?

A compliant portable gas detector for confined space entry on a ship must, at minimum, be capable of measuring oxygen (O2), flammable gases (LEL), carbon monoxide (CO), hydrogen sulfide (H2S), and carbon dioxide (CO2) at ppm-level resolution. Following MSC 581(110), detectors relying solely on electrochemical CO2 sensors are no longer adequate for full compliance. NDIR (Non-Dispersive Infrared) technology is now the benchmark.

Beyond the sensor types, a compliant detector should also meet these practical requirements:

Approved and certified for use in maritime environments
Capable of logging and recording test results (unrecorded tests are treated as non-existent by Port State Control)
Regularly calibrated and maintained in working order
Suitable for the specific cargo types and spaces on your vessel

It is worth checking whether your current detector is compatible with the recording and documentation requirements of MSC 581(110). Many older four-gas monitors do not have the data logging capability or the NDIR sensor needed to meet the current standard. Our service and repair team can assess whether your existing equipment meets today’s requirements or whether an upgrade is needed.

When is continuous gas monitoring required instead of a single pre-entry test?

Continuous gas monitoring is required when the work being carried out inside the confined space is likely to disturb the atmosphere, when ventilation is intermittent or cannot be guaranteed throughout the entire task, or when the nature of the cargo or space creates an ongoing risk of gas generation. A single pre-entry test only confirms conditions at the moment of entry, not what happens once work begins.

MSC 581(110) tightens the rules around entry permits significantly. An entry permit is valid for a maximum of 8 hours. If the work team takes a break or ventilation stops for any reason, the permit becomes immediately void and all personnel must evacuate. Before re-entry, the atmosphere must be re-tested, and the results must be formally recorded. This effectively means that any space where work conditions change frequently will require repeated atmospheric testing throughout the job.

Continuous personal gas monitors worn by crew members inside the space provide an additional layer of protection, particularly in spaces where conditions can change rapidly, such as holds carrying organic cargo or tanks that have recently been cleaned with chemical agents.

Who is responsible for carrying out gas testing before confined space entry?

The master of the vessel holds overall responsibility for confined space entry safety and must personally verify that a ship-specific Enclosed Space Emergency Response Plan is in place before issuing any entry permit. The actual atmospheric testing must be carried out by a trained, competent person using calibrated equipment, and the results must be officially recorded before entry is permitted.

MSC 581(110) also requires that a trained attendant is stationed at the entrance at all times during entry. Solo entry is strictly prohibited. The attendant’s role is to maintain communication with the crew inside, monitor conditions, and coordinate emergency response if needed. Unplanned rescue attempts are explicitly prohibited under the new framework, as more than half of confined space fatalities involve would-be rescuers who entered without proper preparation.

Training requirements have also been strengthened. Enclosed space drills must now be conducted at least once every two months and must include practical use of atmospheric testing instruments and resuscitation techniques, not just a simple muster. Every vessel must also maintain a ship-specific Enclosed Space Register, kept both onboard and ashore, as a dynamic record of physical and atmospheric hazards rather than a static checklist.

How Lavastica helps with portable gas detection for confined spaces

At Lavastica, we supply and support portable gas detection equipment that meets the current requirements for confined space entry on ships, including detectors with NDIR technology for accurate CO2 monitoring at ppm level. We work with more than 100 brands and maintain a large stock in our Rotterdam warehouse, so we can move quickly when you need equipment before a vessel sails.

Supply of compliant portable gas detectors, including multi-gas monitors with NDIR CO2 sensors
Calibration and maintenance of existing equipment to keep it within regulatory requirements
Advice on compatibility with your current onboard systems and documentation processes
Replacement support for older detectors that no longer meet MSC 581(110) standards
Fast worldwide delivery to minimize time in port

Want to know whether your current detectors are still compliant, or do you need a reliable replacement fast? Learn more about us or get in touch directly and we will help you find the right solution for your vessel. You can reach us by phone at +31 (0) 10 265 5070 or by email at [email protected].