When must you use a gas detector for tank entry?
You must use a gas detector for tank entry on any ship before a crew member enters an enclosed space. This applies to cargo tanks, void spaces, pump rooms, cofferdams, and any other space where the atmosphere cannot be confirmed safe by visual inspection alone. Since IMO Resolution MSC.581(110) entered into force on 3 December 2025, the requirements for gas detection have become significantly more stringent. Below we answer the most important practical questions about gas detection for tank entry.
What gases must be tested before entering a ship’s tank?
Before entering any enclosed space or tank on board a ship, you must test for at least five gases. These are: oxygen (O₂), flammable gases or vapours expressed as a percentage of the Lower Explosive Limit (LEL), carbon monoxide (CO), carbon dioxide (CO₂), and at least one additional toxic gas identified through risk assessment, most commonly hydrogen sulphide (H₂S).
Entry is only permitted when all measurements fall within the following safe limits:
- O₂: at or above 20.9% by volume
- CO₂: below 0.5% by volume (5,000 ppm)
- Flammable gases: below 1% of the Lower Flammability Limit (LFL)
- Toxic gases: below 50% of the applicable Occupational Exposure Limit (OEL)
One important point that many crews overlook: ordinary rusting in a damp, enclosed space can rapidly consume oxygen and raise CO₂ levels to dangerous concentrations. A tank does not need to contain hazardous cargo to become lethal. Moisture and bare steel are enough to create an asphyxiation risk within days, which is why CO₂ testing is now mandatory and must be measured at ppm resolution rather than in broad percentage ranges.
What regulations require gas detection for tank entry?
The primary regulation governing gas detection for tank entry is IMO Resolution MSC.581(110), which entered into force on 3 December 2025. This resolution replaces the older Resolution A.1050(27) and sets a new framework for enclosed space safety across the maritime industry. It is a high-priority safety mandate, not a routine administrative update.
In addition, SOLAS Regulation XI-1/7 requires ships to carry portable atmospheric testing instruments, and SOLAS Regulation III/19 mandates enclosed space entry and rescue drills. MSC.581(110) closes the gap between these baseline SOLAS requirements and the technical reality of modern risk assessment.
Inspection bodies are already applying the new standard. RightShip RiSQ version 3.2 has integrated MSC.581(110) into its inspection scope, meaning Port State Control inspectors and vetting inspectors will look for evidence of compliance, including recorded atmospheric test results. An unrecorded test is treated as a test that did not take place.
Who is responsible for carrying out gas testing onboard?
The Master is ultimately responsible for ensuring that gas testing is carried out correctly before any enclosed space entry. In practice, the testing is performed by a designated competent person, typically a senior officer with training in atmospheric testing equipment and risk assessment procedures.
Under MSC.581(110), the Master must personally verify that a ship-specific Enclosed Space Emergency Response Plan exists before issuing any entry permit. No permit may be issued without this plan in place. The permit itself carries a maximum validity of 8 hours. If work stops, the team takes a break, or ventilation is interrupted, the permit becomes void immediately and all personnel must evacuate. Before re-entry, the atmosphere must be re-tested and the results formally recorded.
A trained attendant must always be stationed at the entrance during entry. Solo entry is strictly prohibited under the new rules. Enclosed space drills must now be conducted at least once every two months and must include practical use of gas detection equipment and resuscitation techniques.
What type of gas detector is required for tank entry?
A compliant 5-gas portable detector is now required for tank entry. Traditional 4-gas monitors measuring O₂, LEL, CO, and H₂S are no longer sufficient under MSC.581(110) because they cannot detect CO₂. The new minimum configuration must include a dedicated CO₂ sensor capable of measuring at ppm resolution in the 0 to 5% volume range.
Standard electrochemical sensors lack the resolution to accurately measure the 5,000 ppm regulatory limit for CO₂. Non-Dispersive Infrared (NDIR) technology is now the benchmark for compliance, as it provides the stability and high-resolution ppm monitoring needed to detect dangerous CO₂ buildup before it becomes fatal.
If your vessel currently operates with calibrated 4-gas detectors, a practical compliance path is to supplement existing equipment with a dedicated standalone CO₂ detector. This bridges the regulatory gap without the capital expenditure of replacing an entire fleet of functional instruments. Always verify that any new detector is compatible with your existing calibration and maintenance procedures to avoid gaps in your Safety Management System.
How often must gas levels be monitored during tank entry?
Gas levels must be tested before entry and continuously monitored throughout the entire duration of work inside a tank or enclosed space. A single pre-entry measurement is not enough. Atmospheric conditions can change rapidly, particularly in spaces where ventilation is limited or where chemical processes such as oxidation are ongoing.
Under MSC.581(110), the entry permit is valid for a maximum of 8 hours. After that period, or whenever work is interrupted, a full re-assessment and re-test are required before work may continue. The results of every re-test must be formally recorded. Connected spaces, such as pipe tunnels or stairways linked to the primary space, and adjacent spaces sharing a common bulkhead must each be independently tested and ventilated. Gas does not respect physical boundaries, and structural gaps, corroded bulkheads, or imperfect seals can allow hazardous atmospheres to migrate.
What should you do if the gas detector alarms during tank entry?
If the gas detector alarms during tank entry, all personnel must evacuate the space immediately and calmly. Do not attempt to investigate the source of the alarm from inside the space. Once outside, the entry permit is automatically void and no one may re-enter until the atmosphere has been re-tested and confirmed safe, and a new permit has been issued.
MSC.581(110) is explicit on one point that directly saves lives: Emergency Escape Breathing Apparatuses (EEBD) are for escape only and must never be used during a rescue entry. More than half of enclosed space fatalities are would-be rescuers who entered without proper equipment or a plan. Any rescue attempt must follow the ship-specific Enclosed Space Emergency Response Plan, use appropriate rescue breathing apparatus, and involve only personnel assigned to the rescue team.
Immediate actions after an alarm and evacuation should include:
- Account for all personnel and confirm no one remains inside
- Increase ventilation if safe to do so from outside
- Re-test the atmosphere before any re-entry
- Investigate the cause of the alarm and update the risk assessment
- Record the event in the vessel’s logs and the Enclosed Space Register
The Enclosed Space Register must be a live document. Update it whenever cargo changes, the medium in a tank changes, or the purpose of a space is altered. Keeping this register synchronized between the vessel and the shore-side office is a direct requirement under MSC.581(110).
How Lavastica helps with gas detection for tank entry
We understand that compliance with MSC.581(110) puts real pressure on fleet engineers and technical superintendents, especially when ships are already in port and time is limited. Lavastica supplies portable and fixed gas detection equipment that meets the new 5-gas requirements, including detectors with NDIR CO₂ sensors capable of ppm-resolution measurement. We can also advise on supplementing existing 4-gas detectors with standalone CO₂ units, so you avoid unnecessary replacement costs while closing the compliance gap.
- Supply of compliant 5-gas portable detectors and standalone CO₂ detectors
- Advice on compatibility with your existing onboard detection systems
- Calibration, repair, and maintenance of gas detection equipment
- Fast worldwide delivery to minimize port days
- Technical support on regulatory compliance and product selection
Need help selecting the right gas detector for your fleet? Learn more about us and how we support maritime safety, or contact our team directly for fast, expert advice. You can reach us by phone at +31 (0) 10 265 5070 or by email at info@lavastica.com.
