What gases must be measured before entering a cargo tank?

Before entering a cargo tank, you must measure at least five gases: oxygen (O₂), flammable gases or vapours (expressed as a percentage of the Lower Explosive Limit), carbon monoxide (CO), carbon dioxide (CO₂), and at least one additional toxic gas identified by risk assessment, typically hydrogen sulphide (H₂S). This requirement reflects the updated IMO recommendations under Resolution MSC.581(110), which entered into force on 3 December 2025. Below, we break down why each gas matters, what safe limits look like, and how gas testing works in practice.

Which gases are most dangerous in a cargo tank?

The most dangerous gases in a cargo tank are those that can kill without warning: oxygen-depleting atmospheres, flammable vapours, carbon monoxide, carbon dioxide, and toxic gases like hydrogen sulphide. What makes confined spaces particularly deadly is that most of these hazards are invisible and odourless at dangerous concentrations, leaving crew with no natural warning before they lose consciousness.

Here is a closer look at each category:

Oxygen deficiency or enrichment: A normal atmosphere contains around 20.9% oxygen. Below roughly 19.5%, cognitive function drops rapidly. Above 23%, flammable materials ignite far more easily. Both extremes are life-threatening.
Flammable gases and vapours: Cargo residues, fuel vapours, and solvents can accumulate to concentrations within the explosive range. Even a small ignition source is enough to trigger a blast.
Carbon monoxide (CO): A colourless, odourless gas produced by incomplete combustion or certain cargo types. It binds to haemoglobin and causes rapid incapacitation at relatively low concentrations.
Carbon dioxide (CO₂): Often overlooked but increasingly recognised as a serious hazard. CO₂ builds up through rusting steel, biological decay in cargo, and oxygen displacement. At concentrations above 0.5% (5,000 ppm), it can cause headaches and disorientation. At higher levels, it is fatal. Importantly, CO₂ is heavier than air and accumulates at the bottom of tanks.
Hydrogen sulphide (H₂S): A highly toxic gas associated with certain bulk cargoes, sewage spaces, and organic material. At very low concentrations, it smells of rotten eggs, but at dangerous levels it paralyses the sense of smell, giving a false impression of safety.

A particularly underappreciated hazard is ordinary rust. In a damp, enclosed steel space, the oxidation of iron consumes oxygen and releases CO₂. A tank does not need to carry inherently hazardous cargo to become lethal. Moisture and bare steel alone can create a dangerous atmosphere within days.

What oxygen level is safe to enter a cargo tank?

Entry into a cargo tank is considered safe when the oxygen level is at or above 20.9% by volume. Some flag states permit a minimum of 19.5%, but 20.9% is the benchmark set by IMO Resolution MSC.581(110). Any reading below the applicable threshold means entry is prohibited until the atmosphere has been ventilated and re-tested.

Oxygen measurement alone is not sufficient to declare a space safe. A tank can show a normal oxygen reading while still containing dangerous concentrations of CO, CO₂, or flammable vapours. This is why the full set of required measurements must be completed before any entry is authorised.

The complete set of acceptable atmospheric limits under MSC.581(110) is as follows:

O₂: At or above 20.9% by volume (minimum 19.5% in some flag state jurisdictions)
CO₂: Below 0.5% by volume (5,000 ppm)
Flammable gases or vapours: Below 1% of the Lower Explosive Limit (LEL)
Toxic gases: Below 50% of the applicable Occupational Exposure Limit (OEL)

All four parameters must fall within safe limits simultaneously. If any single reading exceeds its threshold, entry is not permitted.

How is gas testing carried out before cargo tank entry?

Gas testing before cargo tank entry follows a structured process governed by a Permit to Work system. A trained crew member uses a calibrated portable gas detector to sample the atmosphere at multiple points within the tank before anyone enters. Readings must be taken at different heights, since gases like CO₂ and H₂S are heavier than air and settle near the bottom, while lighter flammable vapours may concentrate near the top.

The process typically works as follows:

Risk assessment: Identify the hazards specific to that tank, including cargo history, adjacent spaces, and the potential for gas migration through connected areas.
Ventilation: Ventilate the space thoroughly before testing begins. Ventilation time depends on the tank’s volume and the nature of the hazard.
Atmospheric testing: Lower the gas detector into the space on a line or use an extension probe to sample at top, middle, and bottom levels without entering.
Recording results: Document all readings. Under MSC.581(110), an unrecorded test is treated by Port State Control as a test that did not happen.
Issuing the permit: If all readings are within safe limits, the Permit to Work is issued. The permit is valid for a maximum of 8 hours. If work stops, ventilation ceases, or crew take a break, the permit is immediately void, and re-testing is required before re-entry.

Connected and adjacent spaces must also be assessed independently. A door or bulkhead cannot be assumed to be airtight, and gas can migrate from a hazardous space into nominally safe areas through gaps, pipe penetrations, or corroded welds.

For calibration and maintenance of gas detection equipment, regular servicing is important to ensure readings remain accurate and compliant during inspections.

What equipment is used to measure gases in a cargo tank?

The standard equipment for measuring gases before cargo tank entry is a hand-held portable multi-gas detector. Since the introduction of MSC.581(110) in December 2025, these detectors must be capable of measuring at least five gases simultaneously, including CO₂ as a mandatory parameter. Traditional four-gas detectors measuring only O₂, LEL, CO, and H₂S are no longer sufficient on their own.

A compliant gas detector for cargo tank entry should include the following sensors:

Electrochemical O₂ sensor for oxygen measurement
Catalytic bead or infrared LEL sensor for flammable gas detection
Electrochemical CO sensor for carbon monoxide
Non-Dispersive Infrared (NDIR) CO₂ sensor for carbon dioxide at ppm resolution
Electrochemical H₂S sensor (or another toxic gas sensor as identified by risk assessment)

The NDIR technology for CO₂ detection is worth highlighting. Standard percentage-range sensors lack the resolution to accurately detect the 5,000 ppm regulatory limit. NDIR sensors provide the high-resolution ppm monitoring needed to alert crew to dangerous CO₂ build-up before it becomes fatal.

For ships that currently operate calibrated four-gas detectors, the practical compliance path recommended by industry is to supplement existing equipment with a dedicated standalone CO₂ detector rather than replacing entire fleets of functional instruments. This bridges the regulatory gap without unnecessary capital expenditure, and it works alongside your existing onboard detection setup.

Detectors should be calibrated regularly in line with the manufacturer’s schedule and classification society requirements. All instruments used for confined space entry must be in calibration at the time of testing. You can find more information about our maritime gas detection systems to see which options are compatible with your current setup.

What regulations govern gas testing before confined space entry on ships?

Gas testing before confined space entry on ships is governed primarily by IMO Resolution MSC.581(110), which entered into force on 3 December 2025. This resolution replaces the previous Resolution A.1050(27) and sets the current technical and procedural framework for enclosed space entry safety, including mandatory CO₂ measurement as a new requirement.

The regulatory landscape includes several overlapping instruments:

MSC.581(110): The primary recommendation covering atmospheric testing requirements, gas detector specifications, Permit to Work rules, the Enclosed Space Register, and emergency response planning.
SOLAS Regulation III/19: Requires enclosed space entry and rescue drills on board ships.
SOLAS XI-1/7: Requires ships to carry portable gas detectors.
Classification society requirements: Each major classification society may have additional or complementary requirements that apply to the vessel’s class notation.
RightShip RiSQ version 3.2: Has already integrated MSC.581(110) requirements into its inspection scope, meaning vetting inspections now check for compliance with the new standard.

The Safety Management System (SMS) must be updated to reflect the new requirements. This includes revised risk assessment forms, updated confined space entry procedures, and a maintained Enclosed Space Register that lists every enclosed space on board, including connected and adjacent spaces, their hazard sources, ventilation methods, and gas testing points.

What should happen if gas readings are outside safe limits?

If gas readings are outside safe limits, entry must not take place under any circumstances. The space must be re-ventilated, and atmospheric testing must be repeated until all parameters fall within the required thresholds. No exceptions apply, regardless of operational pressure or time constraints.

The steps to follow when readings are unsafe are straightforward:

Do not enter: No work may begin and no one may enter the space, including for a “quick check.” Unplanned entries are a leading cause of confined space fatalities.
Ventilate: Increase or extend mechanical ventilation. The ventilation method and duration should match the specific hazard identified in the risk assessment.
Re-test: After ventilation, re-test the atmosphere at multiple levels. Document all readings.
Investigate the source: If readings remain elevated despite ventilation, investigate the source of the hazard. Check connected spaces, adjacent areas, and potential gas migration pathways.
Revise the plan: If the hazard cannot be resolved, the entry plan must be revised. This may involve additional PPE, breathing apparatus, or postponing entry until the cause is identified and controlled.

MSC.581(110) makes one point very clear regarding rescue: Emergency Escape Breathing Apparatuses (EEBDs) are strictly for escape and must never be used during a rescue entry. More than half of confined space fatalities on ships are would-be rescuers who entered without proper equipment or a plan. Every vessel must maintain a ship-specific Enclosed Space Emergency Response Plan, including mapped rescue routes, communication channels, and evacuation signals, before any entry permit is issued.

If an entry permit expires during work, all personnel must evacuate immediately. Re-entry requires a full re-assessment and re-test, with results recorded before the new permit is issued.

How Lavastica helps with gas detection for cargo tank entry

We understand the pressure of keeping your vessels compliant and your crew safe, especially when regulations change and you need to act quickly. At Lavastica, we supply portable and fixed gas detection equipment for maritime applications, including five-gas detectors that meet the requirements of MSC.581(110). Whether you need to upgrade existing equipment or add a standalone CO₂ detector to complement your current four-gas units, we can advise on the most practical and cost-effective solution for your fleet.

Supply of compliant multi-gas detectors, including NDIR CO₂ sensors
Standalone CO₂ detectors to supplement existing four-gas equipment
Calibration and maintenance support through our in-house workshop
Advice on compatibility with your existing onboard detection systems
Fast worldwide delivery to minimise time in port
Technical support on regulatory compliance and equipment selection

Get in touch with us to discuss your specific situation. Learn more about who we are or contact our team directly for fast, practical advice. You can reach us by phone at +31 (0) 10 265 5070 or by email at info@lavastica.com.