What should a gas detection procedure include for tank entry?

A gas detection procedure for tank entry must test for at least five atmospheric hazards: oxygen levels, flammable gases, carbon monoxide, carbon dioxide, and at least one additional toxic gas identified through risk assessment. All readings must fall within safe limits before anyone enters. The sections below walk through exactly how that procedure works in practice, who is responsible, and what to do when readings fall outside acceptable ranges.

What gases must be tested before entering a ship’s tank?

Before entering any enclosed space or tank on board, you must test for at least five gases: oxygen (O₂), flammable gases expressed as a percentage of the Lower Explosive Limit (LEL), carbon monoxide (CO), carbon dioxide (CO₂), and at least one additional toxic gas identified by your vessel’s risk assessment, most commonly hydrogen sulphide (H₂S).

The acceptable limits under current IMO recommendations are clear:

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 worth highlighting: CO₂ is now a mandatory testing parameter under IMO Resolution MSC.581(110), which entered into force in December 2025. Traditional four-gas detectors measuring only O₂, LEL, CO, and H₂S are no longer sufficient. A space does not need to carry dangerous cargo to become lethal. Ordinary rusting in a damp, confined environment can rapidly consume oxygen and raise CO₂ to fatal levels within days. Your gas detection procedure must account for these passive chemical changes, not just obvious cargo hazards.

It is also important to test connected and adjacent spaces, not just the primary space you plan to enter. Any area linked by a door, manhole, pipe, or even a small gap to a hazardous space must be treated as potentially hazardous until independently tested and confirmed safe.

How should gas testing be carried out in an enclosed space?

Gas testing in an enclosed space should be carried out from outside the space before any person enters. A calibrated portable gas detector is lowered into the space on a line, or extended using a probe and sampling tube, to measure the atmosphere at multiple levels: top, middle, and bottom. Different gases behave differently, with heavier gases like CO₂ settling low and lighter gases accumulating at the top.

The testing sequence matters. Always begin with oxygen, then flammable gases, then toxic gases including CO₂. Record every reading formally, because an unrecorded test is treated by Port State Control as a test that did not occur.

After the initial test, ventilate the space thoroughly and re-test before entry. During the work period, continuous or repeated atmospheric monitoring is required. If the work team takes a break or ventilation is interrupted for any reason, the entry permit is immediately void and all personnel must leave. Before re-entry, the atmosphere must be fully re-tested and the results recorded as acceptable.

Pay particular attention to dead-end areas, vertical trunks, and structural pockets where gas can remain trapped even after the main compartment has been ventilated and cleared. Each connection point must be independently ventilated and confirmed safe.

Who is responsible for gas detection during tank entry?

Responsibility for gas detection during tank entry sits with the Master, who must verify that all conditions are met before issuing an entry permit. In practice, a competent officer designated by the Master carries out or supervises the actual atmospheric testing. A trained attendant must be stationed at the entrance at all times while anyone is inside. Solo entry is prohibited under current regulations.

The person conducting the gas test must be trained in the use of the specific detector being used, including understanding how to interpret readings, recognise sensor limitations, and respond to alarm signals. Training requirements have been strengthened under MSC.581(110): enclosed space drills must now be held at least once every two months and must include practical use of atmospheric testing instruments.

The Master also carries responsibility for maintaining the vessel’s Enclosed Space Register, which must be kept both on board and synchronised with the shore-side office in real time. This register must be updated whenever cargo changes, the medium in a tank changes, or the purpose of a space is altered.

What equipment is required for safe tank entry gas detection?

Safe tank entry gas detection requires a calibrated five-gas portable detector capable of measuring O₂, LEL, CO, CO₂, and at least one additional toxic gas such as H₂S. For CO₂ specifically, Non-Dispersive Infrared (NDIR) technology is now the benchmark for compliance, because it provides the high-resolution ppm monitoring needed to accurately detect the 5,000 ppm regulatory limit. Standard electrochemical sensors lack the resolution to measure CO₂ at this level reliably.

Additional equipment required includes:

Emergency Escape Breathing Apparatuses (EEBD) for escape purposes only, never for rescue entry
Rescue equipment as specified in the vessel’s ship-specific Enclosed Space Emergency Response Plan
Physical SAFE / UNSAFE signs at all entrances
Locked manholes and access points when no active permit is in place
A functioning communication system between the person inside and the attendant outside

If your existing fleet of four-gas detectors is functional and calibrated, you do not necessarily need to replace everything immediately. A practical compliance path is to supplement existing equipment with a dedicated standalone CO₂ detector. This bridges the regulatory gap without the full capital expenditure of replacing an entire fleet. Our fire and gas detection equipment range includes options that work alongside existing onboard installations.

What should happen if gas readings are out of safe limits?

If any gas reading falls outside the safe limits, entry must not proceed under any circumstances. All personnel must remain outside, and the space must be ventilated further before re-testing. Do not assume a single reading was a sensor error. Record the out-of-limit result, investigate the source, and re-test only after the cause has been identified and addressed.

If readings are dangerously high, the space must be locked and clearly marked as UNSAFE. No one may enter for any reason, including rescue, without a fully equipped and briefed team following the ship-specific Emergency Response Plan. This is a firm rule: unplanned rescue attempts are prohibited. More than half of enclosed space fatalities are would-be rescuers who entered without proper equipment or a coordinated plan.

Once ventilation has been completed and the atmosphere re-tested with acceptable results, a new entry permit must be issued. The previous permit is void and cannot simply be extended or reactivated.

How does a gas detection procedure fit into the permit-to-work system?

Gas detection is a mandatory pre-condition for issuing a permit to work for tank or enclosed space entry. No permit may be issued until atmospheric testing confirms that all parameters are within safe limits and those results are formally recorded. The permit itself now carries a maximum validity of eight hours, after which a full re-assessment and re-test are required before work may continue.

The permit-to-work system connects directly to the vessel’s Safety Management System (SMS). Masters are required to revise their SMS to align shipboard procedures and risk assessment forms with current regulatory requirements. The gas test results, the names of the tester and the attendant, the time of testing, and the specific readings must all be documented as part of the permit record.

Entrance control is also part of the permit system. All manholes and access points must be physically locked or sealed unless an active permit is in place. Entrances must be marked with SAFE or UNSAFE signs. The permit-to-work process is not a formality; it is the operational framework that connects atmospheric testing, responsible persons, time limits, and emergency procedures into one verifiable system. Our service and repair support can assist with calibration and maintenance of gas detection equipment to keep that system functioning reliably.

How Lavastica helps with gas detection for tank entry

We understand that finding the right gas detection equipment quickly, especially equipment that works with what you already have on board, is exactly what fleet engineers need when a vessel is in port. At Lavastica, we supply portable and fixed gas detection solutions for maritime applications, including five-gas detectors with NDIR CO₂ sensors that meet current IMO requirements. We also carry standalone CO₂ detectors for vessels that want to supplement existing four-gas units rather than replace an entire fleet.

Supply of compliant five-gas portable detectors and standalone CO₂ monitors
Advice on compatibility with your existing detection systems and onboard installations
Calibration, maintenance, and repair support through our in-house workshop
Fast worldwide delivery to minimise time in port
Technical support on regulatory requirements and product selection

Need help finding the right gas detector for your fleet? Learn more about us or get in touch directly via our contact page. You can also reach us by phone at +31 (0) 10 265 5070 or by email at info@lavastica.com. We respond quickly, because we know every hour in port counts.