Start hereRules checked 20 October 2026
Every ship has a gas meter. Far fewer have a crew who know when it is lying. A catalytic flammable gas sensor can read zero in a nitrogen-filled tank full of hydrocarbon vapour. A four-gas meter that meets SOLAS perfectly says nothing about the carbon dioxide that investigators found, alongside 3.5% oxygen, in the hold access where the officers of Sally Ann C died. A two-metre hose held at the manhole tells you about the top of a tank, not the bottom where the casualty will lie.
This lesson is about the instrument that stands between the crew and those spaces: what the rule requires, what the IMO guidelines expect of it, how its sensors work, and how to take a reading you can trust.
Try it
Testing before entry on a bulk carrier and a product tanker
In plain words
The ship must carry a portable gas meter that, at the least, measures four things before anyone goes into an enclosed space:1
- Oxygen, because too little kills and too much feeds fire.
- Flammable gas or vapour, shown as a percentage of the lower flammable limit (LFL). 100% LFL is the weakest mixture that can burn.
- Hydrogen sulphide (H₂S), a poison from crude oil, sewage and some cargoes.
- Carbon monoxide (CO), a poison from coal, fires and engines.
The ship must also have a way to calibrate it.1 The IMO has written guidelines on choosing it: it should sample from a distance through a hose, test itself on switch-on, run at least 10 hours on fresh batteries, be safe to use where gas may explode, and come with a manual in the ship’s working language.2
Two things the meter is not. It is not the small personal detector clipped to the chest of the person inside; that is a separate device that warns its wearer.2 And it is not enough on its own for every ship: if your cargo can give off carbon dioxide, fumigant or a chemical vapour, you need instruments for those as well.23
In detail
What the regulation says and who it covers
SOLAS regulation XI-1/7 was added by resolution MSC.380(94) and entered into force on 1 July 2016. It applies to “every ship to which chapter I applies”, so to new and existing cargo and passenger ships on international voyages alike. It allows instruments carried under other rules (for example the tanker instruments of SOLAS II-2/4.5.7) to count towards it.1
The guidelines in MSC.1/Circ.1477 (9 June 2014) help the ship choose the instrument. They are expressly not a performance standard.2 For a performance standard, ISO 19891-1:2017 specifies portable gas detectors intended to meet XI-1/7, including the measured gases, alarm levels and environmental protection; it specifies the flammable gas as either methane or propane and butane, and requires clear marking of which flammable gases the detector can measure.4
What MSC.1/Circ.1477 asks of the instrument
| Paragraph | Feature |
|---|---|
| 3 | It is the pre-entry and periodic test instrument, not a personal detector worn inside |
| 4 | Remote sampling of all its gases without interference from the space in between |
| 5 | Self-test on switch-on |
| 7 and 8 | Measures and displays O₂, flammable gas (% LFL), CO and H₂S, clearly showing which gas is shown |
| 9 | If it has alarms, they activate at levels set by the flag Administration |
| 10 to 12 | Protected against the environment, dust and water; easily carried |
| 13 | At least 10 hours on fresh recommended batteries |
| 14 | Intrinsically safe |
| 15 | Display readable in all lighting |
| 16 and 17 | Clear calibration requirements in the maker’s instructions; an over-due alarm must not stop it working in use |
| 18 | Manual covering calibration, use and maintenance, in the ship’s working language |
Tankers, chemical tankers and gas carriers carry more
- Tankers must have at least one portable instrument for oxygen and one for flammable vapour, with a sufficient set of spares and a means of calibration (SOLAS II-2/4.5.7.1). For double-hull and double-bottom spaces they must have portable instruments that work with the fixed sampling lines; where flexible hoses cannot reliably reach, permanent sampling lines must be fitted, and plastic lines must be electrically conductive (II-2/4.5.7.2).5
- Chemical tankers carrying toxic or flammable products must have at least two instruments designed and calibrated for the specific vapours, and two separate sets if one instrument cannot test both toxic and flammable concentrations. If a fixed system is fitted, at least one portable instrument is still required (IBC Code 13.2.1 and 13.2.2).6
- Gas carriers built to the 2016 IGC Code must have at least two sets of portable gas detection equipment and a suitable instrument for measuring oxygen in inert atmospheres (IGC Code 13.6.19 and 13.6.20).7
MSC.581(110) adds a recommendation for all ships: carry at least two sets of gas detection equipment, and two more sets on ships whose cargo may give off hazardous vapours and whose cargo spaces are regularly entered for cleaning or inspection.3 Note that XI-1/7 itself says “instrument or instruments”; the two-set figure comes from the recommendation, the chemical tanker and gas carrier rules, and some flags.
How the sensors work, and where they fail
This is general instrument practice. Always check your instrument’s manual for its own sensors and limits.
| Gas | Common sensor | Watch out for |
|---|---|---|
| Oxygen | Electrochemical cell | Slow response when cold; cell life limited; reads % by volume |
| Flammable gas, % LFL | Catalytic bead (pellistor) | Needs oxygen to work, so unreliable in inerted or oxygen-poor air; can be poisoned by silicones, lead or sulphur compounds; calibrated for one gas (often methane) and reads other gases differently |
| Hydrocarbon, % volume | Infrared | Works without oxygen, which is why it is used in inert atmospheres; does not see hydrogen |
| CO, H₂S | Electrochemical cells | Cross-sensitivity to other gases; limited life |
| CO₂ | Infrared, or detector tubes | Not part of the XI-1/7 minimum; needed where cargo or fumigation can produce it |
| Specific toxic vapours | Detector tubes, photo-ionisation detector | Each tube or sensor reads one family of substances; check the cargo safety data sheet |
MSC.581(110) section 7.7 makes the key point in one sentence: testing for flammability or oxygen “does not provide a suitable means of measuring for toxicity, nor vice versa”.3 An LFL sensor that reads 1% may be showing a vapour that is already well above its toxic limit.
The flag can accept other tools. The Bahamas, for example, accepts colorimetric detector tubes for one or more of the Circ.1477 gases, provided they are in date and stored as the maker says.8
Taking a reading you can trust
- Check the instrument. Switch on in fresh air, let it complete its self-test and zero, and bump test it if the maker requires (lesson 3). Check the calibration date.
- Stop the ventilation and let the space settle. The IMO permit example suggests stopping ventilation for about 10 minutes before the pre-entry test.3
- Sample remotely, from outside, through as many openings and at as many levels as needed to deal with stratification. Heavy gases such as CO₂ and H₂S collect low; methane rises.3
- Allow for the hose. The sample takes time to travel through a long hose; the manual gives the pump flow and the wait per metre. Read only steady values.3
- Record each reading, the place, the level and the time on the permit.
- Keep testing while people are inside, and after every break.3
Before entry, MSC.581(110) looks for steady readings of 20.9% oxygen, CO₂ below 0.5%, flammable gas below 1% LFL and toxic gases below 50% of their exposure limit; national rules may differ.3 Lesson 5 shows how these readings fit into the permit.
What the rules say
- SOLAS regulation XI-1/7 requires every ship to which chapter I applies to carry appropriate portable instruments able to measure oxygen, flammable gases or vapours, hydrogen sulphide and carbon monoxide “prior to entry into enclosed spaces”, and suitable means to calibrate them.1
- MSC.1/Circ.1477 paragraph 3 says the guidelines cover the instrument used before entry and at intervals until work is complete, not a personal gas detector carried inside the space.2
- MSC.1/Circ.1477 paragraphs 4, 5, 13 and 14 recommend remote sampling, a self-test on activation, a battery life of at least 10 hours and an intrinsically safe instrument.2
- MSC.1/Circ.1477 paragraph 2 says additional instruments should be carried where the ship’s operations create hazards the basic instrument cannot detect.2
- SOLAS regulation II-2/4.5.7.1 requires tankers to carry at least one portable instrument for oxygen and one for flammable vapour, with spares and calibration means; II-2/4.5.7.2 requires suitable instruments, and if needed permanent sampling lines, for double-hull and double-bottom spaces.5
- IBC Code paragraph 13.2.1 requires ships carrying toxic or flammable products to have at least two instruments designed and calibrated for the specific vapours.6
- IGC Code paragraphs 13.6.19 and 13.6.20 require at least two sets of portable gas detection equipment and an instrument for oxygen in inert atmospheres.7
- MSC.581(110) section 7.1 recommends gas detection suited to the cargo carried, including CO₂ testing, that works correctly in oxygen-depleted air.3
- MSC.581(110) section 7.6 recommends testing with ventilation stopped and after conditions have stabilised.3
Flag and class notes
- The Bahamas Maritime Authority accepts colorimetric tubes for detecting one or more of the Circ.1477 gases if they are within their expiry date and correctly stored, and accepts that personal detectors may be the same instruments as those carried under XI-1/7 (other than tubes) or additional ones.8
- The Marshall Islands Marine Notice 7-041-1 adds a national requirement that enclosed space drills check the suitability, including length, of sampling hoses for measuring at all levels in double-bottom spaces, based on SOLAS II-2/4.5.7.9
- Antigua and Barbuda notes that MSC.581(110) does not create a new mandatory requirement to carry dedicated CO₂ instruments, but that where CO₂ is a potential hazard, means to verify a safe concentration must be provided before entry.10
Key numbers
| What | Value |
|---|---|
| SOLAS XI-1/7 in force | 1 July 20161 |
| Minimum gases measured | O₂, flammable (% LFL), H₂S, CO1 |
| Minimum battery life (fresh batteries) | 10 h2 |
| Portable sets recommended for all ships | at least 23 |
| IBC instruments for specific vapours | at least 26 |
| IGC portable gas detection sets | at least 27 |
| Ventilation stopped before pre-entry test | about 10 min3 |
| Oxygen for entry | steady 20.9%3 |
| Flammable gas for entry | below 1% LFL3 |
What inspectors find
- The Marshall Islands Administrator warned operators after a rise in US Coast Guard deficiencies for portable atmosphere testing instruments that were not properly calibrated, and reminded them to calibrate to the maker’s instructions and train the crew.11
- The joint 2015 Paris and Tokyo MoU campaign checked, among other things, the atmosphere testing equipment on board and the crew’s training to use it; the Tokyo MoU raised 1,584 deficiencies from unsatisfactory answers.12
- On Sally Ann C the ship had a tube pump with a 10 m hose, tubes for oxygen, carbon monoxide, carbon dioxide and other gases, and two oxygen meters, all working; yet there was no evidence of a permit for the chief officer’s entry into the hold access.13
- On Berge Mawson the MAIB found that cargo space atmospheres were not routinely tested before stevedores entered.14
FAQ
Does a single four-gas meter satisfy SOLAS XI-1/7?
Legally one instrument can, since the rule says “instrument or instruments”. In practice you also need a calibrated spare for when it is out of service, and MSC.581(110), the chemical tanker and gas carrier rules and some flags expect two. Some flags require an alternative instrument on board while one is ashore for calibration.
Can the personal detector worn by the entrant be used for the pre-entry test?
Circ.1477 treats them as different jobs. The pre-entry instrument samples remotely through a hose from outside; a personal detector warns its wearer once inside. Some flags accept that the same model may do both roles, but the pre-entry test must still be done from outside, before anyone enters.
Why is flammable gas shown as % LFL and not % volume?
Because the danger depends on how close the mixture is to burning. 100% LFL means the mixture can ignite. For methane that is roughly 4.4 to 5% by volume, depending on the data the maker uses. In inerted tanks, where a catalytic sensor does not work, hydrocarbon is measured in % by volume with an infrared instrument.
Do we need to measure carbon dioxide?
SOLAS does not list CO₂, but MSC.581(110) recommends checking it is below 0.5% before entry, and testing it on ships with organic cargoes before entry and frequently afterwards. If your risk assessment says CO₂ is possible, carry the means to measure it.
Quiz5 questions
Sources
- 1
IMO Resolution MSC.380(94), Amendments to SOLAS (adopted 21 November 2014, in force 1 July 2016), Annex, new regulation XI-1/7, Atmosphere testing instrument for enclosed spaces. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MSCResolutions/MSC.380(94).pdf
- 2
IMO MSC.1/Circ.1477, Guidelines to facilitate the selection of portable atmosphere testing instruments for enclosed spaces as required by SOLAS regulation XI-1/7 (9 June 2014), Annex paragraphs 1 to 18. Copy published by the Vietnam Maritime Administration: https://vimawa.gov.vn/f/circ%201477.PDF
- 3
IMO Resolution MSC.581(110), Revised recommendations for entering enclosed spaces aboard ships (adopted 27 June 2025), Annex, sections 7.1 to 7.7, 8.1 to 8.3, 10.5 and appendix 2 note 2. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MSCResolutions/MSC.581(110).pdf
- 4
ISO 19891-1:2017, Ships and marine technology: Specifications for gas detectors intended for use on board ships, Part 1: Portable gas detectors for atmosphere testing of enclosed spaces, scope (as listed by the Estonian Centre for Standardisation). https://www.evs.ee/en/iso-19891-1-2017
- 5
IMO Resolution MSC.291(87), Amendments to SOLAS (adopted 21 May 2010, in force 1 January 2012), Annex, regulation II-2/4 paragraph 5.7 (5.7.1 and 5.7.2). https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MSCResolutions/MSC.291(87).pdf
- 6
IMO Resolution MSC.176(79), 2004 amendments to the IBC Code (adopted 10 December 2004, in force 1 January 2007), Annex, paragraphs 13.2.1 to 13.2.4. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MSCResolutions/MSC.176(79).pdf
- 7
IMO Resolution MSC.370(93), Amendments to the IGC Code (adopted 22 May 2014; applies to gas carriers constructed on or after 1 July 2016), Annex, paragraphs 13.6.19 and 13.6.20. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MSCResolutions/MSC.370(93).pdf
- 8
Indian Register of Shipping, Technical Circular 011/2017, The Bahamas National Requirements, Revision 08 (17 January 2017): atmosphere testing instruments, calibration and colorimetric tubes. https://www.irclass.org/media/2668/technical-circular-no-11.pdf
- 9
Republic of the Marshall Islands Maritime Administrator, Marine Notice 7-041-1, Entering enclosed spaces: safety precautions (Rev. Jan/2026), paragraph 1.4.4 and footnote 3. https://www.register-iri.com/wp-content/uploads/MN-7-041-1.pdf
- 10
Antigua and Barbuda Department of Marine Services and Merchant Shipping, SOLAS Circular 2026-002, Enclosed space entry (17 March 2026), paragraph 4.4. https://abregistry.ag/wp-content/uploads/2026/03/SOLAS-Circular-2026-002-Enclosed-Space-Entry.pdf
- 11
Republic of the Marshall Islands Maritime Administrator advisory on improperly calibrated portable atmosphere testing instruments, as reported by SAFETY4SEA. https://safety4sea.com/rmi-beware-of-improperly-calibrated-portable-atmosphere-testing-instruments/
- 12
Tokyo MoU results of the 2015 concentrated inspection campaign on crew familiarization for enclosed space entry, as reported by The Maritime Executive. https://maritime-executive.com/article/strong-commitment-to-enclosed-spaces-safety
- 13
Isle of Man Ship Registry, Casualty investigation report: Sally Ann C, enclosed space fatalities and near fatality, 13 March 2015, sections 2.6 and 2.9. https://www.iomshipregistry.com/media/1124/cr-sally-ann-c-enclosed-space-fatalities-and-near-fatality.pdf
- 14
MAIB on behalf of the Isle of Man Administration, Report 5/2025, Berge Mawson (March 2025), Synopsis. https://www.iomshipregistry.com/media/qoucqgn3/2025-5-bergemawson-reportandannexes.pdf
Rules checked 20 October 2026. VSL Safety is not an approved training provider and doesn't issue certificates. These lessons don't replace STCW training, or the manufacturer's training and certification that service technicians need.
