A cylinder can look perfect from the outside and be corroding from the inside. The mechanism is simple and it is almost always water. A bail-out cylinder that loses all pressure above ambient underwater can draw seawater past the valve, and once salt water is sitting inside a steel cylinder at 200 bar or more, the wall starts thinning where nobody can see it. The cylinders most exposed to this are the ones a diver depends on in an emergency, which is why the commercial sector inspects them far more often than the headline figures most guidance quotes.
Summary: Periodic inspection and testing of diving cylinders is governed by BS EN ISO 18119:2018+A2:2024 for seamless steel and seamless aluminium-alloy cylinders, and BS EN ISO 11623:2023 for composite cylinders. In UK commercial diving, both offshore and inshore, the working intervals are a periodic inspection and test every 5 years and an internal examination every 2.5 years. Cylinders used for bail-out, suit inflation or buoyancy control are internally examined every 6 months because they are at far greater risk of water ingress. Cylinders used only on the surface to supply a diver through an umbilical are inspected and tested every 10 years.
Which standards apply to diving cylinders
Two standards carry the periodic inspection and testing requirements, and which one applies depends on how the cylinder is built.
| Cylinder type | Standard |
|---|---|
| Seamless steel and seamless aluminium-alloy cylinders and tubes | BS EN ISO 18119:2018+A2:2024, Gas cylinders. Periodic inspection and testing |
| Composite cylinders and tubes | BS EN ISO 11623:2023, Gas cylinders. Composite cylinders and tubes. Periodic inspection and testing |
| All cylinders, at the point of filling | BS EN ISO 24431:2016, Inspection at time of filling |
| All cylinders, marking and identification | BS EN 1089-3, Transportable gas cylinders. Gas cylinder identification |
If your procedures still reference BS EN 1968:2002 for steel or BS EN 1802:2002 for aluminium, they are out of date. Those two European standards were merged with ISO 6406 and ISO 10461 to create BS EN ISO 18119, and the UK Diving Industry Committee describes both as superseded. They are worth knowing about only because they specified a fixed 2.5 year period between internal inspections, and that figure has outlived the documents it came from. It is still widely quoted as though it were a rule.
What the law actually requires, and what it does not
This is where most published guidance overstates the position, so it is worth being precise.
There is no statutory interval for the in-service inspection and testing of diving cylinders in the UK. What exists is a general duty. Health and safety law requires that equipment used at work, or in connection with work, is properly maintained, so cylinders used at work or filled by someone at work must be subject to a suitable inspection and test regime. That duty applies to recreational divers’ cylinders too, where they are filled by a person who is at work.
The periodicity figures themselves sit in an informative annex within the standards. Informative means guidance rather than mandate. The standard for seamless steel and aluminium-alloy cylinders says that local regulations will specify the interval, and that in the absence of any local regulation an internal examination every year with a periodic inspection and test at 5 yearly intervals should be carried out, with the internal examination interval extendable to a maximum of 2.5 years where a risk assessment shows a low risk of internal degradation. The composite standard says that for cylinders used for underwater operations and self-contained breathing apparatus, the retest period should not exceed 5 years.
One place regulation does bite directly is road transport. Where cylinders are carried in the course of a work activity they fall within the scope of ADR, the agreement on the international carriage of dangerous goods by road, under the Carriage of Dangerous Goods and Use of Transportable Pressure Equipment Regulations 2009. For cylinders over 10 years old transported by road for work, inspection must be carried out by an organisation appointed by the Vehicle Certification Agency, every 10 years after manufacture. That is a separate requirement sitting alongside your in-service regime, not a replacement for it.
The intervals commercial diving works to
Because the standards defer to local arrangements, the UK diving industry produced its own risk assessment to give the sector clear direction. It is reviewed annually by the UK Diving Industry Committee, and the current version is dated 16 October 2025. These are the intervals in use, and they are not one number.
| Cylinder role | Internal examination | Periodic inspection and test |
|---|---|---|
| General diving cylinders, offshore and inshore commercial | Every 2.5 years | Every 5 years |
| Bail-out cylinders, or part of a bail-out system | Every 6 months | Every 5 years |
| Suit inflation and buoyancy control device cylinders | Every 6 months | Every 5 years |
| Cylinders used only on the surface to supply a diver via umbilical, individually or in bundles | Covered by the 10 year cycle | Every 10 years |
The same 5 year and 2.5 year pattern is applied across the media, scientific, defence, police and recreational sectors. What changes between sectors is the treatment of cylinders at elevated risk of water ingress, and in commercial diving those are on six months.
It is worth noting what the industry risk assessment concluded, because it is reassuring rather than alarming. Across an estimated 100,000 cylinders in UK use, no bursting failure involving a cylinder used underwater has been recorded in the last ten years, and the residual risk at current intervals is assessed as broadly acceptable against HSE’s own criteria. The regime works when it is followed.
Why bail-out cylinders sit on a six month cycle
A bail-out cylinder is not continuously breathed underwater. It sits on the diver’s back, at depth, in salt water, often for its whole working life without being opened. If it loses all pressure above ambient while submerged, water can get in, and nothing about the outside of the cylinder will tell you that has happened.
The same logic applies to suit inflation and buoyancy control cylinders, and to marker buoy and side-mount cylinders, all of which can normally lose pressure underwater during use. Oxygen content raises the stakes further. A cylinder holding a gas with an oxygen content above 21 per cent will corrode faster if water does get in, because the higher oxygen content supports a more rapid rate of oxidisation.
This is also the reason for a rule that catches people out: if a cylinder is suspected of having taken on water, it comes out of service immediately and stays out until a competent person has inspected it. That is not a scheduled event. It happens the moment the suspicion arises.
The aluminium alloys that need eddy-current testing
Two aluminium alloys carry a specific additional requirement. Cylinders manufactured from AA6082, also known as HE30, and from AA6351 must be tested with an eddy-current device at each periodic and internal examination. The device must be approved by the manufacturer of the cylinder, and if the cylinder is fitted with a thread adaptor, that adaptor has to be removed before examination and eddy-current testing.
The reason is sustained load cracking in the neck and shoulder region, which visual inspection alone will not reliably find. HSE issued a safety bulletin on cylinders made from these alloys and used primarily for underwater breathing apparatus, following a catastrophic failure of an aluminium alloy cylinder in 2017 that caused serious harm. That cylinder had not been used underwater.
If you operate a mixed fleet, knowing which of your cylinders are HE30 or AA6351 is not optional detail. It changes what a valid examination looks like.
What should happen every time a cylinder is filled
Periodic examination is not the only control. BS EN ISO 24431:2016 governs inspection at the time of filling, and two of its provisions matter every single fill.
- Interior condition. Before filling, the presence of liquid that could have caused internal corrosion must be established.
- No residual pressure. A cylinder presented for filling with no residual pressure must be internally examined before it is filled.
There is also a direct link back to gas quality. Cylinders are only to be filled with gases that comply with BS EN 12021:2014, and any cylinder known to have been repeatedly filled with gas carrying a higher moisture content than that standard permits should be internally examined. We covered the composition limits and testing frequency for breathing gas in our article on divers’ breathing air quality and EN 12021.
Competence, marking and records
There is no unique legal definition of competence for cylinder testing. In practice, most filling companies and dive centres will only accept cylinders examined and tested by organisations that are appointed by the Secretary of State for Transport for the purposes of gas cylinder inspection, or working within a UKAS accredited scheme, or both. The UK diving industry risk assessment goes further and expects periodic and internal examinations to be carried out by competent persons within organisations whose procedures are subject to third party independent audit.
On paperwork, national standards require a cylinder to be marked to show it has been subject to the relevant inspection and test, and labelled to show the next test date. A test report should be given to the owner, and owners are advised to keep those reports as evidence that the work was done by someone suitably competent. HSE recommends that any inspection and testing regime is itself subject to independent assurance.
If you transport cylinders by road, label them to help the emergency services: a green compressed-gas hazard diamond, plus a yellow hazard diamond where the oxygen content exceeds 23.5 per cent.
Keeping water out in the first place
Every interval above is a detection measure. The cheaper control is prevention, and the industry good practice list is short and specific.
- Keep or store cylinders with a residual pressure of at least 2 bar.
- Do not empty a cylinder underwater to the point where there is no residual internal pressure above ambient.
- Any cylinder that has lost all pressure above ambient while underwater should be internally examined.
- Do not charge cylinders that have been emptied underwater by decanting from another diving cylinder.
- Blow through all charging hoses before use, and blow through cylinder valves before use or charging.
- Leave any empty cylinder with the valve closed, and dry and cap cylinder valves when a regulator is not connected.
One measure that might look like an obvious fix is specifically ruled out. Residual pressure valves are standard practice across the wider compressed gas industry for preventing contamination and water ingress, and they have been considered for diving apparatus. They are not considered appropriate for diving cylinders, because they introduce a potential single point of failure that could prevent the supply of breathing gas, and may restrict breathing gas flow. In a bail-out scenario that trade-off is the wrong way round.
How we can help
We supply the cylinders and the equipment used to test them, rather than operating a test station. Our range covers steel and carbon composite cylinders and gas quads, including 20 litre and 50 litre high pressure steel cylinders, carbon composite cylinders, chamber breathing air quads and multi element gas containers. For teams testing in house, the Octopus Test Systems hydrostatic test system in our NDT equipment range uses the standard jacket method with a coned lid to avoid trapped air, and is supplied with HY-PAC over-pressure control.
We also stock diver bailout equipment , and much of our range is available new, used, or for hire where a project does not justify a purchase. Where you are buying used, treat the inspection and test history as part of the specification rather than something to check afterwards. Our technician’s service and calibration training supports teams who maintain and test their own equipment.
We understand commercial diving is fast paced and time critical, so we pride ourselves on extensive high quality stock, honest and realistic delivery timeframes, and flexible operating procedures across global time zones. We ship worldwide by sea or air freight, door to door. To talk through a requirement, no matter how big or small, call +44 1253 767 788, email info@isubc.com, or request a quote.
