ALUMINIUM TANKS – WHAT EVERY DIVER SHOULD KNOW

Readers Views

In the August /September issue we had a special three page feature entitled ‘Are aluminium tanks safe?’ In this issue we follow on with an interview with Bill High, president of PSI Inc in the USA who are regarded as international experts on testing aluminium cylinders.

We have had a huge response to the issues raised in the feature and have printed some of the letters we received together with answers to the issues raised. On 16 and 17 August Luxfer representatives held meetings in Wellington and Auckland to allow open discussions on scuba cylinder testing stations’ concerns regarding certain Luxfer aluminium cylinders. Congratulations to Luxfer for fronting up to the New Zealand dive industry so that issues could be discussed.

I came away from the meeting feeling that the majority of the audience were dissatisfied with the answers provided by Luxfer, and wanted Luxfer to admit there is a problem with their cylinders manufactured from 6351 alloy, or that problems with cracking in the neck area of a cylinder may be due to their method of manufacture. A total recall of the cylinders in question was rejected by Luxfer as unnecessary. Some discussion centered on the cylinder owner’s responsibility to maintain their own cylinders in good working condition. The average diver does not know what is required to care for his/her cylinder other than washing it with fresh water after use. Diver training agencies may well see a need to provide more in-depth education to ‘open water’ divers on how to care for cylinders and what is regarded as dangerous or unacceptable practice when handling and filling cylinders.

It became obvious that the expertise of testing stations to visually detect a crack varies with testing personnel involved in the process of checking cylinders. Is it time to have central testing stations that have expensive high tech equipment to virtually eliminate the human factor when testing a tank? Some in the dive industry naturally say, ‘why should the industry go that way when the current system has been satisfactory? Why go to that expense when the problem, as they see it, is a manufacturers problem, not an industry problem?

Some dive shops are taking a stand by not filling the suspected series of tanks. (Luxfer E6498, SP6498 and DOT3AL). Their main concern is that if a cylinder does explode and injure someone, they would be held responsible and they do not want to take that risk. Another practice that has slowly crept into the industry is the filling of tanks in an unprotected area that is within or very close to the person filling the tanks and the customers in the shop.

OSH will be looking into this practice over the coming months as the current laws (HSC Act) are adequate regarding dangerous work practices ie filling of high pressure cylinders. OSH have also put in place two initiatives. Firstly, that all scuba cylinder manufactures must place on all cylinders coming into NZ a sticker advising the user how to look after the cylinder. Secondly, all importers of tanks must affix a green diamond-shaped sticker advising the owner that the cylinder is potentially dangerous and must be cared for accordingly. Testing stations will also be required to place this sticker on all tanks that they test if the tank does not already have a sticker.

During the meetings Luxfer indicated that the current replacement programme may be extended past 31 December 1999. They are considering a ten year replacement guarantee for new Luxfer cylinders. After ten years if your cylinder develops a fault, they will replace it with a new cylinder at half price. As we go to press, this has not been confirmed. Luxfer are currently the only scuba cylinder manufacturer that I am aware of that are standing up to be counted and are looking at long term replacement guarantees for their cylinders. As someone commented to me recently, ‘at least this issue has brought to people’s attention the issues of properly testing cylinders and the consequences if one slips through the system.’ I am sure divers and the dive industry are the winners of these current issues. In the final analysis we will have a safer sport.

Dave Moran, Editor


Dear Dive New Zealand,

Congratulations on the well-presented special feature on aluminium cylinders in issue 53. I would like to add to this ongoing saga by making an analogy between this gas cylinder hazard and an electrical hazard, which each in their own ways have the potential to cause serious injury or even death.

Regulation 102 of the 1997 Electricity Regulations states in part that ‘The Secretary may prohibit the manufacture, importation, sale, installation, and use of any fittings or electrical appliance that constitutes or may constitute an electrical hazard.’ Regulation 105 states in part that ‘In case of urgency, the Secretary may issue instructions, orders, or requirements for securing the protection of persons from injuries caused, directly or indirectly, by electricity.’ In recent times there have been many examples of recalls of unsafe electrical appliances for either modifications, replacements or money back, without all the rigmarole that the potential hazard was caused by the user! This has also been done with other items: children’s clothes and toys, to mention a couple.

From this analogy, my question is: if OSH (Occupational Safety & Health) is that well organised, and as a government department must have its own set of regulations, why is it that the OSH management do not enjoy the same powers as the secretary of the Electrical Registration Board, to recall these aluminium cylinders? Maybe OSH regulations do allow such action, and for reasons of their own OSH have decided not to enforce them.

I own one of the aluminium cylinders concerned, which I purchased new in 1976 with a lifetime guarantee. For my part, as the owner of a potentially hazardous gas cylinder, in April this year I made the decision to take a responsible approach and voluntarily condemn this cylinder, so that I could have a clear conscience and be able to sleep at night. As it was due for a visual inspection, I requested that it be failed, but this was declined and returned as inspected OK with a full charge. This then placed me in the dilemma of how to mechanically render the cylinder unusable and not jeopardise the lifetime guarantee or any possible future recall.

It is therefore being stored in a secure place, with a minimal charge in it, until Luxfer and/or Tank Test Laboratories also take an equally responsible approach, come back down to Earth from whatever cloud they are on, and do the consumer a service by voluntarily introducing a cylinder recall and replacement programme, as any other reputable organisation would do.

Derek Olson, Matapouri Bay, Northland


The Dangerous Good Act 1974 and the Dangerous Goods Class 2 Gases Regulations 1980 set minimum standards for control of gas cylinders. This includes the manufacture, the filling and the testing of cylinders. Similar controls may be applied to what has been described by Mr Olson in respect of the 1997 Electricity Regulations.

In respect of aluminium alloy gas cylinders, and in particular the failed Tairua cylinder, the cylinder meets the chemical and mechanical property requirements of the current US DOT-3AL specification in 49 CFR 178.46-5 for the alloy from which it was made. The defect of the failed cylinder developed over a number of years and theoretically this defect should have been detected at the time of test. Therefore, the cylinder should have been removed from service before reaching a critical failure state.

The Department of Labour is looking at a longer term strategy to ensure that cylinders being used are in a safe state. One of the strategies that is being contemplated is a finite life being placed on aluminium alloy cylinders, in addition to ensuring that gas cylinder testing stations are able to detect cracks in cylinders whilst they are at an early stage in their development. The matter of the cylinder replacement programme for condemned aluminium cylinders is a decision that must be made by the manufacturer. The Department’s role is to ensure that the product is safe for owners, testers and fillers.

Kim Comben, Chief Adviser of Explosive and Dangerous Good – Occupational Safety and Health Service (OSH).


Dear Dave,

I think it’s a shame you did not ask all six commenters the same questions, like ‘Are Luxfer and OSH aware of the hydrostatic test fill rate?’ To exceed the manufacturer’s recommendation by ten times is most certainly asking for trouble. I can understand the block wall. If all else fails, read the instructions.

Why does the test fill rate have to be so fast? If I had a Luxfer cylinder fail a test I think I would be saying to the tester ‘You destroyed it, you replace it.’ As I see it, the problem is the testers, not the makers. I am not surprised that one of the testers thinks such severe shock loading every two years is detrimental to a cylinder. I think you have opened a can of worms here.

Neville Harris, Bay of Plenty

Thanks for your comments, Neville. Our attempt to address this problem is just that – an attempt. It’s great that people like yourself wish to add to the debate. We would rather open the can of worms than have it burst due to the gas buildup! Thanks again for your input. Regards, Dave


Dear Sir,

What price a human life? There are those in the dive industry who apparently place it below that of an aluminium dive tank. On the one hand we see a manufacturer denying all responsibility for an inherently faulty product: attempting to cover their collective corporate asses by blaming cylinder service centres and divers themselves – their customers – for what is obviously, even to the most mentally challenged of persons, a manufacturing deficiency.

And on the other hand we have an industrial safety department reluctant to put the safety of people above economic considerations, preferring to take a sit-on-the-fence approach that in reality amounts to no more than ‘Let’s just wait and see what happens.’

I am not a diver with hundreds of hours in my logbook, nor do I work for a living within the dive industry. I am simply a recreational diver who loves it, and wants to still be diving many years from now. There may be ‘millions’ of cylinders still in use that satisfy testing standards. So what? That’s no consolation to the surviving partner and children of the diver unfortunate enough to be using the one that goes bang, or the filling station technician who ends up having a really bad day.

I have an E6498 series cylinder. Nobody wants to fill it. Nobody wants to buy it. And I sure as hell won’t be using it. The only way it’s going to move is when the manufacturer has the balls to take ownership of the problem and do the right thing by their customers. We are each given only this one life – I wouldn’t trade it for a Luxfer dollar.

Kim Nankivell, Auckland



Dive shops take a stand

This is a sample of a notice to customers that some dive shops have posted in their shops. The shops that we are aware of are The Dive Centre, Takapuna, Auckland, The Air Shop, Mairangi Bay, Auckland, Western Underwater, New Lynn, Auckland and Panmure Underwater, Panmure, Auckland.


Dear Customer,


Re: Luxfer DOT-E6498 / SP6498 / 3AL Cylinders manufactured up to 01/89

You are aware that many of the US Luxfer dive cylinders have failed due to cracking in the neck. Some test stations will no longer test them, many air filling stations will no longer fill them. The problem appears to be in cylinders made from the aluminium alloy 6351-T6. These cylinders were made and imported up to 1988. The incidence of cracking and catastrophic failure (explosion) seems to be increasing with age, and we no longer have any confidence in these cylinders. Luxfer has put the responsibility for the failure of these cylinders back to:

  • The Owner – for abuse, leaving in the sun and failure to maintain
  • Air Fill Stations – for overfilling and fast filling
  • Test Stations – for incorrect testing.

The detection of these cracks is very difficult and, rather than risk your safety as well as our own, ‘WE WILL NO LONGER FILL OR TEST THESE CYLINDERS.’

We will continue to fill these cylinders only until the ‘current’ visual or hydrostatic test date expires – whichever is the sooner. Should you wish to trade in these cylinders we will offer you a $65.00 reduction off the retail cost of a new cylinder until 31st December 1999 (NB retail depends on size etc). Should you wish to take this matter up with Luxfer I suggest you contact them at their local area office in Australia. Contact Ms Leonie Reguson, phone +61-2-9830 0999, fax +61-2-9622 9227.



US dive stores also take a stand

Florida Down Under, Inc., will not fill any of the following ‘suspect’ cylinders until they have undergone an eletronic thread check. We will provide this check FREE OF CHARGE for any cylinder in current visual inspection. This has become a STANDARD part of the FDU visual cylinder inspection process. Here is ‘The List’ of scuba tanks that DOT says are most likely made from the 6351-T6 aluminium alloy:

  • All DOT-3AL tanks manufactured under one of the following exemptions or special permits: 6498, 7042, 8107, 8364, 8422.
  • All composite cylinders manufactured under one of the following exemptions: 7235, 8023, 8115.  All Walter Kidde DOT-3AL scuba tanks.
  • All Cliff Impact DOT-3AL scuba tanks made before July 1990.
  • All Luxfer 80.8 cu. ft. scuba tanks made before May 1987.
  • All Luxfer scuba tanks made before August 1987.
  • All other scuba tanks made in the US before February 1990 (except Catalina).
  • All scuba tanks not made in the US. Unless proven otherwise, all scuba tanks in the above list should be assumed as being made using the 6351-T6 alloy.

Also it should be noted that Catalina cylinders were NEVER made from the 6351-T6 alloy. They were made using alloy 6061-T6, which as of yet has not been known to fail explosively. All of the manufacturers in the above list, except Walter Kidde, switched to the 6061-T6 alloy on the dates shown.




Interview with Bill High, President of PSI Inc.



by Dave Moran


How long has PSI been involved with cylinder testing and investigations?


I formed PSI, Inc. (initially called Professional Scuba Inspectors) in 1982, following three years of research. I had completed four years as NAUI’s president and was looking for another way to serve the diving community. Our first seminar was conducted in January 1983. We have trained over 10,000 inspectors for military, government, hydrostatic retesting, dive and fire industries in North America, and at numerous international locations. We have 65 instructors in ten countries conducting nearly 200 seminars each year.


What are the main reasons for an aluminium cylinder being condemned and withdrawn from service?


Aluminium cylinders have received a great deal of attention in the USA since an explosion in Riviera Beach, Florida in February 1998. This was widely discussed on the Internet. Before that event, the occasional explosion of either steel or aluminium cylinders was largely ignored by the dive industry. Internal and external corrosion are the main causes of steel cylinders failing inspection or test. Aluminium cylinders fail less often, but when they do the cause is either excessive corrosion, often in the thread area or beneath the boot, or from a crack discovered during a visual inspection.


What does your organisation consider to be the main cause of these problems?


Corrosion damage is usually the fault of the cylinder owner or the fill station. Water, especially salt water, causes metal to corrode. Water lies in contact with the cylinder for days beneath a boot. If air fill stations have poorly maintained filters and water separators, water will be pumped into the cylinder during fills. Sustained load cracking is a defect that appears in some cylinders made from 6351 alloy. In the USA, this alloy was used by Luxfer Gas Cylinders from 1971 through to 1987. All Walter Kidde scuba cylinders were also made from 6351.


Is it hydro testing or visual testing that shows if there is a problem with a cylinder?


Hydro testing does not appear to be the best means to detect a sustained load crack in at-risk cylinders. Most cracks in cylinders are found by visual inspectors. Considering both steel and aluminium, over 90% of the cylinders failed by a hydro retester are failed by visual inspection prior to a pressure test.


Do you have an idea of the percentage of cylinders which fail out of those tested?


If you mean hydrostatic tested cylinders which failed that pressure test, then the number is very small, since most cylinders are failed based upon damage observed by visual inspection. The following example from one of our PSI affiliate instructors who is also a federally licensed hydrostatic retester may help. His retest business is primarily limited to scuba cylinders. He tests about 2000 scuba cylinders each year – these cylinders are visually inspected annually and hydro tested with visual inspection every five years. Of those 2000, he condemns nearly 12% prior to the actual pressure test, due to damage observed that exceeds the allowable limits. Of those that pass the pre-test visual examination and are then pressure tested, he condemns only one or two aluminium cylinders and about ten steel cylinders. This example is consistent with enquiries we have made of other hydrostatic retesters on other occasions.


What can divers do to ensure their aluminium cylinders are safe?


The final responsibility for cylinder safety lies with the cylinder owner. They must realise that an 80 cubic foot capacity cylinder contains more than one million foot/lbs of potential energy, very likely equal to a hand grenade. Divers should clean their cylinders, dry them and store them away from excessive heat, always with some internal pressure. They can demand that the fill station have properly functioning filter/separator systems, fill the cylinder slowly, and not overfill it. Cylinders should be inspected frequently, both internally and externally, by a technician who actually knows what to look for in the way of damage. In North America we found that, regardless of how many years of experience an inspector may have handling cylinders, without formal training they cannot properly assess damage.


Do you have any recommendations for filling/test stations to ensure the safety of their staff and customers?


I have written several articles and produced a booklet that details safe practices for fill station operations. There are many things fill stations can do. Unfortunately, many dive stores in North America fail to appreciate the power of compressed air. Since most cylinder explosions occur during the filling process, there needs to be a separation of the operator and the cylinder. Separation can be accomplished by either distance or a barrier. Only cylinders that have been inspected by a trained visual inspector and are within the retest period should be accepted for filling.


What staff training procedures are recommended in the USA to ensure that people testing and filling cylinders are competent?


In Canada and the USA there are federal requirements and licensing for hydrostatic retesters. The test procedures are established by law, and enforcement is administered by the Department of Transportation (DOT) or Transport Canada (TC). By law, the retester must take training every three years and be examined by federal enforcement personnel every five years to retain their license. Unfortunately, enforcement is incomplete and there are many hydro retesters who are inadequately trained and certify improperly evaluated cylinders. The retester must conduct a visual inspection as part of the hydro retest, but many don’t know what that entails. The North American dive industry has established its own standard of an annual or more frequent visual inspection. This standard is rather well enforced by dive stores and other air fill stations. There is no legal mandate that the inspectors be trained. However, untrained inspectors have no legal defence if a cylinder they inspected fails explosively. Most dive industry entities – including PADI, NAUI, Luxfer, Catalina, Compair/Mako, most dive stores and several court jurisdictions – accept the PSI training protocol as the industry standard.


What is the number one thing that divers do to affect the structural strength of their steel or aluminium cylinders?


Cylinder owners are probably most guilty of not regularly removing and cleaning the cylinder beneath the boot; and, for salt water divers, not washing and drying the valve/cylinder interface.


What are your recommendations to divers for maintaining their cylinders?


Cylinder owners should insist that a trained visual inspector inspect both the interior and exterior of the cylinder at least annually. In North America we have to recommend that a visual inspector look into the cylinder after the hydro retest to ensure no water or other foreign material has been left inside. Store the cylinder in a dry, cool location with some pressure. Secure the cylinder during transport. Don’t drop the cylinder or allow it to bang against hard objects. If corrosion is noted, clean it away, assess the damage against allowable limits and protect the area from future deterioration.


Does storing a full cylinder for long periods – several weeks or months – have a detrimental effect on its structural strength?


There are several considerations when making this decision. In a clean dry cylinder, clean dry air will store quite nicely for long periods, certainly a year or more. Poorly filtered air containing various hydrocarbons or other contaminants could deteriorate in quality over weeks or months. The cylinder itself is designed for a sustained pressure load during its entire life. If moisture is left in the cylinder, then the increased oxygen in a full cylinder will contribute to more rapid corrosion that in turn could weaken the cylinder dramatically over a matter of months.

More importantly, internal corrosion activity will reduce the oxygen content of the air. In the USA we have two examples of oxygen depletion in steel cylinders (corrosion is more rapid in steel). Following three months of storage, a corroded cylinder contained only 15% oxygen, a near-deadly deficiency. Analysis of a dead diver’s air supply in a heavily corroded steel cylinder revealed a deadly 6% oxygen content. The University of Rhode Island’s study of cylinder internal corrosion reveals that under equal conditions of moisture in cylinders, aluminium cylinders fare much better, so air stored in aluminium cylinders may retain its quality longer.

There is one additional consideration for aluminium cylinders. Because of its low tolerance for heat, and where there is potential for the aluminium cylinder to be involved in a fire, there is merit in storing the cylinder either full or nearly empty (about 20 psig). A 3000 psig aluminium cylinder containing less than about 1500 psig is at risk of exploding in a fire. The metal will lose its strength before the heat causes a rise in internal pressure sufficient to activate the pressure relief device (burst disc).


Do you have any comment on the effect of hydrotesting on the material of aluminium cylinders?


I am not an expert in metallurgy, so can say very little about material integrity. However, there are specialists who believe that the hydro test procedure may not adequately test aluminium cylinders. The manufacturers cycle many test cylinders to hydro test pressure thousands of times with no apparent damage. I have not heard any expert suggest that the hydro test causes damage to the cylinder.


In New Zealand and Australia, cylinders are currently tested every two years, and this may change to every year. In the USA, it is currently every five years and may change to every ten years! What are your thoughts on the wide difference in the period over which cylinders must be hydro-tested?



The hydro test alone is not a very good assessment of a cylinder’s condition. Cylinders with obvious cracks and pinholes have passed hydro retest. US rules for retest were written more than 50 years ago, when cylinders were used very differently to how they are used by divers today. Experience in the USA shows that a five year interval seems to work for most gas industry cylinders. I can’t speak about the logic used by the rulemakers in New Zealand and Australia. Whether the retest is one year, two years, or five years is irrelevant for scuba cylinders unless a technical visual inspection is part of the test. Remember, 90% of the cylinders failed by a retester fail the visual inspection, not the pressure test.

In a test conducted by the University of Rhode Island in 1971, a steel cylinder with a small amount of salt water inside was damaged in 100 days to a point of imminent explosion. If I were only given a choice of my cylinder being either hydro retested by one of our less diligent retesters or by a PSI trained visual inspector, I would take a frequent visual inspection and skip the hydro test. There is a proposal before DOT to eliminate hydro retest and replace it with an ultrasound examination at ten year intervals. The vast majority of gas industry users oppose this dramatic change. Ultrasound cannot test for heat damage, the impact of continual overpressurisation, or a number of other forms of damage. Because of the outcry from the dive industry and others, DOT has delayed its decision on this matter. Scuba cylinders cannot possibly remain in service for ten years without a formal examination. Should this rule pass, the frequency of cylinder explosions will increase rapidly.


Do you think aluminium cylinders should have a finite life, then be removed from service?


I have followed closely the issue of sustained load cracking of aluminium cylinders since 1985. As yet, I have not found a correlation between age and the formation of a crack. We do see more cracked cylinders now than we saw ten years ago. That probably means that we now have more people looking, and we have better tools such as the Visual Plus apparatus to conduct the assessment. Some of the cracked cylinders were made in the 1980s, while many thousands from the 1970s remain in service.

Since the cracks propagate slowly over several years, technical visual inspection of the suspect area at least annually should identify defective cylinders. Our records show less than ten aluminium scuba cylinders exploded in North America and several international locations, including Australia, that were most likely caused by sustained load cracking. Statistically, that is a very small number (several million were made), although tragic for those persons injured.

We know some of those cracks should have been found by the visual inspectors who last looked at the cylinder. Unfortunately they were not properly trained. PSI does not have information that would cause us to urge a condemnation of scuba cylinders over a certain age. We believe that no cylinder owner should imagine that his cylinder must last forever. It could last a lifetime, or as little as a few months. It all depends on its treatment while in service. Scuba cylinders are very well made and pass through a variety of rigorous tests both by the manufacturer and by third party examiners. Almost without exception, those that have exploded would have been removed from service by trained inspectors well before the failure, if the opportunity to inspect them had been there. This suggests that the opportunity for technical visual inspections should exist and be available to all cylinder owners. The cylinder owners then must be willing to have their cylinders condemned when damage beyond allowable limits is found.


Thanks, Bill, for your time in providing our readers with this invaluable information. You have brough up some important points that all divers and cylinder owners should be aware of.

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